Q. Searching through CMOS, I can’t determine if this sentence is properly capitalized: “It is the sign that sat squarely on the Earth’s eastern horizon when you were born.” (It’s for an astrological publication.) Specifically, should the words earth, eastern, and horizon be capitalized, and is the “the” before Earth correct? Thank you.
A. Considered as a planet among other planets and bodies in our own solar system, “Earth” may be capitalized. In such contexts, “Sun” and “Moon” may also be capitalized, and “Earth” often appears without the definite article—like Mars and the other planets, but unlike the Sun and the Moon:
The Moon is much closer to Earth than the Sun is to Mercury.
If you (or your publication) prefer instead to write “the Earth” (as in your example)—and to use lowercase for the sun and the moon—that’s okay too. Just be consistent.
Questions like yours wouldn’t come up if not for the fact that there are many moons and suns besides our own, and the earth to us is both a planet and the substance on its surface (and the model for other earthlike planets). In ordinary prose—or in any generic reference that doesn’t depend on the identity of a specific astronomical body among other such objects, or where our own earth and sun and moon may be assumed—lowercase is almost always appropriate:
We learned that the moon is round, the earth is flat, and the sun is a golden orb.
Why on earth would anyone under the sun believe the moon is made of cheese?
Ganymede is Jupiter’s largest moon.
Circumbinary planets are planets that orbit two suns.
As for “eastern horizon,” that’s a relatively generic description, so lowercase is your best option. See CMOS 8.140 and 8.141 for a few additional considerations.