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ASTRONOMY TERMINOLOGY
DECLINATION (DEC.) - The astronomical equivalent of latitude. Declination describes the
angle of a celestial object above or below the celestial equator. The sky over the northern
hemisphere has a positive declination. The sky over the Southern hemisphere has a negative
declination. For example, Polaris (the North Star) which lies nearly directly over the North
Pole, has a declination value of 90°.
RIGHT ASCENSION (R.A.) - The astronomical equivalent of longitude. Right Ascension mea-
sures the degree of distance of a star to the east of where the ecliptic crosses the celestial
equator. R.A. is measured in hours, minutes, and seconds as opposed to degrees. As oposed
to the term meridian which is used in referring to lines of longitude, right ascension is
referred to as hour circles. There are 24 hour circles of right ascension which run from the
north to south celestial poles.
CELESTIAL EQUATOR - The celestial equator is the line of declination which lies directly above
the Earth’s equator. The celestial equator lies halfway between the north and south celestial
poles and serves as the 0° point in measuring declination.
ECLIPTIC - The ecliptic is the apparent path of the sun through the sky over the course of the
year. Since we view the sun from different angles throughout the year, it appears to move
in relation to other stars. The vernal (spring) and autumnal (fall) equinoxes lie at the points
where the ecliptic intersects the celestial equator. The vernal equinox is where right ascension
is at 0 h (hours). The autumnal equinox can be found at 12 h R.A..
ZENITH - The zenith is the point in the celestial sphere directly above your head. The zenith
varies depending upon your location. In general, the declination point of your zenith is
equal to the latitude at which you are standing on Earth.
EPHEMERIS - The ephemeris of a planet or the sun or the moon is a table giving the coordi-
nates of the object at regular intervals of time. The coordinates will be listed using declina-
tion and right ascension. Other information such as distance and magnitude may be listed
in ephemerides (plural of ephemeris).
ALTITUDE - The altitude of a celestial object is the angular distance of that object above the
horizon. The maximum possible altitude is the altitude of an object at the zenith, 90°. The
altitude of an object on the horizon is 0°. Altitude is measured from your point of observa-
tion and does not directly correlate to points on the celestial sphere.
AZIMUTH - Azimuth is the angular distance around the horizon measured eastward in de-
grees from the North Horizon Point. Thus the North Horizon Point lies at an azimuth of
0°, while the East Horizon Point lies at 90°, and the South Horizon Point at 180°. Azimuth
is measured from the point of observation and does not directly correspond to points on
the celestial sphere.
ANGULAR DISTANCE - Angular distance is the size of the angle through which a telescope
tube aiming at one object must be turned in order to aim at the another object. If you must
rotate the telescope from the zenith to the horizon, the angular distance between the two
points would be 90°.