Quote:
Originally Posted by adman
the sidereal rate is the rate at which the earth spins on its axis ...
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Not quite.
The Earth rotates around its own axis very nearly once every 24 hours.
This determines civil time.
However, the Earth is orbiting around the Sun as well and this changes the apparent
position of the stars every day. During the course of the year, the Earth will thus
rotate exactly one more time with respect to the stars that it does to the Sun.
This determines sidereal time and it results in a solar day being approximately
3 minutes, 54.6 seconds longer than a sidereal day.
To make matters more complex, the tracking rate is simply not the first
differential. The effects of atmospheric refraction distort things and the tracking
rate varies continually from horizon to zenith and then from zenith back down
to the horizon. Some older telescopes attempted to kludge for this by what is
known as the King Rate.
Best Regards
Gary Kopff
Mt. Kuring-Gai NSW