Of, relating to, or measured by the apparent motion of the stars; determined by reference to the stars rather than the sun.
From Latin 'sidereus' (of the stars), from 'sidus' (star/constellation). A sidereal day is about 4 minutes shorter than a solar day — the difference between star-time and sun-time.
There are TWO kinds of day! Solar day (sun) and SIDEREAL day (stars) — and they're 4 minutes different! Over a year, that adds up to an entire extra star-day! The universe has two clocks and they don't agree! âðŸ•
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