- Chris Kronberg's Astronomical Glossary
- Our small Deep Sky Glossary
- Bill Arnett's Web Nebula Glossary
- Chris Kronberg's "Scholar" collection of Astronomical Teaching Texts
- or our (soon to come) Brief Introduction to Astronomy.

**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 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.

**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: *exactly*

1 ly = 9.460536207068016 * 10^12 km = 9,460,536,207,068.016 kmThis exact value is valid

Similarly, a

**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

**Orientation**

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