The Moon

About how many days does it take for the moon to make a full revolution around the Earth?

And the answer: 27.

Photo credit: Gregory H. Revera, CC BY-SA 3.0, via Wikimedia Commons

The moon travels around the Earth in an elliptical orbit, not in a perfect circle. It takes 27.3 days for the moon to complete one revolution around the Earth. It also takes approximately 27 days for the moon to rotate once on its own axis.

As the moon revolves around the Earth, it goes through phases. The beginning of this cycle is marked by the New Moon, a phase in which the mass of the Earth aligns with the sun in a way to cast the moon in complete shadow. The moon then orbits around the Earth at a rate of about 13 degrees every 24 hours. After about 3 days, it will have travelled 45 degrees, and becomes visible as a waxing crescent moon. "Waxing" refers to how the amount of visible sunlight is getting larger, and "crescent" is the apparent shape of the moon at this time.

After 10 days, the moon will have moved 135 degrees – earning the title of waxing gibbous (no, not the monkey). Fourteen days in, the moon has travelled 180 degrees, and is finally a full moon. From there, it begins to "wane," the phase in which the moon slowly orbits back into full darkness, only to begin its 27 day cycle anew.

Did you know?

As the moon travels, it rotates on its own axis. It takes roughly the same amount of time for the Moon to make a full rotation as it does for it to complete its orbit, which means that we only ever see around 60% of the Moon’s surface from Earth. The part that faces Earth is known as the "near side" and the other, the "far side" (see: Pink Floyd's Dark Side of the Moon).

And, check out the video below for one of the first expeditions into the world of science-fiction filmmaking: George Melies' 1902 film A Trip to the Moon.

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