Why isn't "ecliptic" a proper noun?

The first three citations in the OED (1625, 1646 and 1698) have the word starting with a capital letter, which may suggest it was indeed once thought of as a proper noun. However, the OED gives two definitions for the noun – the great circle of the celestial sphere and the great circle of the terrestial sphere – so there are thus more than one, making the classification as a common noun appropriate. It might also be argued that the noun is derived by conversion from the adjective, which predates it by a couple of hundred years, and so has suspect breeding to qualify for proper noun status.

For the same reason that equator isn't a proper noun. Names for abstract concepts are seldom capitalized (unless they involve a person's name), unlike names for planets, comets, stars, and the like.