"weakness, debility," 1788, medical Latin, from Greek astheneia "want of strength, weakness, feebleness, sickness; a sickness, a disease," from asthenes "weak, without strength, feeble," from a- "not, without" (see a- (3)) + sthenos "strength, power, ability, might," which is of uncertain origin, perhaps from PIE root *segh- "to have, hold," on the notion of "steadfastness, toughness."
mid-15c., Latinized spelling 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.
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.). Also from late 14c. as "a globe; object of spherical form, a ball," and the geometric sense "solid figure with all points equidistant from the center." Meaning "range of something, place or scene of activity" is first recorded c. 1600 (as in sphere of influence, 1885, originally in reference to Anglo-German colonial rivalry in Africa).
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sahate "he masters, overcomes," sahah "power, victory;" Avestan hazah "power, victory;" Greek skhema "figure, appearance, the nature of a thing," related to skhein "to get," ekhein "to have, hold; be in a given state or condition;" Gothic sigis, Old High German sigu, Old Norse sigr, Old English sige "victory."