noun[mass noun] Astronomy
Time reckoned from the motion of the earth (or a planet) relative to the distant stars (rather than with respect to the sun).
- Two clocks were used, one keeping mean time i.e. 24 hours a day, the other sidereal time of 23 hours, 56 minutes and 4 seconds to the day (the length of time until the stars reach the same position as the previous day).
- Today we would use a telescope in an accurately-calibrated equatorial mounting to find the planet's altitude and azimuth (compass-bearing) and then, given the correct sidereal time, its latitude and longitude could be calculated.
- Flamsteed used the star Sirius as a timekeeper correcting the sidereal time obtained from successive transits of the star into solar time, the difference of course being due to the rotation of the Earth round the Sun.
Definition of sidereal time in:
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