Astronomy Answers
Astronomical Dictionary

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This dictionary explains a number of difficult astronomical words that are also used elsewhere in these web pages. If you find a word in these pages that you do not understand, then you can check if the word is explained in this dictionary.

If the first sentence of the explanation of a word begins with this kind of letters, then that sentence explains the origin of the word. The language of origin of the word is shown between square brackets; usually this is [Greek] or [Latin].

Letter sections: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


to B

the absorbtion line
absorptio = [Latin] drink

An absorption line is a spectral line (a narrow band of colors) in which an object shines less brightly than in other similar colors.

the active region

An active region is an area at the surface of the Sun with a lot of magnetic field in the form of sunspots, pores, and plage. Large active regions can grow to be 160,000 km long (equal to four times around the Earth) and can last for two or more months, but small active regions appear and disappear in a matter of days.

the albedo
albedo = [Latin] white paint or color

The albedo of an object is a number between 0 and 1 that indicates which fraction of incoming light is immediately reflected by the object. The albedo of the Earth is about 0.3: the Earth and its atmosphere reflect about 30 percent of the sunlight that hits them. All other things being equal, a planet looks brighter and has a lower temperature when its albedo is greater.


Altitude and elevation measure how far something is above some reference plane. In daily life, they are used to indicate how far above sea level something is. In astronomy, they are used for that as well, but also to indicate how far (in degrees) something like a planet or a star is above the horizon. This can be confusing, so the preferred use is for elevation to be used for the height above sea level (in meters or feet), and altitude for height above the horizon (in degrees).

In the horizontal coordinate system, altitude is the coordinate that measures the height above the horizon (in degrees). The other coordinate is the azimuth. Because the true horizon depends on the local landscape and the exact location of the observer, astronomers often use an "artificial" horizon that runs exactly midway between the zenith and nadir. If you read about astronomical altitudes, then you may assume they are measured relative to the artificial horizon, unless the accompanying text says otherwise.

the angle
In geometry, an angle is a difference between directions. Angles are commonly measured in degrees. A full circle is equivalent to 360 degrees: If you turn around completely and end up looking in the same direction as you did before, then you've turned over 360 degrees. A right angle is equal to 90 degrees, and the angles between the sides of an equilateral triangle (one with three sides of equal length) is 60 degrees.

A degree is also written as °, so 31° is 31 degrees. A degree is further divided into 60 arcminutes (also written as 60' or 60 minutes of arc), and an arcminute into 60 arcseconds (also written as 60" or 60 seconds of arc), so one degree is equal to 3600 arcseconds. Astronomical objects often appear so small in the sky that their apparent angular sizes are expressed in arcminutes or arcseconds. For instance, the Sun and Moon have angular diameters of about 30 arcminutes, the planet Jupiter has an angular diameter of about 40 arcseconds, and the planet Pluto of 0.1 arcseconds.

The smallest detail a telescope can possible see (which is called its resolution) is also often measured in arcseconds. It is about equal to 0.13 arcseconds divided by the diameter of the primary entrance of the telescope in meters, or to 130 arcseconds over the diameter in centimeters, or to 5.0 arcseconds divided by the diameter in inches. With a perfect 6-inch telescope, you may be able to see details as small as 0.8 arcseconds - so in such a telescope Jupiter would appear as a small disk, but Pluto would look like a point, just like the stars. Often, the resolution of telescopes is worse than what this formula yields, because the mirrors or lenses are not perfect and because the atmosphere of the Earth tends to blur the images.

anoomalia = [Grieks] deviation, roughness

In astronomy, anomalistic means that it has something to do with the apsides of the orbit of a celestial body. The anomalistic month is the time between two passages of the Moon through its perigee.

the anomaly
anoomalia = [Greek] deviation, roughness

In astronomy, anomaly is used for different kinds of angles that are important when calculating the position of objects in their orbits. Below, several different anomalies are explained for the orbit of the Earth around the Sun (or rather: of the Earth around the barycenter of the Solar System, but that is almost the same thing), but they are also used for orbits of other celestial objects.

  1. The true anomaly is the angle (as seen from the Sun) between the Earth and the perihelion of the orbit of the Earth. When the true anomaly is equal to 0 degrees, then the Earth is closest to the Sun (or: in its perihelion). When the true anomaly is equal to 180 degrees, then the Earth is furthest from the Sun (in the aphelion).
  2. The mean anomaly is what the true anomaly would be if the Earth moved with constant speed along a perfectly circular orbit (with an eccentricity equal to zero) around the Sun in the same time. Just as for the true anomaly, the mean anomaly is equal to 0 in the perihelion and to 180 degrees in the aphelion, but at other points along the Earth's orbit the true and mean anomalies are not equal to one another. The mean anomaly is often used for one of the orbital elements.
  3. The eccentric anomaly is an angle that is related to both the mean and the true anomaly. You encounter this angle if you solve Kepler's Equation to find the true anomaly from the mean anomaly.

the aphelion
apo = [Latin] from, Helios = [Greek] Sun

The aphelion is the point in an orbit around the Sun that is furthest from the Sun. The opposite point is called perihelion. The generic word for the furthest point in an orbit around some other object is apoapsis.

the apoapsis
The apoapsis of an orbit of one object around another is the point at which the one object is furthest away from the other object. Apoapsis is the general term for such a point, but there are also many specific terms for specific cases: the aphelion is the furthest point from the Sun in an orbit around the Sun. Likewise, apoastron is linked to other stars, apogee to the Earth, and apojove to Jupiter. The opposite is periapsis.

the apsis
hapsis = [Greek] connection

An apsis is a position in an orbit that is at an extreme distance (either a minimum or a maximum) to the central object. The minimal distance is attained in the periapsis and the maximal distance in the apoapsis.

Ares = [Greek] Mars, graphia = [Greek] description

Arographic means it describes the planet Mars. The arographic coordinate system has the martian equator for base plane, and uses the longitude and latitude for coordinates.

the asteroid
asteroid, plural asteroids; astron = [Greek] star, -oid = [Greek] -like

An asteroid is a rock that orbits the Sun. The largest asteroid has a diameter of about 1000 km, but most are much smaller than that. If they get small enough, then they are sometimes called meteoroids. Most asteroids orbit the Sun at distances between than of the orbits of Mars and Jupiter.

astron = [Greek] star, nomos = [Greek] knowledge

Astronomy is the science that studies everything outside of the Earth. This includes, among other things, the Moon, planets, Sun, stars, black holes, Milky Way, and Universe. People who do astronomy are called astronomers.

the AU or Astronomical Unit

An AU is very close to the average distance between the Sun and the Earth. Distances between the planets and the Sun are often expressed in AU. 1 AU equals about 93 million miles or about 150 million km.

the azimuth
as-sumut = [Arab] the roads

The azimuth is the coordinate from the horizontal coordinate system that indicates the direction along the horizon. The azimuth is measured in degrees, but not everyone uses the same range of azimuth or the same zero point. Sometimes the azimuth is measured between -180 and +180°, sometimes between 0 and 360°, and sometimes with 0° in the south, and sometimes with 0° in the north. For astronomical application it is convenient to set 0° in the south and to measure azimuth between -180 and +180°: that provides the best fit to the hour angle.


to C

the barred spiral galaxy

A barred spiral galaxy is a spiral galaxy with a bar-shaped structure through the center.

the barycenter
barus = [Greek] heavy

The barycenter is the same as the center of mass.

the blueshift

A blueshift is a doppler shift of distinguishing marks (such as spectral lines) in the frequency spectrum of light to greater frequencies, so that yellow light changes in the direction of blue. The opposite of blueshift is redshift.


