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A lunar day is the length of time it takes for the Moon to make one complete rotation on its axis compared to the Sun. This is important because the Moon is tidally locked with respect to the Earth. So it always points the same face towards the Earth as it goes around the planet. So, how long is a day on the Moon?
The lunar day lasts 27 days, 7 hours and 43.2 minutes. And this the same time it takes for the Moon to orbit around the Earth.
But things get a little more complex. While the Moon is orbiting around the Earth, the Earth and the Moon are orbiting around the Sun. While the lunar day lasts 27 days and 7 hours, it actually takes longer for the Moon to get back to the same phase from our perspective here on Earth; from full Moon to full Moon takes 29 days, 12 hours, and 44 minutes.
If you ever get the opportunity to stand on the surface of the Moon, and look at the Earth, our planet would always remain in the exact same position in the sky. The Sun, on the other hand, will still rise, move across the sky and then set. Of course, an average day will last 27 days and 7 hours until the Sun returns to the same position in the sky.
Astronomers say that the Moon is tidally locked to the Earth. At some point in the distant past, the Moon rotated more rapidly than it currently done. The Earth’s gravity caused part of the Moon to bulge out. The pull of gravity caused the rotation of the Moon to slow down until this bulge was pointing directly at the Earth. At this point, the Moon was tidally locked to the Earth; this is why it shows the same face to us.
And it’s also why a lunar day lasts the same as it takes the Moon to go around the Earth.
NASA Moon Facts