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.
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Line breaks: si|der¦eal time
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