'Solar' is Latin for 'of the sun' — from PIE *soh-wl-, one of the most stable reconstructed words.
Of, relating to, or determined by the sun.
From Latin 'sōlāris' (of or pertaining to the sun), from 'sōl' (the sun), from PIE *séh₂wl̥ (the sun) — one of the most ancient and well-attested PIE roots, appearing across the entire family. Sanskrit 'sūryaḥ' (sun), Greek 'hēlios' (sun, giving 'helium' and 'heliocentric'), Old English 'sol' and 'sunne' (sun), Gothic 'sauil,' Welsh 'haul' — all reflect the same PIE root. Latin 'sōl' gives English 'solar,' 'solstice' (sun-standing
The PIE root *sóh₂wl̥ for 'sun' is one of the most confidently reconstructed words in historical linguistics, with reflexes in nearly every branch of the Indo-European family — Latin 'sōl,' Greek 'hēlios,' Sanskrit 'sūrya,' Gothic 'sauil,' Welsh 'haul,' and Lithuanian 'saulė' all descend from this single prehistoric word.