to D

the calendar
Kalendae = [Latin] first day of the month

A calendar is

  1. a method to combine days into larger units of time, such as weeks, months, and years.
  2. a representation of a division of time, for example on paper.
For more information about calendars, see the Calendars Page of the Astronomical Answer Book.

the center of mass

The center of mass of a system is the average of the positions of all objects in the system, each position weighted with the mass of the object. The sum of all the forces from outside the system on the objects of the system is the same as the force on a single particle with the total mass of the system at the center of mass. Forces between the objects in the system cannot influence the center of mass.

the chromosphere
chrooma = [Greek] color; sphairos = [Greek] ball

The chromosphere is a layer in the Sun that is roughly between about 250 miles (400 km) and 1300 miles (2100 km) above the solar surface. The temperature in the chromosphere varies between about 4000 K at the bottom (the so-called temperature minimum) and 8000 K at the top (6700 and 14,000 degrees F, 3700 and 7700 degrees C), so in this layer (and higher layers) it actually gets hotter if you go further away from the Sun. The density in the chromosphere is much, much smaller than the density of air at sea level on Earth. At the top of the chromosphere there are only about 10 thousand million atoms in each cubic centimetre (100 thousand million atoms per cubic inch). The chromosphere shows up in images taken in the center of the H-alpha spectral line and also (briefly) near the beginning and end of a total solar eclipse.

the coma
komè = [Greek] hair

The coma of a comet is a cloud of water vapor and dust particules around the still frozen center of the comet. The coma develops when the comet gets close enough to the Sun (within about 2.5 AU) that the water ice at the surface of the comet sublimates (changes into water vapor). The coma can grow to be many hundreds of thousands of kilometers (or miles) in size.

the comet
komètès = [Greek] with long hair

A comet is a block of ice and dust with perhaps a rock in the middle, that orbits around the Sun. Most comets come from far beyond the furtherst planet and fly through the inner part of the Solar System in a short time. When comets come close enough to the Sun, then they usually develop a coma and one or more tails.

the conjunction

When two heavenly bodies are in conjunction, then they are very close together in the sky. When astronomers say something like "Jupiter is in conjunction" without mentioning a second heavenly body, then they mean "with the Sun". In such a case Jupiter is not visible at any time of the night. The planets that are further away from the Sun than the Earth (the superior planets) have one conjunction each synodical orbital period. The planets that are closer to the Sun (the inferior planets, Mercury and Venus) have two conjunctions per synodical orbital period: one when they pass between the Sun and the Earth (the inferior conjunction), and one when they pass behind the Sun (the superior conjunction).

For more information, read the Conjunctions Page.

the constellation
con- = [Latin] together, with; stella = [Latin] star

A constellation is

  1. a group of stars that form a pattern that made someone think of a certain person, animal, or object, which makes the pattern easier to remember and find again.
  2. an area of the sky, designated by the International Astronomical Union (IAU), in which there is a group of stars that form a pattern as described above. The area of the sky has the same name as the group of stars.

Professional astronomical publications throughout the world always use the constellations as defined by the IAU, but in the past many other constellations were proposed, and in different areas of the world completely different sets of constellations were (and are still) used.

the continuum
continuum = [Latin] something without breaks

Continuum is the name astronomers use for the combination of all colors that an object such as the Sun emits, and also for the broad variation from color to color in how much light is emitted. Broad means: without looking at the little details, such as spectral lines. The continuum is determined mostly by the temperature of the object. The hotter the object is, the brighter it shines. The color at which an object shines brightest also depends on the temperature. Hot objects such as the Sun shine brightest in yellow light; less hot objects shine most in red light, and cool objects shine brightest in (invisible) infrared light.

the convection
convectio = [Latin] bring together, from con- = with, and vectio = carry

Convection is a form of energy transport in which the material with the energy in it moves. The hotter material moves toward the cooler area and the cooler material toward the hotter area. On Earth, you can see convection in a pan of boiling water (where hot water moves up and cooler water moves down), and also in a thunderstorm (where warmer, moist air moves up and forms clouds). You can also find convection in the convection zone of the Sun, and in granulation.

the convection zone
convectio = [Latin] bring together, from con- = with, and vectio = carry; zona [Latin] from zoone = [Greek] girdle

The convection zone is a layer in the Sun that reaches from just below the surface down to about 130,000 miles (183,000 km) below the surface. The convection zone contains about 2/3 of the Sun's volume (up to the visible surface) but only about 1/60 of the Sun's mass. In this layer the energy of the Sun is transported to the surface by convection. The temperature inside this layer is thought to vary between 2.0 million and 6500 K (4 million and 12,000 degrees F, 2.2 million and 6200 degrees Centigrade), and the density between 100 times more and 4,000 times less than that of air at the Earth's surface.

the coordinate system
co- = [Latin] together, ordinatio = [Latin] ordering, arrangement

A coordinate system is a tool that allows fixing of positions by measuring distances in different directions. Those distances are the coordinates. There are two often-used classes of coordinate systems: those that use rectangular (or cartesian) coordinates, and those that use polar coordinates. Polar coordinates use angles (for the direction) and one distance, and usually have a base plane that the angles are tied to. Rectangular coordinates use only distances, measured in mutually orthogonal directions from a common origin. Coordinate systems are often named for the thing in or on which they measure positions, or for the base plane or origin with which they are associated.

Here are a number of polar coordinate systems that are used in astronomy:

system object base plane longitude latitude
geographical Earth equator longitude latitude
equatorial sky equator right ascension declination
ecliptical sky ecliptic longitude latitude
galactic sky milky way longitude latitude
horizontal sky horizon azimuth altitude
selenographical Moon equator longitude latitude
heliographical Sun equator longitude latitude
arographical Mars equator longitude latitude
jovigraphical Jupiter equator longitude latitude

The column marked "system" indicates the name of the system; "object" lists the object to which the coordinates apply; "base plane" shows the plane that has a latitude of 0; "longitude" and "latitude" provide the names of the coordinates that correspond to longitude and latitude.

The names of polar coordinate systems that fix positions on a celestial body are often made up of the Latin or Greek name of the body, followed by graphical, for example geographical for the Earth. The names of the corresponding rectangular coordinate systems (with the center of the body as origin) usually have the same first part, but followed by centric, for example geocentric for the Earth, planetocentric for planets in general, or heliocentric for the Sun.

the core
core, plural cores

The core of the Sun is the centermost part of the Sun, where all the Sun's energy is produced by nuclear processes. It has a radius of about 86,000 miles (140,000 km). It contains about 1/120 of the Sun's volume (up to the visible surface), and about 1/3 of the Sun's total mass. At the very center of the Sun, the temperature is thought to be about 16 million K (28 million °F, 16 million °C), and the density about 150 times that of water. At the outer edge of the core, the temperature is thought to be 9 million K (17 million °F, 9 million °C), and the density 34 times that of water. The layer immediately around the core is the convection layer.

the corona
corona = [Latin] from koroonè = [Greek] crown

The corona is the outermost layer of the Sun, starting at about 1300 miles (2100 km) above the surface. The temperature in the corona is 500,000 K (900,000 degrees F, 500,000 degrees C) or more, up to a few million K. The corona is very dilute indeed (less than 1000 million atoms per cubic centimetre or 10,000 million atoms per cubic inch) and cannot be seen with the naked eye except during a total solar eclipse, or with the use of a coronagraph.

the coronagraph
corona = [Latin] from koroonè = [Greek] crown; graphia = [Greek] description

A coronagraph is an instrument that can look at the faint outer layers (the corona) of the Sun, by covering the bright solar disk.

the coronal hole
koroonè = [Greek] crown

A coronal hole is an area in the corona of the Sun that appears dark in pictures taken with coronagraphs or during total solar eclipses. There are often coronal holes around the north and south poles of the Sun, especially near the minimum of the solar cycle.

