phase, in astronomy, the measure of how much of the illuminated surface of a planet or satellite can be seen from a point at a distance from that body; the term is most often used to describe the moon as seen from the earth. When the moon is between the earth and the sun, we cannot see the lighted half at all, and the moon is said to be new. For a few days before and after the new moon we can see a small part of the lighted half, which appears as a crescent with the horns, or cusps, pointing away from the sun. When the moon has completed half its orbit from new moon to new moon, it is on the opposite side of the earth from the sun and we see the entire lighted half; this phase is called the full moon. When the moon is at quadrature with the sun, having completed either one quarter or three quarters of its orbit from new moon to new moon, half the lighted side is visible; this phase is called the half-moon. The half-moon between the new moon and the full moon is known as the first quarter and that between the full moon and new moon is known as the last quarter. Between the first quarter and the full moon and between the full moon and the last quarter we see more than half the lighted side; this phase is called gibbous. Of the planets, only Mercury and Venus, whose orbits pass between the earth and sun, show all the phases that the moon shows; the other planets are always either gibbous or full.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.