What is the X in X-mas?
Old English sunne, from Proto-Germanic *sunnon (cf. Old Norse, Old Saxon, Old High German sunna, Middle Dutch sonne, Dutch zon, German Sonne, Gothic sunno), from PIE *s(u)wen- (cf. Avestan xueng "sun," Old Irish fur-sunnud "lighting up"), alternative form of root *saewel- "to shine, sun" (see Sol).
Old English sunne was fem., and the fem. pronoun was used until 16c.; since then masc. has prevailed. The empire on which the sun never sets (1630) originally was the Spanish, later the British. To have one's place in the sun (1680s) is from Pascal's "Pensées"; the German imperial foreign policy sense (1897) is from a speech by von Bülow.
1510s, "to set something in the sun," from sun (n.). Meaning "to expose oneself to the sun" is recorded from c.1600. Sun-bathing is attested from c.1600.
Often Sun. A medium-sized, main-sequence star located in a spiral arm of the Milky Way galaxy, orbited by all of the planets and other bodies in our solar system and supplying the heat and light that sustain life on Earth. Its diameter is approximately 1.4 million km (868,000 mi), and its mass, about 330,000 times that of Earth, comprises more than 99 percent of the matter in the solar system. It has a temperature of some 16 million degrees C (27 million degrees F) at its core, where nuclear fusion produces tremendous amounts of energy, mainly through the series of reactions known as the proton-proton chain. The energy generated in the core radiates through a radiation zone to an opaque convection zone, where it rises to the surface through convection currents of the Sun's plasma. The Sun's surface temperature (at its photosphere) is approximately 6,200 degrees C (11,200 degrees F). Turbulent surface phenomena analogous to the Earth's weather are prevalent, including magnetic storms, sunspots, and solar flares. The Sun was formed along with the rest of the solar system about 4.5 billion years ago and is expected to run out of its current hydrogen fuel in another 5 billion years, at which point it will develop into a red giant and ultimately into a white dwarf. See Table at solar system. See Note at dwarf star.
(Heb. shemesh), first mentioned along with the moon as the two great luminaries of heaven (Gen. 1:14-18). By their motions and influence they were intended to mark and divide times and seasons. The worship of the sun was one of the oldest forms of false religion (Job 31:26,27), and was common among the Egyptians and Chaldeans and other pagan nations. The Jews were warned against this form of idolatry (Deut. 4:19; 17:3; comp. 2 Kings 23:11; Jer. 19:13).