How many days long is the moon's orbit around earth?

Answer 1

The moon's orbital period is roughly 27.32 days. This is the true orbital (sidereal) period of the moon around the earth. The moon's synodic period is roughly 29.53 days. This is the period that is related to the regular pattern of phases of the moon. If the above answers your question, that is good. For a little basic information on how the sidereal and synodic periods differ, read on. It is easier to explain using the earth orbiting the sun as an example, but the same concept applies to the moon. If you observe the earth's movement from far to the north, you will see the earth spinning on its axis. The earth will be orbiting (revolving) counter-clockwise, and also spinning (rotating) counter-clockwise. Just look down from the stars and imagine [or sketch] that you will start observing the earth's spin when the earth is about at the location of '5' on a clock. You can see that the sun is to the left, and higher than the earth in your mental image. Imagine an arrow painted on the earth pointing straight out (exactly straight up, in your mental image) from the earth, in earth's orbital plane, and then when the earth completes exactly one full rotation, the arrow will again be pointing exactly straight up, and you will stop the action; imagine that the earth is now roughly at '2' on a clock. It is ok to exaggerate the orbit in this mental experiment in order to clarify the point. When you stop the action, you realize that not only has the earth been rotating, but it has continued along its orbit around the sun. You notice that the sun is still to the left, but it is now below the earth in your mental image. But the arrow is pointing straight up, indicating that exactly one rotation has taken place. This is the sidereal period; a true rotation relative to the stars. Now, you realize that the spot on the earth that was right under the sun when you started isn't nearly under the sun now that you stopped. The sun is still to the left of the earth, but it is now below the earth in your mental image! When the earth was at '5', a line connecting the earth's center to the center of the sun would slope upwardto the left. At '2', the line would slope downward to the left. The people right under the sun when you started observing don't think that an entire rotation has happened yet, because the sun is not directly overhead! So the earth has to rotate a little longer than one full rotation in order to 'get' the sun to where it 'should' be. This is the synodic period-- the period of rotation based on the earth's position relative to the sun. The very same thing happens with the movement of the moon. The sidereal, or true rotation is a little shorter than the synodic period, because the moon has to orbit (and rotate) a little longer in order to be in the same position relative to the earth and sun. When the moon reaches that point where it is in the same place relative to the earth and sun, you observe exactly the same moon phase as when you started.

Answer 2

The moon's orbit around Earth, known as a synodic month, is approximately 29.5 days long.