We have seen that the rotation of the Earth causes a regular change
in the apparent positions of stars. The
hour angle of a star can
therefore be used as a measure of time. In particular, astronomers
use the hour angle of the first
point of Aries,
as a measure of the
rotation of the Earth with respect to the stars. This is known as
local sidereal time:
LST = HA
and is zero when the first point of Aries crosses the
As its name implies, local sidereal time depends upon the observer's
longitude on the Earth's surface.
From Figure 15
it can be seen that
the HA is equal
to the sum of the right
ascension of the star X (RAX) and
the hour angle of the star X (HAX). Hence,
LST = RAX + HAX.
This is a very important relationship because X can be any
celestial object - star, Sun, Moon, planet or spacecraft.
Since the hour angle of a star is zero when it transits on the observer's
meridian, the star's right ascension at that time is the local sidereal
time (or, equivalently, the local sidereal time gives the right ascension
of the star).
©Vik Dhillon, 30th September 2009