Why isn't ecliptic a proper noun? There is only one, and it has a name.

Example (context):

... 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.

3 Answers 3


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.


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... enter image description here

...and here's the capitalised version the Moon...

enter image description here

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.

The ecliptic shows much the same pattern, in that it was normally capitalised in the 1700s, but normally isn't today.

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.

  • 1
    I would argue that the existence of moons on other planets would be a factor in favor of capitalizing the natural satellite that orbits our own, so that when talking about the discovery of e.g. another moon orbiting Pluto, one can distinguish "the moon" [Pluto's newly-discovered satellite] from "the Moon" [Earth's satellite].
    – supercat
    May 1, 2014 at 0:14

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