How come it take 29.5 days for the moons phases to complete, when there are only 27.3 days in a moon year?

Answer 1

I think that its because the Earth is spinning as the moon spins around it so on the Moon you spin around once in 27.3 days but here on Earth you only see it happen every 29.5 days as the moon has to catch up to the spinning Earth.

Answer 2

The Moon takes 27.3 days to complete one orbit of the Earth, but the phases are depndent on the angle between the Earth, Moon and Sun.

The Moon moves roughly 13 degrees in its orbit per Earth day, while Earth moves about 1 degree around the sun per day. Since one full lunar orbit takes about 27 days, that means Earth moves about 27 degrees around the sun per lunar orbit.

Now, imagine starting at full Moon. The Earth, Moon and Sun are all aligned. That is to say the angle at the Earth between the Moon and Sun is 180 degrees. One full lunar orbit, 27 days, later the Moon has travelled 360 degrees around Earth, but Earth has also moved 27 degrees around the Sun. The angle between Sun and Moon from Earth is now therefore less than 180 degrees so the Moon will not appear full from Earth. It needs to move about 27 degrees further on in its orbit to again line up with the Earth and Sun, and, as we saw above, this takes about two days. Hence the Moon orbits in about 27 days, but the phases take roughly 2 extra days, making a total of about 29 days, to complete a full cycle.

(This would be so much easier if we could attach diagrams....)

Answer 3

Gene got it.

In the lunar month, you have to consider that the Earth has moved about 8% around its own orbit around the sun. That orbit is almost circular, so there is about an 8 % change in its moon/sun aspect.

That 8% takes care of the 2 days difference between the synodic period (29.5 days) and the sidereal period (27.3 days).

Answer 4

The earth is orbiting the sun so the sun is not in the same celestial position on a subsequent full moon as it was the last.

Answer 5

Because in the time it takes for the moon to go around once the sun also moves (but alot slower).

Answer 6

Because the moon is revoulving, causing it's phases to ocur when it is in certain positions. hovever, years refer to the time it takes for something to rotate on it's axis. so it rotates faster that it revolves. another example of this is mars. it's years are much shortr than it's days.