Astro Glossary

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the brightest star in Taurus (alpha Tau). It is orange-red and has a brightness of 0.8
mag. It is located near the ecliptic and is frequently occulted by the moon.

Algol = beta Persei

a star with variable brightness in the constellation of Perseus. It is an eclipsing variable star, i.e. a binary system in which the two stars occult each other periodically as they revolve in their orbits. The brightness of Algol varies from 2.1
mag to 3.4 mag within a period of 68.8 hours.

At the minimum time, Algol should be at its dimmest, magnitude 3.4 instead of its usual 2.1. It is nearly this faint for two hours, and it takes several hours to fade and brighten.
(see also variable stars)

Aphelion, Perihelion

these are the points in a planet's orbit where the planet is farthest from (aphelion) or closest to (perihelion) the Sun.

Asteroid, Minor Planet, Planetoid

Small rocky solar-system bodies that orbit the Sun.Most know asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter at distances of 1.7 to 4.0
astronomical units from the Sun.

AU = Astronomical Unit

A unit of distance. The distance of the Earth from the Sun. One AU is about 150 million kms.

Celestial coordinate systems

Coordinate systems, resembling that of latitude and longitude on the Earth, by which the position of a celestial body in the sky can be specified. The main coordinate systems include the equatorial and the horizontal coordinate system:

The equatorial coordinate system is the most widely used astronomical coordinate system in which the fundamental reference circle is the celestial equator and the zero point is the vernal
equinox. The coordinates are right ascension and declination.

The horizontal coordinate system is a system in which the fundamental reference circle is the observer's astronomical horizon and the zero point is the north point. The coordinates are altitude and azimuth.

The altitude of a celestial body is its angular distance north (counted positive) or south (negative) of the horizon. It is measured along the vertical circle through the body and ranges from 0°, when the object rises or sets, to 90°, when the object is directly overhead at the zenith. The azimuth of a body is its angular distance measured eastwards along the horizon from the north point (or sometimes from the south point) to the intersection of the object's vertical circle.

Circumpolar Stars

Stars that are permanently above the horizon from a given observational point on earth, i.e. they never set. For stars to be circumpolar from a given geographical latitude phi, their
declination (angular distance from the celestial equator) must be greater than 90-phi.

For an observer at the north pole all stars in the northern hemisphere of the celestial sphere will be circumpolar, while the stars of the southern celestial hemisphere will never be seen. At the equator there are no circumpolar stars although almost all of the stars will be seen at some time during a year.

Culmination, Meridian Passage

The passage of a celestial body across an observer's
meridian. As a result of the Earth's rotation this occurs twice daily, although both culminations can only be observed for circumpolar stars. Upper culmination (or transit) is the meridian crossing closer to the observer's zenith; for circumpolar stars this is also called culmination above pole. Lower culmination, or culmination below pole, for circumplar stars is the meridian crossing further from zenith (i.e. between the celestial north pole and the north point on the horizon for observers on the northern hemisphere).


Symbol: Greek Delta or DE. A coordinate used with
right ascension in a celestial coordinate system. The declination of a celestial object is its angular distance from the celestial equator, expressed in degrees. Declination is positive when measured from the equator towards the north pole, and negative towards the south pole. Declination corresponds to the geographic latitude in the terrestrial coordinate system.

Delta Cephei

A pulsating variable star in the constellation Cepheus. This star shows periodical variations in brightness from 3.6
mag to 4.3 mag within a period of 5 days 8 hours and 48 minutes. The variations in brightness are easily observable.
(see also variable stars)


The apparent yearly path of the Sun projected onto the celestial sphere. It is also the approximate path of the Moon and the planets. The ecliptic is tilted at an angle of 23½° from the celestial equator and intersects the equator at two points called "
equinoxes" or "nodes". The Sun passes the equinoxes at March 21 and September 23.

The twelve constelllations lying along the ecliptic form a band around the sky called the Zodiac. All the planets, the Moon and the Sun will always be found somewhere within this band. The twelve zodiacal constellations are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces.


