Solarium

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mid 19th century: from Latin, literally ‘sundial, place for sunning oneself’, from Latin sol ‘sun’.

wiktionary

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From Latin sōlārium( sunroom, sundial), from sōl( sun) + -ārium(thing or place used for).

etymonline

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solarium (n.)

1891, "part of a house arranged to receive the sun's rays," earlier "sundial" (1842), from Latin solarium "sundial," also "a flat housetop," literally "that which is exposed to the sun," from sol "the sun" (from PIE root *sawel- "the sun").