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").
Sidereal time is measured by the apparent diurnal motion of the fixed stars. 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.
Old English hærfest "autumn," as one of the four seasons, "period between August and November," from Proto-Germanic *harbitas (source also of Old Saxon hervist, Old Frisian and Dutch herfst, German Herbst "autumn," Old Norse haust "harvest"), from PIE root *kerp- "to gather, pluck, harvest."
In Old English and Middle English it was primarily a season name, with only an implied reference to the gathering of crops. The meaning "the time of gathering crops" is attested by mid-13c., and the sense was extended to the action itself and the product of the action (after c. 1300). After c. 1500 these were the main senses and the borrowed autumn and repurposed fall (n.) supplied the season name.
The figurative uses begin by 1530s. As an adjective, from late 14c. Harvest home (1570s) was a festive celebration of the bringing home the last of the harvest; harvest moon (1704) is that which is full within a fortnight of the autumnal equinox.
season after summer and before winter, late 14c., autumpne (modern form from 16c.), from Old French autumpne, automne (13c.), from Latin autumnus (also auctumnus, perhaps influenced by auctus "increase"), which is of unknown origin. Perhaps from Etruscan, but Tucker suggests a meaning "drying-up season" and a root in *auq- (which would suggest the form in -c- was the original) and compares archaic English sere-month "August." De Vaan writes, "Although 'summer', 'winter' and 'spring' are inherited IE words in Latin, a foreign origin of autumnus is conceivable, since we cannot reconstruct a PIE word for 'autumn'".
Harvest (n.) was the English name for the season until autumn began to displace it 16c. Astronomically, from the descending equinox to the winter solstice; in Britain, the season is popularly August through October; in U.S., September through November. Compare Italian autunno, Spanish otoño, Portuguese outono, all from the Latin word.
As de Vaan notes, autumn's names across the Indo-European languages leave no evidence that there ever was a common word for it. Many "autumn" words mean "end, end of summer," or "harvest." Compare Greek phthinoporon "waning of summer;" Lithuanian ruduo "autumn," from rudas "reddish," in reference to leaves; Old Irish fogamar, literally "under-winter."