User`s guide

56 Starry Night User’s Guide
measures the compass direction of an
object. An object which is due north in the
sky has an azimuth of 0°, one that is due
east has an azimuth of 90°, and one that is
due south has an azimuth of 180°. The
local meridian is the line passing directly
overhead in the sky that runs from 0°
azimuth (due north) to 180° azimuth (due
south).
Equatorial (aka Celestial): The equatorial
co-ordinate system is the most common
system for describing the position of
objects in the sky. Its two co-ordinates are
declination and right ascension.
Declination is the astronomical equivalent
of latitude. It measures an object’s angular
distance north or south of the
celestial
equator
, which is simply a projection of
Earth’s equator into space (an object of 0°
declination is directly on the celestial
equator). Because of this, objects with
positive declination can be seen more
easily in the Northern Hemisphere, and
objects with negative declination can be
seen more easily from the Southern
Hemisphere. Declination is usually
measured in degrees, arcminutes, and
arcseconds (°, ', "). Polaris, the North Star,
has a declination of almost 90 degrees.
The east-west measurement is called the
right ascension (RA), and is most often
measured in hours, minutes, and seconds
(h, m, s), from 0 to 24 hours. Because
Earth rotates, it is not possible to equate
longitude on earth with right ascension.
Think of lines of right ascension as
longitude lines which are fixed in space,
not rotating with the Earth-centred lines of
longitude. The zero-point of right
ascension (RA) is defined to be the right
ascension of the sun at the
Vernal Equinox,
which is the first day of spring in the
Northern Hemisphere.
Earth is precessing on its axis of rotation
(picture a spinning top which does not
point straight up but instead moves in an
arc around the vertical) with a period of
26 000 years. Because of this, the Vernal
Equinox slowly changes over time and so
do the equatorial co-ordinates of an object.
These changes are quite small and often
unimportant for the amateur astronomer.
Note (Pro and Pro Plus only): To
standardize astronomical positions,
astronomers often refer to an object’s
position using the co-ordinate system of a
particular date. For example you will
often see positions given in J2000 co-
ordinates, using the equatorial co-ordinate
system of Jan 1, 2000. Starry Night’s Info
pane displays equatorial co-ordinates for
the current time (JNow) and for the year
2000 (J2000).
Note: The book Starry Night Companion
(included with your copy of Starry Night)
has more information on using the alt/az
and equatorial co-ordinate systems.
Ecliptic: This reference system uses
ecliptic latitude and longitude as its two
co-ordinates. Ecliptic latitude is similar to
declination, except the line of 0° latitude is
the
ecliptic line (a projection onto the
celestial sphere of the plane of Earth’s
revolution around the Sun), instead of the
plane of Earth’s equator. Notice the
constellations which the ecliptic line
passes through - these are the familiar
zodiac constellations. The Sun will always
be found directly on the ecliptic line,