Earth phase

The Earth phase, or Terra phase, is the shape of the directly sunlit portion of the Earth as viewed from the Moon (or elsewhere extraterrestrially).

From the Moon, the Earth phases gradually and cyclically change over a period of about a month, as the orbital positions of the Moon around the Earth and of the Earth around the Sun shift.^[1]^[2]^[3]^[4]^[5]^[6]

Overview

NASA astronaut deploying American flag during a half phase of the Earth.

Among the most prominent features of the Moon's sky is Earth. Earth's apparent size is four times the Moon's as seen from Earth. However, because the Moon's orbit is eccentric, Earth's size in the sky may be a bit smaller or larger at times. Earth shows phases, just like the Moon does for observers on the Earth. The phases, however, are opposite; when the observer on the Earth sees the "full Moon", the observer on the Moon sees a "new Earth", and vice versa. Earth's albedo is three times as high as that of the Moon (due in part to its whitish cloud cover), and coupled with the wider area, the "full Earth" glows over 50 times brighter than the "full Moon" does for the observer on Earth. This Earth light reflected on the Moon's darkened near-side half is bright enough to be visible from Earth – a phenomenon known as Earthshine.

As a result of the Moon's tidal locking, one side of the Moon (the "near side") is permanently turned towards Earth, and the other side, the "far side", mostly cannot be seen from Earth. This means, conversely, that Earth can be seen only from the near side of the Moon and would always be invisible from the far side.

If the Moon's rotation were purely synchronous, Earth would not have any noticeable movement in the Moon's sky. However, due to the Moon's libration, Earth does perform a slow and complex wobbling movement. Once a month, as seen from the Moon, Earth traces out a small oval. The exact shape and orientation of this oval depend on one's location on the Moon. As a result, near the boundary of the near and far sides of the Moon, Earth is sometimes below the horizon and sometimes above it.

Although genuine photographs of the Earth viewed from the Moon exist, many from NASA and some shared on social media, may not be real.^[7]

Eclipses from the Moon

Earth and the Sun sometimes meet in the lunar sky, causing an eclipse. On Earth, one would see a lunar eclipse, when the Moon passes through the Earth's shadow; meanwhile on the Moon, one would see a solar eclipse, when the Sun goes behind the Earth. Since the apparent diameter of the Earth is four times as large as that of the Sun, the Sun would be hidden behind the Earth for hours. Earth's atmosphere would be visible as a reddish ring. During the Apollo 15 mission, an attempt was made to use the Lunar Roving Vehicle's TV camera to view such an eclipse, but the camera or its power source failed after the astronauts left for Earth.^[8]

Solar eclipse shadows, when the Moon blocks sunlight to the Earth, on the other hand, would not be as spectacular for observers on the Moon viewing the Earth: the Moon's umbra nearly tapers out at the Earth's surface. A blurry dark patch would be barely visible. The effect would be comparable to the shadow of a golf ball cast by sunlight on an object 5 m (16 ft) away. Observers on the Moon with telescopes might be able to discern the umbral shadow as a black spot at the center of a less dark region (penumbra) traveling across the full Earth's disk. It would look essentially the same as it does to the Deep Space Climate Observatory, which orbits Earth at the L1 Lagrangian point in the Sun-Earth system, 1.5 million km (0.93 million mi) from Earth.

In summary, whenever an eclipse of some sort is occurring on Earth, an eclipse of another sort is occurring on the Moon. Eclipses occur for observers on both Earth and the Moon whenever the two bodies and the Sun align in a straight line, or syzygy.