Why does it take the moon 27.3 days to complete one rotation?

Answer 1

Well - this is an interesting one as it requires a term of reference.

Whilst the Moon is orbiting the Earth, the Earth is also orbiting the Sun and the Sun is in orbit around the center of our galaxy, and, the galaxy, is careening at a substantial velocity through the cosmos!

The Moon's orbit about Earth has it, on average, 390 917kms from Earth's center, as such it travels in one full orbit ~2 456 522kms.

Over the same period however, the course of the 27-odd days, the Earth has moved ~69 465 600kms around the sun in its orbit.

Over the same period, the Sun has travelled ~513 216 000kms in its orbit about the center of the galaxy.

Over the same period, the Milky way galaxy has travelled ~1 217 721 600kms in relation to the cosmic background radiation which is used as a universal reference frame.

So, I guess if you could somehow determine the average radial velocities for all these and bring to back to the Moons point of view, you have the real distance travelled by the Moon as it undertakes one orbit.

Answer 2

The Moon's rotation period is the same as its orbit period around Earth, which is called synchronous rotation. This is caused by the gravitational forces between Earth and the Moon, which have led to the Moon's rotation and orbital period being synchronized over time.

Answer 3

Because the Earth-Moon system moves significantly over that time frame relative to the Sun.

Over the course of a month, the position of the Sun relative to the Earth and Moon changes by about thirty degrees. Assume we start with full moon, where the Moon, Earth, and Sun are all more or less lined up. In order to get back to that same configuration, the Moon would have to make a complete circle around the Earth but, as I said, by that time the Sun has moved out of line by a noticeable amount, so it takes more than one complete orbit of the moon for the three to be lined up in the same way again.

Answer 4

While the Moon is orbiting around the Earth, the Earth is ALSO moving - orbiting around the Sun. In the time that it takes for the Moon to go once around the Earth, the Earth has moved so far along in its own orbit that it takes the Moon another 2 days to "catch up" to be in the same relative position of Sun-Earth-Moon.

Answer 5

Sunspots. The Sun goes through an 11-year "sunspot cycle" in which the Sun goes from having very few sunspots to LOTS of spots and back to a minimum. Astronomers using special cameras can watch the sunspots travel from one side of the Sun to the other, and during active phases, watch the same sunspot cluster reappear again 13 days later.

Because the Earth is solid, when the Earth rotates, it rotates all together. The Sun, however, is a plasma, not a solid. So the various parts of the Sun don't have to stay together, and they don't. The "equator" of the Sun rotates in 25 days, but the areas near the poles rotate in about 33 days, and the mid-latitudes rotate at an in-between speed.

Answer 6

That's the orbital period that results from the gravitational coupling of two bodies
with the masses of the Earth and Moon, separated by the distance at which they're
presently orbiting their common center of mass.

Answer 7

Because the Apollo missions went to the Moon on the barest minimum amount of fuel. If you had a little more fuel, you could get there in one day, and if you had a LOT more fuel, you could get there in one HOUR.

Answer 8

That's how long it takes for the Moon to orbit the Earth once.

However, the cycle of the phases of the Moon is 29.5 days. The difference occurs because the Earth is continuing in its orbit while the Moon is orbiting the Earth, and after 27.3 days, the Earth has moved so far in its orbit that it takes the Moon another 2.2 days for the Moon to get back into the same relative Earth-Sun-Moon orientation.

Answer 9

First, "They" are our ancient ancestors. For example, early farmers needed to tell the time of the year to best plant their crops, for this and other reasons we began to pay attention to the place where the sun rose and set, and where the stars were at different times of the year. Some people noticed patterns from natural formations such as distant mountain features that lined up with the sun, or caves that face the rising sun, only one day a year. Using simple structures like sundials, which can be as simple as a stick in the ground and a way to mark off the time, you can see the shadow's apex trace a pattern over each day, as the earth rotates on it's axis, and, as the earth orbits the sun. Every year, on the same day at the same time, the sun would make the same shadow of the stick on the ground.

As far as we understand, the speed of earth's rotation, which defines our "day", isn't directly related to how fast the earth moves around the sun, our "year". Two different kinds of motion. The two are in fact related but only in very complex, solar-system-forming kinds of ways, and it could be entirely possible that our day might have been half or twice or ten times what our current "day" is. We might be very different if this were the case - no one knows for sure.

It's important to realize that humans figured out the length of the year long before they knew what "caused" the year. Some people may have figured it out, but most people, until the last millennium or so, believed the earth was the center of the universe and everything revolved around it! It took us a long time to reason out what was happening because the way the sky looks to someone standing on the earth shows several kinds of motion happening at once - as the complex motion of a sundial's shadows shows.

Today, we think a year is about 365 and 1/4 days. We ignore the extra 1/4 and save it up for an extra day every 4 years, which we know as February 29 on leap years.

Finally, it is important to realize that time units like day and year are related to reality, something occurring in nature. Other units, like hours, minutes, and seconds, are just made up by humans. A second is a second because we SAY how long a second lasts. But a year is, however long it takes the earth (and has been taking the earth) to orbit the sun, which is something humans obviously could not have made up.

Good question!

Answer 10

I'm sorry, there are some errors here. Our Earth rotates around the Sun in about 365 days. The Earth revolves on it's axis in 24 hours.

Now it is true that it is not exactly 24 hours. There are very, very minor, but measurable, differences. But not so great as 21 minutes.

At no point has our Earth spun in 23.39 minutes in Mankind's history.