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plural sidereal days
Definition of sidereal day
: the interval between two successive transits of a point on the celestial sphere (such as the vernal equinox) over the upper meridian of a place : 23 hours, 56 minutes, 4.1 seconds of mean time In recent experiments, the frequency … was found to vary by not more than 100 μHz over a sidereal day as the rotation of the earth changed the orientation of the ions (fixed by the laboratory magnetic field) with respect to the external universe.— David Golden
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Merriam-Webster unabridgedWords at Play
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Time Traveler for sidereal day
The first known use of sidereal day was circa 1678
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