solar:  Solar comes from Latin sōlāris, a derivative of sōl ‘sun’. This went back to the same ultimate Indo-European base, *su-, that produced English sun. Solarium  was borrowed from another Latin derivative, sōlārium, which denoted a ‘sundial’ as well as a ‘balcony, flat rooftop, or other part of a house exposed to the sun’. Solstice  means etymologically the ‘sun standing still’. It comes from Latin sōlstitium, a compound formed from sōl and stit-, the past participial stem of sistere ‘make stand’. => solarium, solstice
mid-15c., "pertaining to the sun," from Latin solaris "of the sun," from sol "sun" (see sol). Meaning "living room on an upper story" is from Old English, from Latin solarium (see solarium). Old English had sunlic "solar."
Astrological sense from 1620s. Meaning "operated by means of the sun" is from 1740; solar power is attested from 1915, solar cell from 1955, solar panel from 1964. Solar system is attested from c. 1704; solar wind is from 1958. Solar plexus (1771) "complex of nerves in the pit of the stomach," apparently so called from its central position in the body (see plexus).