the coronal mass ejection
koroonè = [Greek] crown

Also called Coronal Mass Eruption or Coronal Transient, and generally abbreviated to CME. A CME is a huge eruption of material from the solar corona into interplanetary space. They can look like bubbles or loops or stranger shapes. When seen close to the Sun, these CMEs can be bigger than the Sun itself, but they are also extremely dilute. Near the Earth, the material typically has a density of at most about 6 protons per cubic inch (100 per cm^3), which is equivalent to about 19 orders of magnitude less than the mass density of air.

the cosmic rays
kosmos = [Greek] arrangement

Cosmic rays are actually particles (mostly helium nuclei, protons, and electrons) that travel through space with a relatively very large amount of energy. Because of their large amount of energy, cosmic rays are about as dangerous as X-rays or gamma-rays. Cosmic rays have many sources. Some are formed in solar flares. Others come from beyond our solar system, such as the galactic cosmic rays.


to E

the declination
declinatio = [Latin] deviation

The declination is the coordinate in the equatorial coordinate system in the sky that is similar to latitude on Earth . It ranges between -90 degrees at the southern celestial pole and +90 degrees at the northern celestial pole and is zero at the celestial equator. The other equatorial coordinate is the right ascension.

the degree

A degree is, in general, a measure of how much or how strongly something is present. In science, "degree" is used more specifically:

  1. as a measure of temperature.
  2. as a measure of angle.

the differential rotation
differentis = [Latin] different; rotatio = [Latin] rotation

Differential rotation is rotation in which not every part of the object has the same period of rotation. Only objects that are not solid can show differential rotation. In the Solar System, differential rotation occurs in, among other things, the Sun, the giant gaseous planets, and the atmosphere of the Earth.

the doppler effect
after C.J. Doppler (1803-1853)

The doppler effect is the phenomenon that the frequency of a wave depends on the speed of the source of the waves relative to the receiver. When the source moves towards the receiver (or the receiver towards the source; only the relative speed matters) then the frequency is higher; if the source and the receiver move apart, then the frequency is lower. The relative change in the frequency is approximately equal to the ratio of the speed difference and the speed of propagation of the waves, as long as the speed difference is much smaller than the propagation speed.

For sound, the doppler effect changes the pitch: if a car, motor cycle, or train passes by you fast then the pitch of the sounds they produce goes down. The speed of sound is about 340 m/s (1200 km/h) at sea level on Earth, so the pitch of the sounds coming from a passing car change by one standard note (about 6 percent of frequency change) for about each 9 m/s (32 km/h) of its speed relative to you.

A speed measured through its doppler effect is called a doppler speed. The shift of characteristics in the frequency spectrum of waves (such as spectral lines) is called doppler shift.

Astronomers can use the doppler effect to fairly simply determine the speeds of stars and galaxies relative to us, and also of material in the photosphere of the Sun .

the doppler shift
after C.J. Doppler (1803-1853)

Doppler shift is a shift of characteristics of a frequency spectrum (such as spectral lines) because of the doppler effect. A doppler speed can be calculated from a doppler shift. A doppler shift of light to smaller frequencies is called redshift, and a doppler shift to greater frequencies blueshift.

the Doppler velocity
Doppler velocity is the speed in the direction of the line of sight, i.e. directed toward you or away from you. The Doppler velocity is named after the Doppler effect (named after the discoverer, Mr Doppler), which is the effect that the frequency of a tone changes when the speed of the source in the line of sight changes. The same effect occurs in frequencies of light and other electromagnetic radiation and enables us to measure velocities on the Sun. An image showing Doppler velocities is called a dopplergram. Redshift is a kind of Doppler shift.


to F

the Earth

The Earth is the third planet of the Solar System, counted from the Sun. It is a terrestrial planet, with an atmosphere and a moon (the Moon), but without any rings.

the eccentricity

The eccentricity of an orbit is one of the orbital elements. It is a number that indicates how much the orbit deviates from a circle. A circular orbit has an eccentricity equal to zero. An elliptical orbit has an eccentricity between zero and one. In this case, the eccentricity is equal to the difference between the lengths of the long and short axes of the ellipse, divided by the sum of those lengths. A parabolic orbit has an eccentricity of one, and a hyperbolic orbit has an eccentricity larger than one.

Orbits with eccentricities less than one are closed, so the objects in such orbits return to the same position regularly. Orbits with eccentricities greater than one are open, which means that objects in such orbits never return to the same position.

the ecliptic

The ecliptic is

  1. the plane of the Earth 's orbit.
  2. the line where the plane of the Earth's orbit intersects the celestial sphere. This is almost the same as the path that the Sun appears to take between the stars, as seen from the Earth.
The ecliptic passes through the middle of the zodiac. All of the planets and the Moon also stay close to the ecliptic.

ekleiptikos = [Greek] belonging to an eclipse

Ecliptical means that it has something to do with the ecliptic. In the ecliptical coordinate system, positions in the sky are indicated with the ecliptical longitude and latitude. The latitude is measured relative to the ecliptic, and the longitude relative to the vernal equinox.

the electromagnetic radiation
elektron = [Greek] gold or amber; he Magnetos lithos = [Greek] stone of Magnesia; radiatio = [Latin] radiate

Electromagnetic radiation is any kind of radiation that consists of alternating electric and magnetic fields and that can propagate even in a vacuum. Electromagnetic radiation is characterized by its wavelength (or, equivalently, its frequency or energy). Some different types of electromagnetic radiation or electromagnetic waves, in increasing order of frequency and energy and decreasing order of wavelength, are: radio waves, microwaves , infrared light, visible light, ultraviolet light, X-rays, and gamma rays. In general, the later in the list the type is, the more dangerous it is. Visible light is the only form of electromagnetic radiation that we can see with our eyes.

the elliptical galaxy

An elliptical galaxy is a galaxy that appears to have an elliptical shape no matter from which side you see it. An elliptical galaxy contains no arms or bar.

the elongation

The elongation of a celestial body is the angular distance, in the sky, of that body to the Sun. The elongation cannot exceed 180 degrees by definition. Superior planets can have any elongation up to the maximum 180 degrees, but inferior planets have a maximum elongation that is less than 180°. The maximum elongation of Mercury is about 28°, and that of Venus about 48°.

the emission line
emissio = [Latin] send away

An emission line is a spectral line (a very narrow set of colors) at which an object shines more brightly than at nearby colors.

the energy transport
energeia = [Greek] ability to do work; trans- = [Latin] over, portare = [Latin] carry

Energy transport can be done in three ways:

  1. by radiation.
  2. by convection (motion of a material).
  3. by conduction (transport through a material).

If you stick one end of a strip of metal in a box such that no light can get in, and then stick the other end of the strip in a fire, then all three kinds of energy transport occur. The hot gases rise, and so transport the energy (heat) by convection. The gases shine brightly both in the visible colors and in the infrared, and so transport energy by radiation. You can feel the infrared radiation on the exposed skin of your face and hands, and if you hold your hand in front of your face, then your face cools off because the radiation no longer reaches it. Conduction occurs in the metal strip. The enclosed end of the strip heats up, even though the end is inside a closed box, so that neither the hot gases nor the radiation can get to it. Instead, the heat moved through the metal.

