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Animated graphs showing the position and phases of the Moon with respect to Earth. At the start, the phase of the Moon is on the y-axis, and time in days is along the x-axis. Starting from a half Moon with the left side lit, called a last quarter Moon, the phase moves down to a new Moon, then returns through a first quarter half moon to a full Moon and then back to the last quarter after 29.53 days. The y-axis then changes to show the position of the Moon in its orbit, with the Sun out of frame to the left. The Moon completes a full orbit around the Earth in 27.32 days. It must travel for another 2.21 days to reach last quarter again, though, as the Earth is also travelling around the Sun. The Earth's movement means the Moon must travel the extra 2.21 days to reach the same alignment with respect to the Earth and Sun, which is what determines its phase as seen from Earth. This is shown in an orbital diagram in clip K006 7160.
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