Elongation of a planet is the apparent distance in degrees of angle between that planet and the Sun, as seen from the Earth. Western elongation or eastern elongation means that the planet is located west or east of the Sun.


1. equinoctial points:

The two points on the celestial sphere at which the
ecliptic intersects the celestial equator. They are thus the two points at which the Sun in its apparent annual motion crosses the celestial equator. The Sun crosses from south to north of the equator at the vernal (spring) equinox, which lies at present in the constellation Pisces. The Sun crosses from north to south of the equator at the autumnal equinox, which at present lies in Virgo. The vernal equinox is the zero point for the equatorial coordinate system.

2. The two instants at which the Sun crosses the equinoctial points, on about March 21, and Sept. 23. On the days of the equinoxes the hours of daylight and of darkness are equal. Compare solstices.

Light year

A unit of distance. The distance that light (with a speed of about 300.000 kms per second) travels in one year. One light year is about kms.

mag = magnitude

The magnitude is a method of expressing the apparent brightness of a celestial object. In the original system, initiated some 2000 years ago, the 20 brightest stars in the sky were collectively grouped together as stars of the first brightness class or "first magnitude". Stars about 2½ times fainter were classified as stars of the "second magnitude", those still fainter were classified as of "third magnitude", and so on. Stars of the sixth magnitude were at the limit of naked-eye visibility.

This system, virtually unaltered, is still in use today. The exact ratio between magnitudes has been set at 2.512, this having the advantage of making a difference of 100 times in brightness for a difference of five magnitudes.

It is evident that a star (or any other object) 2½ times brighter than magnitude 1.00 will have to be assigned a magnitude of 0.00, while one still brighter will actually have a negative or minus value. Thus Vega is of magnitude 0.04; Sirius, the brightest star in the sky, has a magnitude of -1.42; while the planet Jupiter is usually brighter than -2.0, and Venus reaches -4.4 on occasion.

The term "magnitude", when used alone, is understood to mean "apparent magnitude", the apparent brightness of a celestial object as seen from the Earth. This has nothing to do with the actual or real luminosity of the object. The "absolute magnitude" of a star is the magnitude that the star would have if it were brought to a standard distance from the Earth, the distance agreed upon being 10
parsecs or 32½ light years.

In astronomy, the symbol for apparent magnitude is a small letter "m" or "mag", while for the absolute magnitude a capital letter "M" is used.

Magnitude of an eclipse

Solar eclipse: The fraction of the Sun's diameter which is covered by the Moon.

Lunar eclipse: The fraction of the Moon's diameter which is immersed in the penumbra (penumbral eclipse) or umbra (partial or total lunar eclipse) of the Earth's shadow cone.

The magnitude of partial (lunar or solar) eclipses is from greater than 0 to less than 1. Values of eclipse magnitude of 1 or larger mean that the entire disk of the Sun or the Moon is eclipsed (total solar or lunar eclipse, or full immersion in the penumbra at a penumbral lunar eclipse). The magnitude of an eclipse may also be given in percent (magnitude 1 = 100%).

Meridian, Celestial Meridian

The projection of the observer's terrestrial meridian on the celestial sphere. It is thus the great circle passing through the north and south celestial poles and the observer's zenith. It intersects the observer's horizon at the north and south points.


The streak of light seen in the clear sky when a small particle of interplanetary dust (a meteoroid) burns itself out in the Earth's outer atmosphere. Sometimes the Earth passes through a meteoroid stream (e.g. the remainders of a disintegrated comet), then particular meteor showers occur at the same time each year (e.g. the Perseids from 10.-14.August)

Mira = omicron Ceti

A long-period variable star in Cetus. It varies in brightness from 9th
magnitude or less at minimum to 3rd or 4th at maximum. The period averages 331 days but there are often considerable irregularities both in period and light range.