The Sun is not solid, and conduction does not work very well in it. It has to choose between radiation and convection, and picks whichever method works best in a given situation. Basically, if the temperature gradient (change with position) is small enough, then transport by radiation works best, as in the radiative zone. If the temperature gradient is too great, then transport by convection works better, as in the convection zone. Something similar happens in a pan of water that is heated up. At first the temperature difference between top and bottom is small, so conduction and radiation work well and the water is at rest. After a while the temperature gradient becomes too large and then convection takes over: the water boils.

the epoch

An epoch is

  1. the beginning (the start of day 1 of year 1) of a calendar. This need not be the first day on which the calendar is used. For example, the epoch of the Gregorian calendar lies over 1500 years before the first day on which it was used.
  2. the time for which a set of orbital elements is valid. Predictions of positions based on those orbital elements will become less accurate the further from the epoch the time is for which the prediction is made.

the equator
aequatio = [Latin] make equal

The equator is equally far from both geographical poles and divides the Earth into a Northern and a Southern part. By extension, there is also an equator in the sky, which divides the sky into a Northern and a Southern part.

aequatio = [Latin] make equal

Equatorial means it has something to do with the equator. In the equatorial coordinate system, positions in the sky are indicated with the right ascension (relative to the vernal equinox) and the declination (relative to the celestial equator). The celesital equator is the extension of the Earth's equator into the sky, halfway between the celestial poles, which themselves are the extensions of the rotation axis of the Earth.

the equinox
aequinoctium = [Latin] equinox, from aequatio = [Latin] make equal, and noctium, nox = [Latin] night

The equinox is

  1. the moment when the Sun crosses the celestial equator. Close to an equinox, day and night have nearly the same length (12 hours) everywhere on Earth. The equinoxes signal the beginning of the seasons of spring and autumn. The March equinox signals the beginning of spring in the Northern hemisphere, and the beginning of autumn in the Southern hemisphere. The March equinox is also called the vernal equinox, and the September equinox the autumnal equinox. The beginning of the other seasons is governed by the solstices.
  2. the place in the sky between the stars where the Sun is during the March equinox. This location is also called the vernal equinox. In the equatorial and ecliptical coordinate systems, the vernal equinox has longitude and latitude equal to zero.

    Because of the precession of the equinoxes, the equinox slowly moves between the stars, so when one quotes ecliptical or equatorial coordinates, one has to indicate relative to which equinox these coordinates are measured. Three equinoxes that are commonly used in stellar atlases and planetary calculations are those of 1950.0 (the beginning of the year 1950), 2000.0 (the beginning of the year 2000), and the equinox of the date (i.e., the equinox of the same date as the coordinates themselves).

the exponential notation

Exponential notation is a method for writing numbers that is used a lot in science because it is convenient for writing the very large and very small numbers that occur often in science. Exponential notation consists of two numbers, the mantissa and the exponent. The mantissa is a number that may be fractional (with a part after the decimal point) and that is usually between 1 and 10. The exponent is a whole number that indicates by which power of 10 you should multiply the mantissa, so by how many places you should shift the decimal point to the left (if the exponent is negative) or the right (if the exponent is positive).

There are different methods in use for linking the mantissa and exponent of a number. The proper mathematical way is to put "times ten to the power of" between the mantissa and the exponent, but this requires the exponent to be written in superscript, a bit higher than normal, and that cannot be displayed properly (or at all) on simple screens. That's why in computer-related texts people usually put a letter "E" or "e" between the mantissa and the exponent.

For example, three million million million, a three followed by 18 zeros, is written in exponential notation as 3×1018 (the 18 ought to be written a bit higher than the 10, if your screen supports this) or 3e18. So, 3.2e3 is equal to 3200 and 3.2e-3 is equal to 0.0032.

In general, people use either exponential or normal notation, whichever is shorter. Exponential notation is convenient for describing numbers that are many orders of magnitude different from 1.


to G

a falling star

A falling star is the same as a meteor.

the filter
filtrum = [Middle Latin] compacted wool; from feltir = [German] felt

A filter is a device that transmits only certain (ranges of) colors (or frequencies) from the electromagnetic spectrum. A broadband filter transmits a wide range of frequencies, and a narrowband filter only a small range. With a narrowband filter, spectral lines can be measured.


to H

galaktos = [Greek] milk

Galactic means one of the following things:

  1. that it has something to do with a galaxy .
  2. that it has something to do with the Milky Way Galaxy, the galaxy that we are in.
  3. that it has something to do with the location of the Milky Way Galaxy in the sky as seen from Earth.

The galactic coordinate system has the Milky Way (as seen from Earth) for base plane. The galactic latitude is measured relative to the plane of the Milky Way Galaxy, and the galactic longitude relative to the direction to the center of the Milky Way. Because of more accurate information about where the center of the Milky Way is, the old galactic coordinate system was replaced with a new and improved one a few decades ago.

the galaxy
'ga.lax.y; galaktos = [Greek] milk

A galaxy is a region of space with millions to thousands of millions of stars and many clouds of gas that are tied together by their gravity. A typical spiral galaxy or elliptical galaxy (two types of galaxies) contains 100 thousand million stars, has a diameter of 100,000 lightyears, and is at a distance of a few million lightyears from other neighboring spiral or elliptical galaxies. Our Solar System is part of the Milky Way, which is a fairly ordinary spiral galaxy. For more information about galaxies, look at the Galaxy page of the Universe Tree.

the gamma rays
gamma = [Greek] third letter

Gamma rays are a form of electromagnetic radiation with a large amount of energy, and therefore dangerous. Gamma rays are made naturally by material with temperatures of millions of degrees, as occurs in some solar flares .

Gè = [Greek] Earth; kentrum = [Greek] center, middle

Geocentric means it has something to do with the center of the Earth.

Geocentric coordinate systems have the center of the Earth for their origin, and are almost always rectangular coordinate systems, but can vary in the directions of the coordinate axes. Oft-used geocentric coordinate systems are the equatorial geocentric coordinate system and the ecliptical geocentric coordinate system.

The "geocentric world view" or "geocentric model of the Universe" states that the Earth is the center of the Solar System and the Universe, and that all other celestial bodies orbit around the Earth. This model was assumed to be correct for about 2000 years in Europe, until observations made from about the 16th century showed that the heliocentric model of the Solar System was better.

Gè = [Greek] Earth; graphia = [Greek] description

Geographic means that it describes the Earth. The geographical coordinate system measures the latitude relative to the equator, and the longitude relative to the prime meridian which runs, for historical reasons, through Greenwich in London.

Gè = [Greek] Earth; hè Magnetis lithos = [Greek] stone from Magnesia

Geomagnetic means it has something to do with the magnetic field of the Earth.

the granulation
granulus = [Latin] small grain

Granulation covers almost all of the visible surface of the Sun. It resembles rice pudding, with bright "rice grains", called granules, that are separated from one another by a network of dark lanes, called intergranular lanes.

the granule
granulus = [Latin] small grain

A granule is a ball of hot solar gas that has appeared at the surface of the Sun. A typical granule has a roughly round shape and a diameter of about 1,000 km. The gas in a granule cools down when it has reached the surface and flows back down through the intergranular lanes. Granules and intergranular lanes together are called granulation and are a form of convection.


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Helios = [Greek] Sun; kentrum = [Greek] center, middle

Heliocentric means it has the Sun as its center.

Heliocentric coordinate systems have the center of the Sun for their origin, and are almost always rectangular coordinate systems, but vary in the directions of the coordinate axes. Oft-used heliocentric coordinate systems are the equatorial heliocentric coordinate system and the ecliptical heliocentric coordinate system.

The "heliocentric world view" or "heliocentric model of the Solar System" states that the Sun is the center of the Solar System, around which all planets orbit. Since about the 16th century astronomers know that the heliocentric view fits better than the geocentric world view that was taken until then.

