Physical Cosmology

When does a new moon occur?
Answered by Science Channel
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  1. The popular misconception is that a new moon occurs when the Earth blocks the sunlight, causing the moon to appear as dark and shadowy. In fact, the moon is located between the Earth and the sun during the new moon phase, and what we see of the moon is whatever the sun's light -- reflected off the moon -- lets us see. Because of that alignment, what we see in the new moon is the side of the moon that the sun is not lighting -- in effect, during this phase we don't "see" the moon at all, because the sun's light is so strong it obscures our lunar friend. When a full moon occurs, by contrast, we're seeing the moon's sunlit near side during the lunar equivalent of noon.

    The new moon, of course, is only one phase of the lunar cycle. The moon circles the Earth every 29.53 days, perched in orbit around us 237,612 miles away (382,400 kilometers) [source: NASA]. The Earth's gravity pulls on it so that the same side of the moon is always facing us. Nonetheless, we see a different view, or phase, of the moon each night of the month. Just like the Earth, half of the moon is always in sunlight, and half in shadow. When the moon is between us and the sun, the lighted part is pointed away from us, so we don't see it -- this is, as we have seen, the new moon. As the moon orbits the Earth, each night we see a greater part of the lighted side, until the whole sunlit side faces us (again, full moon). As the orbit continues, we then see less of the lighted side until the whole cycle is completed and starts again. As the moon, from our view, becomes more visible during the cycle, it is said to be "waxing," while it is "waning" after the full moon, as it once again becomes less and less visible.

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