The English adjective 'solar' entered the language in the mid-fifteenth century, borrowed from Latin 'sōlāris' through the intermediary of Middle French 'solaire.' The Latin adjective was formed by attaching the relational suffix '-āris' to the noun 'sōl,' meaning 'the sun.' This derivation is straightforward, but the deep history of 'sōl' itself is one of the great success stories of comparative linguistics.
Latin 'sōl' descends from Proto-Italic *swol, which in turn derives from the Proto-Indo-European root *sóh₂wl̥. This reconstructed form is among the most confidently established words in the proto-language, supported by cognates in nearly every major branch of the Indo-European family. Greek 'hēlios' (as in 'heliocentric'), Sanskrit 'sūrya' (the Hindu sun deity), Gothic 'sauil,' Old Norse 'sól,' Lithuanian 'saulė,' Welsh 'haul,' and Old Church Slavonic 'slŭnĭce' all trace back to this single prehistoric word. The PIE sun-word is so well attested that it serves as a textbook example of the comparative method.
The phonological changes that produced such different-looking reflexes are regular and predictable. The initial *s was lost before vowels in Greek (a process called 'psilosis' in certain dialects, though the loss here is actually a regular PIE-to-Greek change involving *s before the laryngeal *h₂). Sanskrit shifted the *l to *r through rhotacism in certain phonological environments, producing 'sūrya.' The Germanic languages preserved the diphthong more faithfully, yielding Gothic 'sauil' and eventually Old English 'sunne' (from the suffixed variant *sun-nōn).
In English, the native Germanic word 'sun' (from Old English 'sunne') has always been the everyday term for the celestial body. 'Solar' entered as a learned, Latinate adjective — the kind of borrowing that characterizes English's massive intake of French and Latin vocabulary during and after the Middle English period. This created one of English's characteristic doublets: the everyday Germanic noun paired with a formal Latinate adjective. English speakers say 'the sun' but 'solar energy,' 'sunshine' but 'solar
The word found early scientific application in the sixteenth century, when Copernican astronomy demanded precise terminology for sun-centered models. 'Solar system' as a phrase is attested from the late seventeenth century. The compound 'solar plexus' was coined in the eighteenth century by anatomists who thought the radiating network of nerves behind the stomach resembled the rays of the sun.
The twentieth and twenty-first centuries have given 'solar' enormous new cultural weight through the technology of photovoltaics. 'Solar panel,' 'solar cell,' 'solar power,' and 'solar farm' are now among the most common collocations of the word. This technological meaning has made 'solar' one of the few Latin-derived adjectives that has crossed back into everyday, non-technical speech — a homeowner discussing 'going solar' is using a fifteenth-century Latinate borrowing as casually as any native English word.
The related Latin derivatives in English are numerous. 'Solstice' comes from Latin 'sōlstitium' (sun-standing), marking the days when the sun appears to pause its north-south migration. 'Parasol' comes from Italian 'parasole,' literally 'against the sun.' 'Insolation' (exposure to sunlight) preserves the Latin verb 'insōlāre.' Even 'solarium' — originally a Roman sundial or sunroom — has been repurposed for modern tanning beds.
What makes 'solar' particularly satisfying as an etymological case study is the transparency of its full lineage: from a PIE root reconstructed by comparing dozens of daughter languages, through Classical Latin, through French, to modern English — each step documented and phonologically regular. It is a word whose history mirrors the history of the science it now most commonly describes.