JOHN SANFORD / SCIENCE PHOTO LIBRARY JOHN SANFORD / SCIENCE PHOTO LIBRARY
Waning gibbous Moon. Optical image of a waning gibbous Moon. A gibbous Moon is one showing over half the surface. The Moon is said to wane when it is diminishing in apparent size. The Moon's phases start with a new Moon, progress through to a full Moon, and then reduce to a new Moon again via this gibbous phase. One cycle takes around 28 days. The phases of the Moon are due to its orbit around Earth. When it lies in between the Earth and the Sun, only the unlit side faces us (new Moon). When it has moved round so that the Earth lies between it and the Sun, we see the full lit face, a full Moon. North is at top. For the sequence of the Moon's phases, see images R340/526-532.
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