Helios = [Greek] Sun; graphia = [Greek] description

Heliographic means it describes the Sun. The heliographic coordinate system has the solar equator for base plane, and uses longitude and latitude for coordinates.

the horizon
horizoon = [Greek] boundary

The horizon is the line at which the sky and the land appear to meet. On a smooth sea with your eye half in the water, the horizon is a straight line midway between the zenith and the nadir, at an altitude of zero degrees. With your eye above the water, the horizon at sea has a slightly negative altitude. If mountains or buildings or other tall things are around, then the horizon is not a straight line.

horizoon = [Greek] boundary

Horizontal means it has something to do with the horizon. The horizontal coordinate system uses the horizon for base plane, and the azimuth and altitude for coordinates.

the hour angle

The hour angle of a celestial body is the difference in right ascension between that body and the meridian (of right ascension) that is due south at that time. The hour angle is usually measured not in degrees but in hours, minutes, and seconds, just like the right ascension. The hour angle indicates (in sidereal time) how long ago the body was due south.


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the inclination
inclinatio = [Latin] slope

In astronomy, inclination is an angle between some direction and a standard plane. Inclination is used as name for

  1. the angle between the orbit of a planet or other celestial body and the base plane of the coordinate system (usually the ecliptic for bodies in the Solar System). The inclination is one of the orbital elements.
  2. the angle that the magnetic field makes with the local surface.

the inferior planet

An inferior planet is a planet that is closer to the Sun than the Earth is. Only inferior planets have an inferior conjunction and an elongation that never exceeds a maximum value that is smaller than 180 degrees. The inferior planets are: Mercury and Venus. The opposite of an inferior planet is a superior planet.

infera = [Latin] under

Infrared radiation is a kind of electromagnetic radiation with wavelengths just larger than those of visible light (on the red side). People sense infrared radiation as "heat rays", which is an example of energy transport.

the intergranular lane
inter- = [Latin] between; granulus = [Latin] little grain

The intergranular lanes are the darker lanes between the granules in granulation where cooler solar gas is flowing down below the surface.


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Jovi- = [Latin] of Jupiter; graphia = [Greek] description

Jovigraphic means that it describes the planet Jupiter. The jovigraphic coordinate system uses Jupiter's equator for base plane, and uses longitude and latitude for coordinates.

The visible surface of Jupiter is not solid but is made up of clouds. How long it takes for a particular cloud to revolve once around the rotation axis of the planet depends on the jovigraphic latitude: The clouds of Jupiter show differential rotation. Astronomers have therefore defined several different jovigraphic coordinate systems, referred to as "System I" and "System II".

Jupiter = [Latin] chief god

Jupiter is the fifth planet of the Solar System, counting from the Sun. Jupiter is a jovian planet, with a very dense and thick atmosphere (of mostly hydrogen and helium), many moons, and narrow rings. Jupiter is the largest and most massive planet in our Solar System.


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the latitude

Latitude is a coordinate that is used to fix positions on a sphere. The latitude of a place is the distance of that place from the equator of the coordinate system, measured in degrees along a meridian. Places at the equator have a latitude of 0; the north pole has a latitude of +90° (or 90° north latitude), and the south pole has latitude -90° (or 90° south latitude). In the sky, latitude is used in the ecliptical and galactic coordinate systems. The corresponding second coordinate is the longitude.

the lightyear
'light.year, plural lightyears

A lightyear is the distance that light travels in a vacuum in a year. So, a lightyear is not a measure of time but of distance. A lightyear is not an official unit and it is therefore not specified which definition of year you should use to calculate its length. You may use the length of the Julian year. With that year, a lightyear corresponds to about 9,460,730,000,000 km or 5,878,640,000,000,000 miles or 9.46073 Pm or 9.46073e15 m or 63241.08 AU or 0.3066 pc

the longitude

The longitude is a coordinate that is used to measure lccations on a sphere or directions in the sky. The longitude of a location is equal to the distance of the meridian of that location from the prime meridian of the coordinate system, measured in degrees along the equator.

Places on the prime meridian have a longitude equal to zero. On almost all celestial bodies in the Solar System and also in the sky, the longitude is measured from 0° to 360° such that it increases towards the west. On Earth and the Moon, the longitude is measured eastward and westward to 180° from the prime meridian.

In the sky, longitude is used in the ecliptical and galactic coordinate systems. The corresponding second coordinate is the latitude.

'lu.nar; lunaire = [French] of the Moon; from [Latin] Luna = Moon

Lunar means it has something to do with the Moon.

Luna = [Latin] Moon, Sol = [Latin] Sun

Lunisolar means it has something to do with both the Sun and the Moon.


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the magnetic field
hè Magnetis lithos = [Greek] stone of Magnesia

The magnetic field is a force field that is associated with moving electrical charge. A magnetic field can influence electrically charged particles and certain metals by attracting or repulsing them. Almost all solar gas is susceptible to the effects of magnetic field.

Magnetic field behaves as if it is made up of closed magnetic field lines (as you can see if you hold a magnet under a transparent plate with iron filings on it and then gently tap the plate). Magnetic field on the Sun appears to exist in only two forms: either it is so weak that it is swept away by the motion of the solar gas, or it is so strong that it inhibits free motion of the gas (for example in convection). In the latter case, the magnetic field is made up of flux tubes: isolated tube-like things in which the magnetic field is strong, while it is weak or absent outside of the flux tube. Most interesting things on the Sun have something to do with magnetic field: sunspots, pores, plage, filaments, solar flares, and prominences. A famous quote in solar physics, attributed to Robert B. Leighton (around 1970), is

If the Sun didn't have a magnetic field, then it would be as boring as most people think it is.

Solar physicists measure the strength of the magnetic field in units of one gauss (G). The magnetic field of the Earth is at most about 1 G strong. The magnetic field in a sunspot at the solar surface can reach a strength of 3000 G.

the major axis

The major axis is the longest straight line that fits in an ellipse. One half of the major axis is called the semimajor axis and that one is used often for one of the orbital elements.

Mars = [Latin] god of war

Mars is the fourth planet of the Solar System, counting from the Sun. Mars is a terrestrial planet with a very thin atmosphere (of mostly carbon dioxide) and two small moons but no rings.

'; Mercurius = [Latin] the messenger of the gods
  1. In astronomy, Mercury is the first planet of our Solar System , counting from the Sun. Mercury is a terrestrial planet without moons, rings, or an atmosphere.
  2. On Earth, mercury is a metal that is liquid at room temperature.

the meridian
meridianus = [Latin] concering noon

A meridian is

  1. a half-circle (semicircle) that runs across the surface between the geographic poles of a planetary body. Meridians intersect the equator at right angles. Observers on the same meridian see the Sun cross the south at the same time.
  2. the half-circle that runs from the equatorial north pole (near the Pole Star) via the zenith to the equatorial south pole. When the Sun goes through the meridian, then it is highest in the sky for the day, and then it is noon. Also called the celestial meridian.

the meteor
meteooros = [Greek] lifted up, floating in the air

Meteors or falling stars are bits of stone, usually of the size of a grain or sand or smaller, that enter the atmosphere, are heated by friction with the air, and then burn up (usually completely) while they shine brightly. Meteors are visible as fast-moving and only momentarily visible points of light, usually soundless. Most meteors have burned up completely when they are still at great heights (70 km or so above the ground). Sometimes they leave behind a glowing trail that lasts for a little while. If a larger meteor breaks up into many pieces during its fall, then this often yields an explosion of light. Large meteors can appear brighter in the sky than the brightest stars and planets.

