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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.

Angle
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.

Brightness
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 magnitudes.

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.

Distance
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)

Epoch
A time or point of time. Used for the definition of celestial coordinate systems which change with time, such as the equatorial coordinate system (Right ascension/Declination) and consequently catalogs and atlasses of celestial objects. Commonly used epochs are J2000.0 (sometimes inacurately called 2000.0) and B1950.0 (sometimes referred as 1950.0).

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 analogously.

Magnitude
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 (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

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Last Modification: 10 Mar 1998, 10:00 MET