Omicron Ceti has been named "Mira" - the wonderful - because of its unusual brightness variations. Mira was the first variable star which has been observed. It was already discovered in 1596. (see also variable stars)

Open Cluster

Open clusters - or galactic clusters - are associations of stars containing from few to thousands of stars. Many of them are embedded in glowing nebulosity or smothered in dark clouds of cosmic dust. Open clusters are usually found along the Milky Way. Examples of open clusters are the Hyades and the Pleiades (the "Seven Sisters") located in Taurus.


the moment at which a body in the solar system lies opposite the sun in the sky and crosses the
meridian at about midnight. It is the most favourable time for observation of planets or asteroids since they are then observable throughout the night and are near their closest point.

PA = Position Angle

The position angle is a method for defining the direction of an object with respect to an other object. The position angle is count in degrees of angle from north (=0°) via east (90°), south (180°) and west (270°) back to north.

This system is used e.g. for defining the direction of the components of double stars, or at which position on the moon's limb an occultation of a star by the moon occurs.


Short for parallax second, a unit of distance. The distance from where the larger axis of the Earth's orbit around the Sun is seen under an angle of one second of arc. 1 Parsec is about 3.26
light years.

Perihelion, Aphelion

these are the points in a planet's orbit where the planet is closest to (perihelion) or farthest from (aphelion) the Sun.

Radiant of a meteor shower

is the point at the celestial sphere from where the meteors appear to originate.


the brightest star in Leo (alpha Leo). It has a brightness of 1.3
mag. It is located near the ecliptic and is frequently occulted by the moon.

Right Ascension

Symbol: Greek alpha or RA. A coordinate used with
declination in the equatorial coordinate system. The right ascension of a celestial body is its angular distance measured eastwards along the celestial equator from the vernal (spring) equinox. It is generally expressed in hours (h), minutes (m) and seconds (s) from 0 to 24 h, one hour equals to 15° of arc.


1. solsticial points

The two points that lie on the
ecliptic midway between the vernal and autumnal equinoxes and at which the Sun, in its apparent annual motion, is at its greatest angular distance (23½°) north or south of the celestial equator.

2. The times at which the Sun reaches these points, on about June 21 and Dec. 21, the hours of daylight and darkness are then at a maximum.

Variable Star

A star whose brightness varies with time. The variation in brightness can be seen on the light curve of the star and may be regular - with a period given by one complete cycle in brightness on the light curve - or may be irregular or semiregular. The periods range from minutes to years. The range between maximum and minimum brightness - the amplitude - is also very wide.

There are three major groups of variables: eclipsing, pulsating and cataclysmic variables.

The eclipsing variables are systems of two or more stars whose light varies as one star is periodically eclipsed by a companion star. A prominent example of an eclipsing variable star is
Algol (beta Persei).
Further examples of eclipsing variable stars are:

StarBrightness range (mag.)Period (days)Right Asc.Decl.
U Cephei6.8 - 9.22.4930801 02+81 53
Beta Persei2.1 - 3.42.86603 08+40 57
BM Orionis (*)7.9 - 8.76.47052505 35-05 23
Delta Librae4.9 - 5.92.32736215 01-08 31
AI Draconis7.1 - 8.11.198815216 56+52 42
Beta Lyrae3.3 - 4.412.93941218 50+33 22

(*) BM Orionis is the northernmost and faintest star in the "Trapezium" in Orion nebula.

Pulsating variables periodically brighten and fade as their surface layers expand and contract. The most prominent representatives of this group of variables are Delta Cephei and Omicron Ceti (Mira).
Further examples of pulsating variables are:

StarBrightness range (mag.)Period (days)Right Asc.Decl.
Zeta Geminorum3.6 - 4.210.1507307 04+20 30
Eta Aquilae3.5 - 4.47.17664119 52+01 00
Delta Cephei3.5 - 4.45.36634122 29+58 25
Omicron Ceti (Mira)3 - 9.533202 17-03 12

Cataclysmic variables are close binary stars where one component is a white dwarf to which mass is transferred from the other component. This class includes e.g. novae in which there is a sudden and unpredictable increase in brightness by 10 magnitudes or more.

ZHR = Zenithal hourly rate

average number of meteors appearing in one hour. Assumed for an ideal case that the
radiant is in the zenith and the limiting magnitude is 6.5

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