Q:

Why does the moon wax and wane?

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Quick Answer

The moon appears to wax and wane through different phases due to how much of its illuminated surface is visible from Earth. At any given time, half the moon is illuminated by the sun's light. Depending on where the moon is in relation to the Earth, the amount of illuminated surface changes. The ratio of illuminated surface to shadowed surface creates the phases.

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Full Answer

When the moon is between the Earth and the sun, its illuminated side faces away from the planet. This is a new moon, when the visible surface is completely dark. As the moon travels through its orbit, the illuminated surface slowly becomes visible, waxing into first a crescent moon and then a half moon. After the half-moon comes a gibbous moon, in which three-quarters of the illuminated surface is visible. Finally, when the moon is directly opposite the sun, it becomes a full moon with its illuminated face entirely visible.

The moon's phase also determines when it becomes visible in the sky. A new moon is in the sky at the same time as the sun, and therefore is not visible with the naked eye. As the moon waxes, it begins to rise later in the day, until the full moon rises at sunset and sets at sunrise. The waning moon continues this pattern, rising later and later, until the new moon rises with the sun once again.

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Related Questions

  • Q:

    What is the difference between a new moon vs. a full moon?

    A:

    The phases of the moon are determined by the angle at which sunlight approaches the moon relative to the position of the Earth. When the sun is behind the Earth, relative to the moon, full sunlight falls on the hemisphere of the moon that faces Earth. When the sun is behind the moon, relative to Earth, sunlight falls on the opposite lunar hemisphere.

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  • Q:

    Why do we have moon phases?

    A:

    Moon phases occur because the angle of the Earth, moon and sun changes as the moon orbits the Earth. One-half of the moon is always illuminated by the sun. As a person's angle of sight changes, different portions of sunlight and shadow are observed on the moon's surface.

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  • Q:

    What causes the phases of the moon?

    A:

    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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  • Q:

    How does the moon change its shape?

    A:

    The changing shapes of the moon, known as phases, are actually just changes in perspective from Earth, which appear as a change in the moon's form. From Earth, only one side of the moon ever appears illuminated by the sun, resulting its apparent change in size.

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