In space, before a meteor reaches the atmosphere, it is called a meteoroid. If a meteor does not burn up completely, so that part of it reaches the ground, then such a piece is called a meteorite.

the meteorite
meteooros = [Greek] lifted up, floating in the air

A meteorite is a piece of rock that came out of space and fell to the ground. Before it entered the atmosphere it was a meteoroid, and when it was passing through the atmosphere it was a meteor.

the meteoroid
meteooros = [Greek] lifted up, floating in the air

A meteoroid is a piece of rock that floats in space. If it enters the atmosphere then it is called a meteor, and if it reaches the ground then it is called a meteorite.

the microwave
mikros = [Greek] small

Microwaves are a kind of electromagnetic radiation with wavelengths between those of infrared light and radio waves.

the month

A month is a period of time that is (historically) tied to the motion of the Moon around the Earth. Quite a few different kinds of months are in use:

the moon
moon, plural moons

A moon is a celestial body that orbits around a bigger celestial body. The Earth not only has a natural moon (the Moon), but also a couple of thousands of artificial moons, launched from the surface of the Earth. Most planets of our Solar System have one or more moons. The biggest planets have more than ten. Another word for moon is satellite. The month is named for the Moon.


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the nadir
nadzir as samt = [Arab] opposite the zenith

The nadir is the direction straight down, at altitude -90°. The opposite direction in the sky is called the zenith.

the names of celestial bodies

The brightest celestial bodies, such as the Sun, Moon, and planets, have their own names in pretty much every language. If a common name is needed, then usually a greek or latin name is used. The following table shows the english, greek, and latin names of a number of celestial bodies, and also the prefixes and postfixes that can be used to refer to those bodies.

English Greek Latin Prefixes Postfixes
Sun Hèlios Sol helio- -helium
Moon Selènè Luna seleno- -selenium
Mercury Hermes Mercurius mecurio- -mercurium
Venus Aphroditè Venus venero- -venerum
Earth Terra geo- -gee
Mars Ares Mars aro- -martium
Jupiter Zeus Iupiter jovi- -jovum
Saturn Chronos Saturnus saturno- -saturnum
Uranus Ouranos Uranus urano- -uranum
Neptune Poseidon Neptunus neptuno- -neptunum
Pluto Ploutoon Pluto pluto- -plutum
Milky Way galaxias kuklos galaxis galacto- -galactium
planet planètès astèr planetus planeto- -planetum
star astèr siderus astro- sider- -astron

Some examples: geography, selenographical, heliocentric, periastron, apogee, sidereal, astronomy.

Neptune = [Latin] God of the Sea

Neptune is the eighth planet of the Solar System, counting from the Sun. Neptune is a jovian planet with a very dense and thick atmosphere (made mostly of hydrogen and helium) and many moons.

the node

The nodes of the orbit of a celestial body are the two places where the orbit passes through the base plane of the coordinate system. The node at which the object goes through the base plane from south to north is called the ascending node, and the node at which the object goes from north to south through the base plane is called the descending node.


Here is a list of British English and American English names of large numbers and small fractions, and the prefixes of units that go with them. In the title row, "om" stands for order of magnitude: this indicates how many zeros are involved. The prefix can be used with units: for example, a kilometer or km is equal to 1,000 meters, and a megameter or Mm is equal to 1,000,000 m or 1,000 kilometers.

British American om prefix number
trillion 18 exa = E
quadrillion 15 peta = P
billion trillion 12 tera = T
billion 9 giga = G
million million 6 mega = M 1.000.000
thousand thousand 3 kilo = k 1.000
hundred hundred 2 hecto = h 100
ten ten 1 deca = da 10
tenth tenth -1 deci = d 1/10
hundredth hundredth -2 centi = c 1/100
thousandth thousandth -3 milli = m 1/1.000
millionth millionth -6 micro = µ 1/1.000.000
billionth -9 nano = n 1/
billionth trillionth -12 pico = p 1/
quadrillionth -15 femto = f 1/
trillionth -18 atto = a 1/
-21 1/


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the opposition
oppositio = [Latin] opposite

Two celestial bodies are in opposition when they are in opposite directions in the sky (as seen from Earth). If only one celestial body is mentioned, then the second body is understood to be the Sun. Only planets or other celestial bodies that are further from the Sun than the Earth is can be in opposition (to the Sun). Around that time, they are above the horizon all night, and therefore the time of opposition is usually the best time to see the object. The opposite of an opposition is a conjunction.

the orbital element

The orbits of things around much more massive celestial bodies (for example, of a planet around the Sun, or of a moon around a planet, or of a spacecraft around a moon, planet, or the Sun) are often very close to conic sections such as circles, ellipses, parabolas, hyperbolas or straight lines. Five numbers are needed to specify the size, shape, and orientation of such an orbit, and with a sixth number you can also fix the position of the thing in the orbit. These six numbers are the orbital elements.

The most common set of orbital elements consists of the length of the semimajor axis and the eccentricity for the size and shape of the orbit, and the inclination, the ecliptical longitude of the ascending node, and the argument of the periapsis (the three of them called the Euler angles) for the orientation of the orbit. For the sixth orbital element, either the mean anomaly at a certain time or a time at which the object goes through its periapsis is often used.

the order of magnitude

An order of magnitude is a factor of about 10. Orders of magnitude are used a lot by astronomers and physicists, who study very small and very large things and so talk about numbers that may not be known very precisely but that have many zeros before or after the decimal point. If an astronomer says that some thing is three orders of magnitude larger than some other thing, then the astronomer means that the second thing is about 1000 times larger than the first thing. It could be 3000 times, or perhaps only 500 times, but not only 100 times or as much as 10,000 times.

For each extra order of magnitude, you must multiply by an extra factor of 10. Orders of magnitude are especially handy for very large numbers. It is easier to talk about 18 orders of magnitude than about the number one trillion (How many zeros did that have again in British English? And wasn't that different in American English?) or one million million million, or 1,000,000,000,000,000,000, or 1e18. 18 orders of magnitude is about the ratio between the diameter of the Earth and the diameter of an atom, and also about the ratio of the diameter of the visible Universe and the diameter of the Earth.


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the parsec
'par.sec, plural parsecs; parallax + second

The parsec is approximately the distance at which the radius of the orbit of the Earth around the Sun covers an angle of 1 second of arc. A star at a distance of 1 parsec shows an annual parallax of 1 second of arc. A parsec corresponds to 648,000/pi = about 206,264.8062 AU or 30,856,780,000,000 km or 19,173,560,000,000 miles or 3.085678e13 m or 30.085678 Pm or 3.2616 lightyears. A parsec may be abbreviated to pc. In astronomy, distances occur that are large even when measured in parsecs. Therefore standard abbreviations with SI-prefixes are used, too, such as kpc for 1000 pc, Mpc for 1000 kpc, and even Gpc for 1000 Mpc.

the penumbra
paene = [Latin] almost; umbra = [Latin] shadow

The penumbra is the outermost part of a sunspot , in which thread-like light and dark filaments mostly point away from the umbra. These structures are formed by strong magnetic field.

the periapsis
peri- = [Greek] around; hapsis = [Greek] connection

The periapsis is the point in an orbit around a celestial body that is closest to the celestial body. The opposite point is the apoapsis. For some celestial bodies more specific names are available that refer to just those bodies, for example perihelion for the Sun, perigee for the Earth, periselene for the Moon, perijove for Jupiter , or periastron for stars.

the perigee
peri- = [Greek] around; Gè = [Greek] Earth

The perigee is the point in an orbit around the Earth that is closest to the Earth. The opposite point is called apogee. The more general word, that can be used also for other bodies, is periapsis.

the perihelion
peri- = [Greek] around; Helios = [Greek] Sun

The perihelion is the point in an orbit around the Sun that is closest to the Sun. The opposite point is called aphelion. The more general word is periapsis.

the photosphere
phootos = [Greek] light; sphairos = [Greek] ball

The photosphere is the deepest layer of the Sun that we can see. This layer reaches from the surface visible in the center of the solar disk to about 500 km above that height. The temperature in the photosphere reaches from about 6500 K at the bottom to about 4000 K at the top, and the density of material in the photosphere reaches from about 4000 to 200,000 times smaller than the density of air at sea level on Earth. Almost the whole photosphere is covered with granulation.

the plage
plage = [French] beach

Plage is an area on the Sun that is brighter than its surroundings if one looks at it in the middle of a spectral line. The greater brightness means that relatively small magnetic flux tubes stick through the surface there. Plage is invisible in images taken in the continuum, except close to the edges of the solar disk, but even there the contrast between plage and its surroundings is quite small.

