What Causes the Phases of the Moon?


Quick Answer

The phases of the moon are caused by the light from the sun and the position of the moon with regard to the Earth and sun. Depending on these positions, the moon appears to be in any one of eight major phases from Earth. These phases are full, waning gibbous, third quarter, waning crescent, new, waxing crescent, first quarter and waxing gibbous. Each phase lasts approximately seven days.

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Full Answer

The moon itself does not actually change during its lunar phases, but its position around the Earth does. Sunlight always shines on and illuminates half of the moon; from Earth, only certain illuminated parts are visible, depending on the moon's position. Therefore, the same phase change appears to happen from the moon to the Earth, but in reverse. For example, the moon appears full from Earth when it is on the opposite side of the Earth from the Sun. From the moon, though, the Earth would appear to be in its new phase. This is due to the sun illuminating the half of the Earth opposite the moon. The opposite effect happens when the moon appears new from Earth. This means that the sun is illuminating the opposite half of the moon and that the Earth would appear full from the moon.

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