An Etymological Dictionary of Astronomy and Astrophysics

Either of the two points on the → ecliptic at which the apparent → longitude of the → Sun is 90° or 270°. Also the time at which the Sun is at either point. Solstices occur when the Earth’s axis is oriented directly toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes. → summer solstice, → winter solstice.

Etymology (EN): M.E., from O.Fr. solstice, from L. solstitium “point at which the sun seems to stand still,” from sol, → sun, cognate with Pers. xor, xoršid, hur, as below, + p.p. stem of sistere “to come to a stop, make stand still,” akin to Pers. istâdan “to stand,” as below.

Etymology (PE): Xoristân, is composed of two components. The first one xor “sun,” variant hur; Mid.Pers. xwar
“sun;” Av. hū-, hvar- “sun;” cf. Skt. surya-, Gk. helios, L. sol, cognate with E. sun, as above;
PIE base *sawel- “sun.” The second component istân “standing,” from istâdan “to stand;” Mid.Pers. êstâtan;
O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthā- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand.”