', plural planets; planètès astèr = [Greek] wandering star

A planet is a large spherical object with a diameter between about 1000 and 300,000 km. A planet is massive enough that its own gravity keeps it round (in contrast to asteroids, comets, and small moons), but too small to generate energy in its core through nuclear fusion (in contrast to stars). For more information about planets, look at the Universe Family Tree.

planètès astèr = [Greek] wandering star; kentrum = [Greek] middle, center

Planetocentric means: with a planet at the center, or relative to the center of a planet. Planetocentric coordinate systems have the center of the planet for origin, and are almost always rectangular coordinate systems, but vary in the directions of the coordinate axes. Oft-used planetocentric coordinate systems are the planetocentric equatorial coordinate system and the planetocentric ecliptical coordinate system.


Pluto is the ninth planet of the Solar System, counting from the Sun. Pluto is a small icy planet with one moon, Charon.

the pole

A pole of a celestial body is one of the two intersections of the surface of the body and the rotation axis of the body. Traditionally, the pole from above where the body appears to rotate counterclockwise is called the north pole, and the pole from above where the body appears to rotate clockwise is called the south pole. Seen from the north pole, the Sun goes through the sky from left to right. Seen from the south pole, the Sun goes from right to left.

the pore
poros = [Latin] opening

A pore is like a sunspot but has no penumbra.

the precession
praecessio = [Latin] going first

The rotation axis of the Earth is slowly spinning around the poles of the ecliptic because of the gravitational attraction between the equatorial bulges of the Earth and the Moon and Sun. It takes about 26,000 years before the axis has spun around once. This spinning motion is called precession.

One result of the precession is that the vernal equinox slowly moves between the stars in the sky, at a speed of one degree each 71.6 years. This motion is called the precession of the equinoxes. The places between the stars in the sky where the Sun is at the beginning of the seasons are tied to the vernal equinox, and so are also influenced by the precession. Since about the year -68, the vernal equinox moves through the modern constellation of Pisces (the Fishes), and it will move into the constellation of Aquarius (the Waterman) around the year 2597.

Another result of the precession is that the poles of the sky also move between the stars. At the moment, the celestial north pole is quite close to the bright star alpha Ursae Minoris (alpha UMi, also called Polaris or the North Star).


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the quasar
quasar, plural quasars; [English] quasi-stellar radio source

A quasar is a celestial body that at first sight looks like a star (i.e., like a small point of light), but that after closer investigation often turns out to be in the center of a galaxy. A quasar is far brighter than the rest of the galaxy, and can therefore be seen at much greater distances than the galaxy itself. Corrected for the distance, quasars are about 100 times brighter than the brightest galaxies.

Astronomers think that quasars are really large black holes that capture great amounts of material from the surrounding galaxy, and that that material emits much energy as it falls into the black hole, not just as visible light, but also as, for example, radio waves, which is where quasars were first discovered.

All quasars are far away, which means they existed long ago only. It seems that many galaxies were quasars for a while in their youth.


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the radio wave
radius = [Latin] stick, radius

Radio waves are electromagnetic waves with wavelengths greater than those of microwaves.

the redshift

A redshift is a doppler shift, of characteristics (such as spectral lines) in the frequency spectrum of light, to smaller frequencies, so that yellow light gets more reddish. The opposite of redshift is blueshift. The ratio of the frequencies of redshifted characteristics and the frequencies of the unshifted characteristics is also called redshift, usually denoted by a variable called "z".

Systematic redshift is found in the spectra of all but the closest of galaxies, which is seen as evidence for the expansion of the Universe.

the right ascension

The right ascension is the coordinate from the equatorial coordinate system in the sky that has the same role as the longitude in other coordinate systems. The right ascension is measured from the vernal equinox. The right ascension is usually measured not in degrees as the other longitudes are, but rather in units of time, such that 360 degrees correspond to 24 hours of right ascension, and 15 degrees to 1 hour of right ascension. Just like for real time, an hour (symbol: h) of right ascension is divided into 60 minutes (symbol: m), and one minute into 60 seconds (symbol: s). Here is an example of a right ascension: 5h23m12s, or 5 hours, 23 minutes, and 12 seconds.


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the satellite
satellite, plural satellites

A satellite is another name for a moon.

Saturn; Saturnus = [Latin] father of Jupiter

Saturn is the sixth planet of the Solar System, counting from the Sun. Saturn is a jovian planet with a very dense and thick atmosphere (made mostly of hydrogen and helium), many moons, and wide rings.

Selènè = [Greek] Moon; graphia = [Greek] description

Selenographic means it describes the Moon. The selenographic coordinate system has the lunar equator for base plane, and uses the longitude and latitude for coordinates.

siderus = [Latin] star

Sidereal means it has something to do with the stars. A sidereal period of time (such as the sidereal month and the sidereal year) is a period of time after which a celestial body reaches the same position relative to the stars.

the sidereal time

The sidereal time is equal to the right ascension that passes through the celestial meridian at that moment. If a given sidereal time (between 0 and 24 hours) returns, then the stars are again in the same places in the sky (seen from the same place on Earth). The sidereal time runs a little faster than ordinary time: 24 hours of sidereal time correspond to 23 hours 56 minutes 4 seconds of ordinary time. Near the equinox of september, the sidereal time and solar time are almost equal.

'so.lar; solaire = [French] of the Sun, from [Latin] Sol = Sun

Solar means it has something to do with the Sun.

the solar cycle

The number of sunspots on the Sun varies fairly regularly with a period of about 11 years which is called the sunspot cycle. When the number of sunspots is smallest (zero or close to it) then the Sun is in the sunspot minimum. When the number of sunspots is greatest, then the Sun is in the sunspot maximum. The strength of other phenomenons changes with the number of sunspots, for example the number of solar flares and the number of coronal mass ejections. The sunspot cycle is therefore also called the solar activity cycle.

the solar eclipse

A solar eclipse is the covering of the disk of the Sun by the Moon. If the Sun is wholly covered by the Moon then it is a total solar eclipse. If only part of the Sun is covered, then it is a partial solar eclipse.

the solar flare

Solar flares are explosions near the solar surface in which the local magnetic field is relaxed. This releases a lot of energy which heats up the solar gas and launches it into space. Solar flares usually occur near active regions.

solar physics
physica = [Latin] physics

Solar physics is the science that studies the Sun. It is part of astronomy. People who do solar physics are called solar physicists.

the solar physicist
physicus = [Latin] physicist

A solar physicist is a physicist or astronomer that studies the Sun using the tools of physics. The science that studies the Sun is called solar physics.

the Solar System
solar system, plural solar systems

The Solar System is the star system of the Sun and consists of the Sun itself and everything that orbits around the Sun, such as planets (e.g., the Earth), comets, and asteroids. Because the Sun is a star, star systems of other stars are also sometimes called solar systems.

the solar time

Solar time is the time measured according to the position of the Sun in the sky. It is 12 o'clock solar time (noon) when the Sun (seen from outside the tropics) is highest in the sky and (seen from the northern hemisphere) exactly to the south. You can measure the solar time with a well-adjusted [zonnewijzer]. Solar time is usually not equal to the official clock time. In the Netherlands and Belgium, solar time is about 40 minutes behind official clock time in winter, and about one hour and 40 minutes behind in summer. For more information, see the time page of the Answer Book.

the solstice

A solstice is a moment when the declination of the Sun changes from increasing to decreasing, or from decreasing to increasing. The solstices mark midsummer's day and midwinter's day, and the beginning of the astronomical seasons of summer and winter.

the spectral line
spectum = [Latin] to watch

A spectral line is a very narrow range of wavelengths (colors) at which a bright object such as the Sun shines more brightly or less brightly than at neighboring wavelengths. A spectral line in which an object shines less brightly is called an absorption line, and a spectral line in which an object shines more brightly is called an emission line.

