Determining whether to refer to geological and astronomical terms with initial uppercase or lowercase letters can be a challenge, because various publications and publishers differ on capitalization style. The following guidelines, however, appear to predominate:
Names of geological time spans are capitalized, but the terms for the magnitude of duration (eons, eras, periods, epochs, and stages, in descending order of length), are not; in scientific and nonscientific prose alike, these terms can be omitted:
“The Mesozoic is also known as the Age of Dinosaurs.”
“Mrs. Wattle has been teaching Freshman Composition since the Mesozoic.”
Whether modifying terms such as early, middle, and late are capitalized depends on whether they are themselves modified:
“Tyrannosaurus rex lived during the Late Cretaceous.”
“The Deccan Traps erupted in the very late Cretaceous.”
“Ice age” is considered a generic term because multiple such events have occurred.
In astronomy, general terms in proper names of celestial bodies are generally capitalized (“Orion’s Belt,” “Barnard’s Star,” “Comet Halley”). Note, however, that comet is lowercased in lay references to “Halley’s comet.”
In nontechnical contexts, sun and moon are often lowercased:
“She shielded her eyes from the bright light of the sun.”
“Beware when the moon is full.”
In works about astronomy, or those in which other celestial bodies are referenced, uppercase them:
“The Sun is merely one of countless stars.”
“The Moon orbits our planet roughly every twenty-eight days.”
The same rule applies to the name of our planet. In idioms such as “where on earth,” “down to earth,” and “move heaven and earth,” the name requires no emphasis, and references to our world from a surface perspective and to its soil are likewise lowercased:
“I traveled to the four corners of the earth to find it.”
“The earth here is rich and loamy.”
But the word as the name of the planet should be emphasized like any other:
“The first four planets, Mercury, Venus, Earth, and Mars, consisting mostly of rock and metals, are called the terrestrial planets.” (Note that Earth, in such contexts, need not be, and rarely is, preceded by the.)
General terms like “solar system,” galaxy, and universe are usually not capitalized; some publications and books uppercase them (especially in references to our own solar system and the Milky Way galaxy). Names of celestial phenomena and objects such as the aurora borealis and the rings orbiting Jupiter and Saturn are lowercased.
Remember, too, when discussing the planets orbiting the Sun, that Pluto was in 2006 demoted to a dwarf planet — one of four in the solar system’s distant Kuiper belt (a fifth dwarf planet lies in the asteroid belt, between the orbits of Mars and Jupiter), and may not even be the largest one. (The scientific jury is still out on whether the similarly sized Eris is larger).
And why is belt capitalized in “Orion’s Belt” and not in “the Kuiper belt”? In the former term, it’s a reference to part of the personification of the Orion constellation, but in the latter, it’s merely a description, just as in “the asteroid belt.”