Basic Astronomical Terms

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Basic Astronomical Terms

Here some basic astronomical terms, relevant for the SEDS Messier

database, are shortly explained, in the hope to help beginners.

For more information you need, please refer to one of the following:

The Terms

Absolute Magnitude

Brightness of a star or celestial object if seen from a standard distance of

10 parsecs (32.6 light years), expressed in

stellar magnitudes.


Two directions, or straight (half) lines through a point, in a plane or in

space form an angle which measures the deviation of the one direction from

the other. Commonly measured in degrees (deg),

arc minutes (arc min, m, ‘) and

arc seconds (arc sec, “);

the full circle (or revolution) is 360 deg, 1 deg = 60′, 1′ = 60”.

Apparent Dimension

The angle under which a (celestial) object appears for

an observer, typically measured in arc minutes.

Arc Minutes (arc min), Arc Seconds (arc sec)

Units for angles.

1 arc min = 1′ = 1/60 deg, 1 arc sec = 1″ = 1/60 arc min.


Intensity of light received from a (celestial) object by an observer (or

apparatus). Because of the sensitivity of the human eye, brightness is

perceived logarithmically, and the perceived intensity is measured in


Declination (Dec)

Celestial coordinate corresponding to latitude in the equatorial celestial

coordinate system. Measured in degrees, arc minuntes and arc seconds,

running from -90 deg to +90 deg.


Length of the direct, shortest way between two points or objects; here,

commonly the distance of a celestial object from Earth, the Solar System,

or the Milky Way Galaxy is considered. Measured in units of length:

feet, miles, centimeters, meters, kilometers, Astronomical Units, or

(more commonly here) light-years (ly),

kilo-light-years (kly), or parsecs (pc),

kiloparsecs (kpc), Megaparsecs (Mpc) and Gigaparsecs (Gpc).

Distance Module

Difference between apparent magnitude m and

absolute magnitude M of an object,

corrected for interstellar absorption. Corresponds to a

distance D by the relation

    m - M = 5 lg (D/10 pc) = 5 lg (D/pc) - 5 = 5 lg (D/kly) + 9.4868

    D = 10 pc * 10^((m-M)/5)

Kilo-Light-Year (kly)

The distance (or length) of 1000 Light Years.

Used as lenght unit in many of our data tables.

Light Speed, Light Velocity, c

Light propagates in vacuum with a constant speed, which according to special

relativity, is measured at the same value by each observer, namely

c = 299,792 km/sec (186,282 miles per second), or more acurately, exactly

299,792,458 m/sec by definition. This value defines, at last, the length

of a meter, from time units, which are measurable by much higher acuracy than

length units.

Light-Year (ly)

The distance for which light, with its speed of

299,792 km/sec (186,282 miles per second) needs one year travel time.

1 ly = 9.46 trillion (10^12) km or 5.88 trillion miles

(more acurately: 9.4608953536 * 10^12 km).

Similarly, a light-second (ls) is defined as the distance for which

light needs one second, and there are light-minutes (lm),

light-days (ld) and even light-weeks and light-months defined



Logarithmical measure for the brightness of celestial objects.

Defined so that a factor 10 in brightness corresponds to 2.5 magnitudes

difference, where the brighter object has the smaller value of magnitudes.

Formally, for objects 1 and 2 with brightness B1 and B2, the difference

of magnitudes m1 and m2 is given by

                     m1 - m2 = - 2.5 lg (B1/B2)

Definition of the zero point is adjusted for the stars of a fundamental

sample. Historically, it was once adjusted for the North Star, Polaris,

but it turned out that this star is variable in light.

One distinguished apparent magnitude (which is measured by an observer)

and absolute magnitude (which is taken at a standard

distance and a measure for the intrinsic luminosity of a celestial object).

The difference between them is a logarithmic measure for the

distance of the object, the so-called

distance module.

Parsec (pc)

The distance from which the mean distance of Earth’s orbit

around the Sun, the astronomical unit, appears under the

angle of 1 arc second.

Here, we prefer to use the light year;

1 pc = 3.26 ly.

Right Ascension (RA)

Celestial coordinate corresponding to longitude in the equatorial celestial

coordinate system. Commonly measured in hours (h), minutes (m) and seconds (s)

where hours are used as measure for angles here, so that

the full revolution of 360 deg corresponds to 24 h, and

1 h = 15 deg .

The Null coordinate is defined by the vernal equinox, the direction where the

Sun apparently crosses the celestial equator to the North around March 21 in

each year, currently situated in Pisces.

Hartmut Frommert

([email protected])

Christine Kronberg

([email protected])





Last Modification: 10 Mar 1998, 10:00 MET

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