The Universe and Galaxies: The Earth-Moon System

Discovery Space: The Earth-Moon System

The Earth and moon were forced into coexistence by the collision that created the moon. Since then their movements have been closely interwoven. They influence each other, creating diverse phenomena.

The diameter of the moon is four times small to than that of the Earth. In comparison, the moons of other planets are proportionately much smaller. Because of this, the Earth and moon are sometimes referred to as a double planet system. The moon has more influence on the Earth than an aver- age planetary moon. However, the gravitational force of the moon is too small to maintain an atmosphere.

Origin of the moon Based on computer simulations and examinations of moon rock, it is thought that the young Earth collided with a smaller proto-planet around 4.5 billion years ago. The rubble from this collision formed a ring around the Earth, which then agglomerated to form the moon.

Moonlight and earthlight

The side of the moon that is illuminated by the sun reflects that light back to the Earth. The amount of moon visible to the Earth depends on its position in relation to the sun. The moon changes its phases over the course of a month as it orbits the Earth At new moon, the dark side of the moon (that part not lit by the sun) is directed at Earth.

At full moon, we see the half of the moon lit by the sun and the sunlight reflected from it. Even the dark side of the moon is never totally black-it is lit by earthlight (sunlight reflected by the Earth’s clouds). By measuring this ash gray light, we can recognize changes in cloud cover and the Earth’s atmosphere.

Synchronous rotation

The moon always shows the same side to the Earth—this means it takes as long for one orbit around the Earth as it does for one revolution around itself. This synchronous rotation is caused by tidal periodicity. Early in the moon’s history, its rotation slowed and became locked in this configuration with the Earth as a result of frictional forces caused by tidal effects from the Earth.

Another consequence of the tidal interaction between the Earth and the moon is that the gravitational force of the Earth’s tidal peaks pulls the moon slightly along its orbit. This causes the moon to travel faster and raise its orbit, so that the moon moves roughly 1.5 inches (3.8 cm) further away from the Earth every year.


The moon darkens when it passes through the shadow of the Earth.

The sun darkens when the shadow of the moon falls onto the Earth In both instances, sun, moon, and Earth are in a straight line However, the moon orbit is inclined with respect to the Earth orbit, so that eclipses do not occur during each orbit of the moon.


CLOSE TO THE HORIZON, the moon looks larger than when it is high in the sky. This is an optical illusion.

THE TIME BETWEEN two equal moon phases, that is from full moon to full moon, lasts 29.5 days.

THE MOON’S ORBIT around the Earth, relative to the Earth’s orbit around the sun. is tilted by 5°.