Sphere shows both a Greek ph and a Renaissance respelling. Middle English had the word about 1300 as spere, from Old French espere, and that is what it sounded like. From the mid-1400s the spelling was re-Latinised to sphere to display its descent from Latin sphaera and Greek sphaira, "globe, ball."
The ph is therefore doing exactly what ph always does — transliterating Greek phi — but it was applied after the fact, to a word that had come in through French wearing plain letters. The Greek word's own origin is unknown; one scholar says there are no certain relatives outside Greek, though it was borrowed onward into Syriac, Ethiopic, Armenian and Georgian. The English meaning was originally cosmological: the concentric transparent crystalline globes that were believed to carry the planets, which is where the music of the spheres comes from.
sphere (n.) a re-Latinized spelling, attested beginning mid-15c., of Middle English spere (c. 1300) "cosmos; space, conceived as a hollow globe about the world," from Anglo-French espiere , Old French espere (13c., Modern French sphère ), from Latin sphaera "globe, ball, celestial sphere" (Medieval Latin spera ), from Greek sphaira "globe, ball, playing ball, terrestrial globe," a word of unknown origin. According to Beekes there are no certain cognates outside Greek, but the Greek word also has been borrowed into Syrian ( espero ), Ethiopian ( spir ), Armenian ( sp'er ), and (non-Indo-European) Georgian ( spero ). From late 14c. in reference to any of the supposed concentric, transparent, hollow, crystalline globes of the cosmos believed to revolve around the earth and contain the planets and the fixed stars; the supposed harmonious sound they made rubbing against one another was the music of the spheres (late 14c.), Milton's sphery chime . Also from late 14c. in the general sense of "a globe; object of spherical form, a ball," and in the geometric sense of "solid figure with all points equidistant from the center." The meaning "range of something, place or scene" of activity, knowledge, etc. is recorded c. 1600 (as in sphere of influence , 1885, originally in reference to Anglo-German colonial rivalry in Africa). also from mid-15c.
sphere came in through Greek, and its spelling follows that family's conventions.
How that family writes these sounds
| /f/ | ph, transliterating φ. English had a perfectly good f and pointedly didn't use itphotograph, philosophy, sphere, amphibian |
What gives it away on the page
| ph | Transliterates φ. English had a perfectly good f and didn't use it — that's the tell.photograph, philosophy, sphere, amphibian |