Which point in the sky do astronomers use to define sidereal time?

Question

Which point in the sky do astronomers use to define sidereal time?

Answer 1

There are many stars that can be used.  The "official" starting point is quasar 3C273B at RA = 12h 29m 6.69966s at epoch 1987.27.  (RA = 12h 29m 32.8s for J2008.5.)

There were originally 212 quasars that were used to define the International Celestial Reference Frame.  In 1999, 59 more were added, and in 2004, 109 more were added.  That makes a total of 380 quasars.

For optical use, the Hipparcos Catalogue has about 100,000 stars with high-precision positions.  But the radio-telescope quasar positions are part of the official definition of sidereal time.

It is interesting that professional astronomers aren't interested in sidereal time very much any more.  Instead they use something called "Earth Rotation Angle", which is something like sidereal time, but simpler.

Answer 2

The Vernal equinox.  Local sidereal time = hour angle of the vernal equinox.

Answer 3

Local sidereal time is the right ascension (RA) of the point passing through the zenith at the current time.  The zenith is the point directly overhead from wherever you are.

Answer 4

TIME

The most natural measure of time for ordinary use, the day; is provided by the movement of the Sun. Each day the Sun reaching its highest point when it is on the meridian (due north when we are south of the tropics). Unfortunately the motion of the Sun in the sky is such that intervals of time between succeeding passages across the meridian are not always exactly equal, so that for convenience the average length is taken, for we could not have clocks going at different rates at different times of the year: This period is a “mean day”, and is divided into 24 equal parts called hours, and these hours into minutes and seconds. This kind of time is called “mean solar time” or more shortly “mean time”. So we have two kinds of time, mean solar time, which is necessary since most human activity is governed by the position of the sun, and sidereal time, which is convenient to astronomers; and also to navigators, or a description of star positions. Just as the Sun passes over the meridian at approximately the same mean time every day, stars will pass the meridian at the same sidereal time every day. There is a simple relation between the two. The Sun, as described in a previous section, is moving eastwards round the celestial sphere, so that each day as it crosses the meridian the stars are a little further west than on the previous day, that is, as far as apparent daily rotation is concerned the stars appear to be gaining on the Sun. Since the Sun makes a complete circuit once in a year, the celestial sphere will appear to gain one whole revolution in a year, and as the Sun crosses the meridian an average of 365¼ times each year the stars rotate 366¾ times, so that the sidereal clock, which has to fit 366¼ days in a year, gains about four minutes a day on the mean time clock, which has to fit only 365½ days in a year.