Personally, I've always suspected that bit of lore is a tale told by copy editors to each other to solace ourselves because the art director and the sales force really decide what's on the covers and everybody knows that.
That said, many folks who don't have anything to do with the interdepartmental machinations of publishing do have questions about whether it's Earth or earth, the Earth or the earth, the Sun or the sun, the Moon or the moon.
That's harder to generalize about than you might think. Of course, when looking at the names of heavenly bodies, the names of planets (and disputed planets--poor Pluto) are always capitalized; they are proper nouns--the proper names of Greek and Roman deities. Mercury, Venus, Mars, Jupiter, Saturn, Neptune, Uranus, and Pluto. Thus, Earth, as the proper noun referring to the planet Earth, is capitalized just like the rest of the planets. When the word is used to describe dirt, soil, the ground we stand on, it is lowercase. "The houses were dug into the earth," "She touched the earth; it was damp," and so on.
When we discuss Apollo missions to the Moon/moon or sending probes into the Sun/sun, more questions come up. We, here on Earth, have one moon and many folks capitalize it. Yet Saturn, Mars, and Jupiter have moons and more moons, and it makes no sense to capitalize them; luckily, they have their own proper names.
Best practice is to pick a style guide and follow it. We still see the Sun, Moon, and Earth capitalized together, especially in some kids' books, but most style guides advise that we lowercase sun and moon, capitalize the Earth in context as one of the planets, and use the lowercase earth for dirt.
But on Earth Day, let's all capitalize on the Earth we have. And treat it well. Upper or lowercase, it's the only Earth we've got.
What's your stand on capitalization? Whether earth, sun, moon, or stars, President or president, do you tend toward capitalizing, or lowercase?
Ellen Scordato has 25 years' book publishing experience as an editor, copy editor, proofreader, and managing editor. She's now a partner in The Stonesong Press, a nonfiction book producer and agency. In addition to her work at Stonesong, Ellen has taught grammar, punctuation, and style at the New School for more than 12 years in the English Language Studies department and is currently teaching English as a Second Language at Cabrini Immigrant Services.