Q:

Why is there a full moon only once a month?

A:

Quick Answer

There is only one full moon each month because the moon only opposes the sun once per month in its orbit. The phases of the moon are caused by how much of its visible surface is illuminated by the sun's light, and a full moon only occurs when the moon is on the opposite side of the Earth from the sun.

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Full Answer

As the moon travels around the Earth, its phase changes. When the moon is on the same side of the planet as the sun, its illuminated surface is facing away from Earth, causing the visible portion of the moon to appear completely dark. As it orbits, more of its illuminated side becomes visible, transitioning from the dark "new moon" to a "crescent," "half" and "gibbous" moon. Only when the moon is in direct opposition to the sun is the entire surface illuminated, producing a full moon. Since this only happens once per orbit, there is only one full moon per month.

Full moons are also the only time lunar eclipses occur. The moon must be in a particular arrangement with the Earth for the Earth's umbral shadow to darken the moon's surface, and even then refracted light through the atmosphere gives it a dark reddish glow instead of obscuring it completely.

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Related Questions

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    How can you tell when the next full moon is?

    A:

    To find out when the next full moon will be, use an online directory of full moon dates or make an educated prediction based upon the moon's current place in its cycle. Some online directories of moon dates include MoonGiant.com, MoonPhases.info and Space.com.

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    What are the moon's phases?

    A:

    The phases of the moon, from darkest to brightest, are the new moon, the crescent moon, the quarter moon, the gibbous moon and the full moon. As the moon fills out in the sky, it passes through the waxing crescent, first quarter and waxing gibbous phases. As it returns to darkness, it passes through the waning gibbous, last quarter and waning crescent phases.

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    A:

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    A:

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