Phases of the Moon

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Sunlight is shown coming in from the right. The earth, of course, is at the center of the diagram. The moon is shown at 8 key stages during its revolution around the earth. The moon phase name is shown alongside the image. The dotted line from the earth to the moon represents your line of sight when looking at the moon. The large moon image shows what you would see at that point in the cycle. For the waning gibbous, third quarter, and waning crescent phases you have to mentally turn yourself upside down when imagining the line of sight. When you do this, you 'll "see" that the illuminated portion is on your left, just as you see in the large image.

One important thing to notice is that exactly one half of the moon is always illuminated
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If you were to view the moon cycling the earth from outside our solar system (the viewpoint of the stars), the time required is 27.3217 days, roughly two days less. This figure is called the sidereal period or orbital period. Why is the synodic period different from the sidereal period? The short answer is because on earth, we are viewing the moon from a moving platform: during the moon cycle, the earth has moved approximately one month along its year-long orbit around the sun, altering our angle of view with respect to the moon, and thus altering the phase. The earth 's orbital direction is such that it lengthens the period for earthbound observers.

Although the synodic and sidereal periods are exact numbers, the moon phase can 't be precisely calculated by simple division of days because the moon 's motion (orbital speed and position) is affected and perturbed by various forces of different strengths. Hence, complex equations are used to determine the exact position and phase of the moon at any given point in time.

Also, looking at the diagram (and imagining it to scale), you may have wondered why, at a new moon, the moon doesn 't block the sun, and at a full moon, why the earth doesn 't block sunlight from reaching the moon. The reason is because the moon 's orbit about the earth is about 5 degrees off from the earth-sun orbital plane.

However, at special times during the year, the earth, moon, and sun do in fact "line

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