This is the time when an observation was made. The stars are pretty
much fixed in the sky, but not exactly... If you were to track the
location (RA and DEC) of a star over many years, you would find that it
slowly moves. This is mostly due to the precession of earth's orbit
around the sun, but also due to the sun's actual movement relative to other
stars. So, when the RA and DEC of a star is specified, the epoch, or
time when they were measured is also specified. An epoch seems to be
good for 40 years or so... The currently used epoch is J2000.0.
Julian Date is commonly used to specify the time in calculations. It
is the number of days since noon GMT on Monday, January 1, 4713 BC.
The integer form is called the Julian Day. As of this writing, the JD
is 2454091.44792. The Julian date of the current epoch, J2000.0, is
Since stars are so far away, their actual distance doesn't matter so much.
So, the stars can all be imagined to lie on the surface of a big sphere, the
celestial sphere. The line between Earth's poles is extended to this
sphere to define a north and south pole. The Earth's equator is
projected to define the celestial equator. In this way, an equivalent
latitude of a star can be defined. Defining an equivalent longitude of
a star is more complex since the Earth is spinning and the celestial sphere
is not. In the equatorial coordinate system, the vernal equinox is
used as a basis for this.
Geographic Coordinates The usual way to give your position on
earth is through your longitude and latitude.
Equatorial Coordinates Most common way to specify the locations of
stars and other bodies in the celestial sphere. This is basically a
projection of earth's system of longitude and latitude onto the celestial
sphere at the instant of the vernal equinox. But, instead of longitude
and latitude, the terms are Right Ascension (α or
RA) and Declination (δ or DEC). RA is like
longitude and can be expressed in terms of hours or degrees. DEC
is like latitude with 0 corresponding to the equator and +90 or North 90,
the north pole, and -90 or South 90, the south pole. Because the RA
and DEC of a star slowly changes over time, the epoch of the coordinates
must be specified.
Equinox The time in spring, ~ March 20th, when the days and nights
are equal length. More precisely, it is the exact time when the sun is
directly on the celestial equator. There are actually two places where
the celestial equator and the plane of the ecliptic cross. The other
place is the Autumnal Equinox that occurs ~September 23. The vernal
equinox is important in astronomy because it defines the equatorial
coordinate system. Midway between equinoxes are the solstices (Winter
and Summer) when the days are either longest or shortest. The vernal
equinox is also called the "first point of Aires" because it also used the
mark the beginning of the zodiac calendar where the sun entered the
constellation Aires. But, because of the precession of the equinoxes,
this point is now in the constellation Pisces. By definition, the
first point of Aires is located at RA=DEC=0.
Horizontal coordinate system Most common way to specify the
apparent location of stars and other bodies. Coordinates are altitude,
Alt, (or elevation) and azimuth, Az. Altitude is the angle of the
object above the horizon. Straight up is 90 degrees and called zenith.
Sometimes the zenith distance, Zd, (really an angle) is used instead, which
is the angle down from the zenith to the object so that Alt=90-Zd.
Azimuth is the angle between the object and the north pole. So east is
90 degrees, south is 180 degrees, and west is 270 degrees.
Local Mean Sidereal
Time or LST indicates your longitudinal position on earth relative to
the celestial sphere. When expressed as hours, it is equal to how many
hours have passed since the vernal equinox point was directly overhead, on
your local meridian (line on celestial sphere passing from north pole to
zenith to south pole). Sidereal time is different than local time by
one day a year because the earth is rotating about the sun.
The hour angle (HA) along with declination can be used to specify the
apparent position of a star. The hour angle indicates how many hours
have passed since the star was on your local meridian (line on celestial
sphere passing from north pole to zenith to south pole). HA=LST-RA.
So, your LST is equal to the HA of the vernal equinox (where RA=0).
(formerly GMT) or Coordinated Universal Time is the official time in
Greenwich, England and is the time normally used by computers. Your
local time is UTC plus your time zone offset plus daylight savings time
offset. UTC is kept by atomic clocks, so seconds are SI standard
seconds, but there are leap second adjustments to keep 365.25*24*60*60
seconds equal to the apparent solar year. These adjustments are
necessary because the Earth's speed around the sun is slowing every year.
Daylight savings are not applied to UTC.
UT1 has a variable
length second so that there are always 86,400 seconds in a day. UTC is
kept close to UT1 by the application of leap seconds. UTC=UT1+sum of
applied leap seconds.
International Atomic Time is the time kept by a system of atomic clocks at
sea level. The second is the SI second and there are no leap
adjustments. UT1 was equal to TAI on January 1, 1958.
Terrestrial Time = TAI + 32.184s
Daylight Savings Time or Summer Time is a 1 hour adjustment made to
local time between spring and fall to save energy. This can be tricky
to deal with because it's application is non-uniform. Fortunately,
most computer systems handle this automatically and can give you the UTC