Moon

The Moon is presumably a natural satellite in orbit around Remnant, the world of RWBY. Apart from being a striking visual feature of the series, it may also bear some symbolic significance.

Astrophysics
The most obvious feature of the planetoid is that it appears to be shattered into several irregularly-formed fragments and a single larger hemispherically-shaped piece. It is not known what caused the moon to exist in this unusual shape.

The moon in the world of RWBY follows a peculiar lunar cycle radically different to that of Earth's. Unlike Earth's moon, which is tidally locked to Earth and thus always presents the same surface when viewed from the planet's surface, the Moon of the RWBY universe exhibits asynchronous rotation.

As a result of this, at different points in the lunar cycle, the Moon presents a different face to the planet: when the hemispherical fragment is directly facing Remnant, the moon appears full; at others the broken parts of the moon are visible.

Different phases of the lunar cycle are visible throughout the series. For example, in Episode 1, the moon appears to be in a broken phase, while in Episode 3 it appears to be full.

Symbolism
The moon is a significant recurring visual motif in the series. The moon is heavily associated with Ruby Rose, who appears dramatically silhouetted against the full moon in both the "Red" Trailer and the Opening, as well as on official merchandise. Yang Xiao Long may also be symbolically connected to the sun in a similar way. The full moon also makes a significant appearance in the "White" Trailer, as Weiss gazes up into the sky after the fight.

It is also notable that the moon in its cracked phase heralds two significant appearances of Roman Torchwick, first in Episode 1 and later in Episode 8. The moon in its cracked phase may foreshadow the unfolding plans of the villains of RWBY.

Reference to the moon is also made on the soundtrack, in the song Gold: "If the stars all fall / When there's no more light / and the moon should crumble / You will be alright."

Trivia

 * Monty Oum has compared the moon of RWBY to the shattered moon of the Klingon homeworld in Star Trek.