15
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 measures 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. It
is different than the term “meridian”, which is used
in referring to lines of longitude. Right ascension
is referred to in “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 0h (hours). The
autumnal equinox can be found at 12h 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 coordinates of the
object at regular intervals of time. The coordinates
will be listed using declination 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 observation and does not directly correlate
to points on the celestial sphere.
AZIMUTH
- Azimuth is the angular distance
around the horizon measured eastward in degrees
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
or binocular aiming at one object must be turned in
order to aim at another object. If you must rotate the
equipment from the zenith to the horizon, the angular
distance between the two points would be 90°.
