Why does it take 27.3 days for the Moon to revolve around the Earth but it takes 29.5 days for the Moon to go through its phases?

Answer 1

27.32 days

Answer 2

Here's a brief answer: Start at the full Moon. The Moon moves through its orbit and finds itself in the same position relative to the Earth that it was in at the last full Moon. But the Earth has also moved, in its orbit around the Sun, over the same period of time. So, the Moon has to go farther than 360 degrees of orbit in order to get to that full Moon position again. The cycle of phases is about 29.5 days. After 27.3 days, the Moon has revolved through a full 360 degrees relative to the stars.

When a body orbits another and moves to the point where it is in the same position relative to the Sun, that is the "synodic period". The Moon phase cycle is a synodic period. When a body orbits another a full 360 degrees relative to the distant stars, that is a "sidereal period".

Here is more detail: Understanding earth's sidereal day will be a great way to start understanding what is happening with the moon. Get ready to do a little work; this isn't easy. Looking down from high above the north pole create a mental image (or use paper and pencil) of the earth orbiting the sun in a counterclockwise movement, while at the same time spinning counterclockwise. This is asking a lot; take some time and give it some thought. You can imagine a point on earth's equator making a tight spiral movement as earth circles the sun.

Now imagine the earth at "5 o'clock" on earth's orbit in your image. There is a point on the earth that is directly under the sun-- the point that is experiencing solar noon. That point is about at 11 o'clock on earth itself. Now imagine another point on earth at 12 o'clock that is pointing straight up to a very distant star. The star is so far away that it will always be pointing straight up, wherever the earth is in its orbit. Now draw the earth at 3 o'clock along its orbit. We'll make believe that this movement in the orbit represents one day. It is exaggerated, but that will help clarify the point.

With the earth at 3 o'clock, imagine that the same 12 o'clock point on earth is pointing straight up in your image. That point is now pointing again directly at the distant star mentioned above. This is a sidereal day-- the length of time it takes for the earth to rotate exactly 360 degrees on its axis. Even if you have a hard time with the idea of the distant star, you can see that the point is pointing straight up to the top edge of your image, and that is has spun 360 degrees.

Notice something here: with the earth at 3 o'clock, the point that was directly under the sun (the point on earth at 11 o'clock) is not now directly under the sun! The earth has rotated exactly 360 degrees, (a sidereal day roughly equalling 23 hours, 56 minutes and 4 seconds) but it still has to rotate a little more for a full solar day to pass. Our 24 hour day is based on the mean solar day, and so it represents a little more than a 360 degree spin of the earth. So the sidereal day is a little shorter than a standard or civil day.

Now it should be easy to see that the moon's orbit around the earth will have a similar characteristic. The sidereal period of the moon is about 27.3 days, the length of time for the moon to travel exactly 360 degrees in its orbit around the earth. However, the earth and moon have both traveled along earth's orbit during this sidereal period of the moon, so in order to orient itself the same way in relation to the earth and the sun (in other words, to show the same phase from earth's point of view) the moon has to move farther than 360 degrees in its orbit-- in a sense to 'catch up' to the earth.

Two diagrams like the ones you did above will help. Show the moon at full moon at one point in the diagram, and then again a month later. You will see that the moon has to orbit more than 360 degrees (in relation to the top edge of your diagram, or in relation to a very distant star) in order to again hit full moon. This period that is in synch with the moon's phases is called the moon's synodic period, and it is about 29.5 days long.

Answer 3

cause it takes long for the moon to get there.

Answer 4

The moon has only a 28 day cyclical pattern because it takes 28 days for the moon to rotate the earth

Answer 5

NO. It takes 29HOURS and 30 MIN