The Phases of the Moon |
(Note: click on any of the moon phases in the image at left to see how that phase appears from Earth.)
As viewed from high above the Earth's North Pole, the Moon travels counterclockwise around our planet. Starting with New Moon (#1 on the diagram), it takes just over a week to get to First Quarter Phase (2), then another week to fill out further to Full Moon (3). About a week later it has become a Last Quarter Moon (4). Finally, exactly 29.53 days after we started at New Moon , it returns to that phase once again. This time period, 29.53 days, is known as the Synodic Month ( Synodic is derived from a Greek word meaning "coming together.") This is really the time the Moon takes to orbit the Earth relative to the Sun, because it is the Sun's light that causes the phases in the first place.
The time it takes the Moon to move through the sky relative to the stars is slightly different. If the Moon were to pass near the star Aldebaran (in Taurus, the Bull) tonight, it would continue in its orbit and return to pass by Aldebaran again in approximately 27.32 days. This is known as the Sidereal Month.
Why is there a discrepancy between the Synodic and Sidereal months? The difference arises because the Earth is also orbiting around the Sun. (The Moon goes along for the ride.) It constantly changes its position relative to the stars. So if the Earth did not orbit the Sun, but rather stood still as the Moon orbited around it, then the Synodic and Sidereal Months would be exactly the same.
(NOTE: The image above is not to scale.)
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