sidereal (adj.)

also siderial, 1630s, "star-like;" 1640s, "of or pertaining to the stars," earlier sideral (1590s), from French sidereal (16c.), from Latin sidereus "starry, astral, of the constellations," from sidus (genitive sideris) "star, group of stars, constellation," which is of uncertain origin, perhaps from PIE root *sweid- "to shine" (source also of Lithuanian svidus "shining, bright").

The sense in sidereal time, motion, etc. is "determined or measured by the apparent motion of the fixed stars," and is attested by 1680s. The sidereal day begins and ends with the passage of the vernal equinox over the meridian and is about four minutes shorter than the solar day, measured by the passage of the sun over the meridian.

updated on September 30, 2022