Moon Geometry
The fraction of the moon that appears illuminated changes since the moon is orbiting the earth. The time it takes the moon to complete one orbit around the earth (with respect to the sun) is also the amount of time it takes to complete one cycle of phases. This period, known as the synodic month, is about 29.5 days.
Thus it takes the moon almost exactly a month to complete one orbit and go through a cycle of phases. You can conclude that if the moon is a waning gibbous on the 15th, then the moon will be waning gibbous (or close to it) on the 15th of the next month. Furthermore, in one week the moon completes about a quarter of its orbit. So if the moon is at first quarter today, then in one week it should be near full.
The diagram below shows the relationship between the position of the moon in its orbit and its appearance from earth. Here we are looking down from far above the earth's North Pole. From this direction, the moon orbits the earth in a counter-clockwise direction.
Moon Geometry
The fraction of the moon that appears illuminated changes since the moon is orbiting the earth. The time it takes the moon to complete one orbit around the earth (with respect to the sun) is also the amount of time it takes to complete one cycle of phases. This period, known as the synodic month, is about 29.5 days.
Thus it takes the moon almost exactly a month to complete one orbit and go through a cycle of phases. You can conclude that if the moon is a waning gibbous on the 15th, then the moon will be waning gibbous (or close to it) on the 15th of the next month. Furthermore, in one week the moon completes about a quarter of its orbit. So if the moon is at first quarter today, then in one week it should be near full.
The diagram below shows the relationship between the position of the moon in its orbit and its appearance from earth. Here we are looking down from far above the earth's North Pole. From this direction, the moon orbits the earth in a counter-clockwise direction.
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