Why isn't ecliptic a proper noun? There is only one, and it has a name.
... the true Sun is not always exactly on the ecliptic for a hypothetical observer at Earth's center, but may be some arcseconds north or south of it.
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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.
Insofar as there are "rules" for English, one that's fairly consistently applied is that proper nouns should be capitalised. Google NGrams are a bit vague about how they treat letter case in search terms, but I think it's meaningful to look at these two charts. Firstly, the moon...
...and here's the capitalised version the Moon...
I think partly this is telling us is that by the end of the 18th century people were more aware that there's more than one moon (even in our own solar system), so they stopped capitalising it so often. This tendency to not capitalise is far less marked in the case of the galaxy, presumably because it wasn't until much later that people realised there are lots of galaxies.
TL;DR The ecliptic, like these other terms, can be considered a proper noun or just "a noun", depending on your point of view. As humanity at large increasingly moves away from geocentric (or even heliocentric) frameworks, we increasingly adopt that second designation. It's just a noun.