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Animation showing some parameters of the orbit of the Moon around Earth, and how they combine to produce a so-called supermoon. The Moon's orbit around Earth is an ellipse. The point at which the Moon is furthest from Earth is called the apogee, and the closest point is called the perigee. The line connecting the points passes through Earth, and is called the line of apsides (blue). A full Moon occurs when the Moon is on the opposite side to the Earth as the Sun. When the full Moon occurs with the Moon at apogee, it appears relatively small in the sky. The Moon's phase and size is seen in the inset at upper left. Some 13.5 days later the Moon is between the Earth and the Sun, and is a new Moon, and not visible. With respect to the distant stars (green line) the Moon orbits Earth every 27.32 days, called the sidereal month. However, it has not returned to the apogee at this point, as the elliptical orbit itself rotates slowly in the same direction as the Moon's orbit, meaning the apogee and perigee also rotate around the Earth. It takes the Moon another 0.23 days to reach the apogee again, a 27.55-day period called the anomalistic month. At this stage it is still not yet back at its original phase, as the Earth has moved around the Sun. It takes a further 1.98 days to reach full again, by which time it has moved past the apogee, and so appears slightly larger than before at full Moon. The phase-to-phase period is the 29.53-day synodic month. The entire lunar orbit rotates around Earth every 3233 days. The combination of these cycles produces full Moons of different sizes over this period. When the full Moon occurs with the Moon near perigee, it appears relatively large in the sky, and is called a supermoon. See clip K006/7165 for a diagram showing the cycle of supermoons.
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