Every substance has its own set of spectral lines, so spectral lines give a sort of fingerprint of the associated substance. The strength of a spectral line (how much absorption or emission it has) depends on many things, including the temperature and gas pressure of the material, and sometimes also of the strength of the magnetic field at that location. The exact wavelength or frequency of a spectral line also depends (through the doppler effect) on the speed of the material relative to the observer. Astronomers often use filters to look at one spectral line in turn, to measure the things that the spectral line is sensitive to.

the spiral system
'spi.ral 'sys.tem

A spiral system is a galaxy with a flat disk in which (when seen from above) bright "arms" appear to come from the center. A spiral system also contains a number of globular clusters, of which most are outside of the disk. In the center of the disk there may be a bar-shaped structure: then the galaxy is a barred spiral galaxy.

the star
astèr = [Greek] star

A star is a ball of (mostly) hydrogen gas that is so massive that it

  1. is held together by its own gravitation.
  2. has such a high gas pressure and temperature in its center that nuclear fusion of hydrogen to helium occurs there, which provides the energy that makes the star shine brightly.

the Sun
sun, plural suns

The Sun is the star in the middel of our Solar System, which provides us here on Earth with light and warmth. Because the Sun is a star, stars are sometimes referred to as suns.

the sunspot

Sunspots are regions on the solar surface where the magnetic field is very strong, up to 3000 gauss in strength. Sunspots look a lot darker than their surroundings in almost all kinds of observations, because they are a few thousand degrees cooler than their surroundings and because they are also large. Sunspots range in diameter from about 2500 to over 50,000 km. A sunspot is often roughly circular in shape, though some have a very irregular appearance. Sunspots have two very distinct parts: the dark umbra in the middle, and the less-dark penumbra around the umbra. The difference between a sunspot and a pore is that a pore has no penumbra.

the superior planet

A superior planet is a planet that is further from the Sun than the Earth is. Only superior planets can be in opposition, and their elongation can have any value up to 180 degrees. The superior planets are: Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. The opposite of a superior planet is an inferior planet.

sunodos = [Greek] meeting; from sun = [Greek] together, and hodos = [Greek] road

A synodical period is the period after which the phases of a celestial body as seen from another celestial body repeat themselves. If only a single body is mentioned, then the synodical period as seen from Earth is usually meant.

After a synodical month, the Moon has reached the same phase again (for example: full moon). The time between two oppositions or (similar) conjunctions of a planet is the synodical period of the planet. After that time, the Earth has caught up again with the planet (or the planet with the Earth) in its orbit around the Sun. The time after which a certain sunspot returns to the same location on the solar disk is the synodical period of the Sun (but that is not the same everywhere on the Sun, because of differential rotation ).

The synodical periods of the planets are, rounded to whole days: Mercury 116, Venus 584, Mars 780, Jupiter 399, Saturn 378, Uranus 370, Neptune 367, Pluto (at present) 367. Calculate it yourself.

Periods of motion measured relative to the stars are called sidereal periods.


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the temperature

The temperature is a measure for how hot something is. There are several units in use for temperature. In Europe, the most common unit is the degree Celsius (written °C). In the USA, the degree Fahrenheit is used (°F). In science, the kelvin (no capital letters; also written K -- without °) is used a lot. The freezing point of water at sea level on Earth is at 0°C, 32°F, and 273 K. The boiling point of water is then at 100°C, 212°F, and 373 K.

the transit
transit, plural transits

A transit is

  1. the moment when a celestial body crosses the meridian (sense 2).
  2. a passage of an apparently smaller body in front of an apparently larger body. More particularly: the crossing of Mercury or Venus in front of the Sun, as seen from Earth.

In astronomy, a tropical period of time is a period of time measured relative to the passage through the vernal equinox, and so is linked to the seasons. The tropical year is the period after which the Sun returns to the vernal equinox, and the tropical month is the period after which the Moon returns to the vernal equinox.


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ultra = [Latin] further

Ultraviolet radiation is a form of electromagnetic radiation with wavelengths just smaller than those of visible light, i.e., just beyond those of violet light.

the umbra
umbra = [Latin] shadow

The umbra is the middle part of a sunspot, which is (relatively) very dark bbecause it is a lot cooler than the rest of the solar surface. The magnetic field in the umbra is very strong and points almost straight uup.

the Universe
the ', plural universes

The Universe is all space put together, with everything that's in it. Because of the finite age of the Universe and the finite speed with which light and other information carriers move, we can have knowledge of even in principle only a finite part of the Universe. That part is also often called the Universe (or the Visible Universe). For more information about the Universe, see the Universe Page of the Universe Family Tree.


Uranus is the seventh planet of the Solar System, counting from the Sun. Uranus is a jovian planet with a very dense and thick atmosphere (made mostly of hydrogen and helium), many moons, and narrow rings.


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Venus = [Latin] goddess of love

Venus is the second planet of the Solar System, counting from the Sun. Venus is a terrestrial planet, with a thick atmosphere of mostly carbon dioxide but no moons or rings.

the vernal equinox

The vernal equinox is the intersection of the equator of the sky and the ecliptic, through which the Sun passes during the March equinox, at the beginning of spring in the northern hemisphere, and autumn in the southern hemisphere. The vernal equinox is the zero point of longitude and latitude of the equatorial and ecliptical coordinate systems. The period between two successive passages of the Sun through the vernal equinox is the tropical year.

visible light

Visible light is a form of electromagnetic radiation with wavelengths between about 400 and 800 nm. It seems silly to talk about "visible" light; after all, you can always see light, can't you? Yet, there are good reasons:

  1. some kinds of electromagnetic radiation that we cannot directly see are yet called light, for example infrared and ultraviolet "light".
  2. the boundary between colors we see well and "colors" we cannot see at all is not sharp but gradual, so we cannot pinpoint an exact border between visible and invisible light.
  3. not everybody is equally sensitive to any given color. A color that is just visible to some may be invisible to some others. Should such a color be considered to be light or not?
  4. visible light does not differ fundamentally from invisible light (such as infrared and ultraviolet light) that is just beyond the visible colors.

Scientists therefore use the phrase "visible light" for the wavelength range of electromagnetic radiation that the average person can see at least a little bit, with vague boundaries on the low and high sides.


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the X-rays

X-rays are a form of electromagnetic radiation that has a relatively large amount of energy per photon and is therefore dangerous. X-rays are emitted naturally by material that is very hot, such as the gases in a solar flare that can have temperatures of hundreds of thousands or even millions of degrees, and by certain radioactive materials.


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the year

The year is a period of time that is related to the motion of the Earth around the Sun. There are quite a few different kinds of years in use:


the zenith
samt ar-ras = [Arab] the way of the head

The zenith is the point in the sky that is straight above your head. The altitude of the zenith is 90°. The opposite point is the nadir, and the horizon is midway between the two.

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Last updated: 2003-5-18

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