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 magnitudes.

Declination (Dec)
Celestial coordinate corresponding to latitude in the equatorial celestial coordinate system. Measured in degrees, arc minutes and arc seconds, running from -90 deg to +90 deg. The declination of -90 deg is the direction of the South Celestial Pole (SCP), +90 deg the (opposite) direction of the North Celestial Pole (NCP), and the circle of declination 0 deg is the Celestial Equator. The longitudinal coordinate in this system, the Celestial Coordinate System, is the Right Ascension.

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)

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: exactly

    1 ly = 9.460536207068016 * 10^12 km
         = 9,460,536,207,068.016 km
This exact value is valid by definition for a "Gregorian" light year, as the Gregorian year is exactly 365.2425 (synodic) days of 86,400 seconds, or 31,556,952.00 sec).
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.

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.

Orientation in the sky, or celestial sphere, is determined by the rotation of Earth, defining North and South by the rotation axis, as well as East and West by the sense of rotation: Like on Earth's surface, North is the direction of the "right thumb" if the fingers of the right hand point in the direction of rotation (South opposite), East is pointed to by the rotation of earth, so that celestial object appear to move from East to West. Therefore, West is also referred to as "Preceding" and East as "Following" direction. A more acurate measure for the orientation of a feature in the sky is the position angle.

Parsec (pc)
The distance from which the mean distance of Earth's orbit around the Sun, the astronomical unit (AU), appears under the angle of 1 arc second. Here, we prefer to use the light year; 1 pc = 3.26 ly. More acurately:

    1 pc = ( 648,000 / pi ) * 1 AU = 206,264.806 * 149,597,892 km
         = 30.856,780 * 10^12 km = 3.261,631 ly

Position Angle (P.A.)
Position Angles give the orientation of a feature in the celestial sphere; they are measured between North and the direction considered, in counterclockwise or direct sense. As an angle, measured in degrees; orientation to North is 0deg, to East ["Following"] 90deg, to South 180deg, to West ["Preceding"] 270 deg.

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. The celestial coordinate corresponding to latitude is the Declination.

Hartmut Frommert
Christine Kronberg

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Last Modification: October 9, 2005