Why are a "day" and a "year" the same on the moon?

Question

I kno other stuff about the moon, but my teacher didn't really explain this...

Answer 1

It depends on what you mean by a "year." Now with respect to the earth, the moon rotates once for every orbit. This is why we always see one side of the moon.

So if you are defining a "lunar year" as one revolution around the earth, then not only is the day the same as the year, but it's the same as the month, too. The reason for this is that the moon had become gravitationally "locked" with the earth. This is not related to ocean tides on the earth. That's something else. It is related to tidal drag on the moon. Most of the satellites (moons), in the solar system do the same thing; one side always facing toward the parent planet.

Re. the answer before mine, not so. The moon does rotate on it's axis about once every 29 of our days, wrt the sun.

Answer 2

A day (in this context) is the time from sunrise to the next sunrise. On the Moon, this is, on average, 29.530589 Earth-days. However, the word day, in relation to the Moon, is already used for something else (see below).

The period the Moon takes to make it around its principal planet is called a "month". The synodic month is the time it takes to go around Earth, relative to the Sun (i.e., the time from New Moon to the next New Moon). Exactly the same value as the time from sunrise to sunrise on the Moon.

A "lunar day" is the time it takes for the Moon to come back over the same longitude on Earth; it is a time measure for an observer on Earth, not on the Moon. It is about 24h48m and it is used for calculations of tides on Earth.


That is why the word "day" is rarely associated with how things would appear from the Moon. We use the word "month" instead.

A lunar year is a whole year for an observer on Earth (not on the Moon), counted relative to the Moon. In the Muslim calendar, the year is always 12 lunar months. Therefore, their year is shorter than the "normal" Earth-year.

In the Chinese calendar, the year begins with the second New Moon after the Winter solstice (and it ends with the start of the next year). Some years have 12 lunar months, others have 13. The cycle is determined by the date of the second New Moon after the solstice.

Other lunar calendars have predetermined cycles of 12 and 13 months per year. The cycle is calculated in order to keep pace with the seasons (which depend on the Sun's relative position). They are called "lunisolar" calendars.

If you were on the Moon and wanted to count the years based on the position of the Sun relative to the "fixed" stars, you could use a lunisolar calendar to determine your year.

or you could pick a star and say that the year begins when the Sun and this star are in line. Because the speed of the Moon relative to the Sun is not regular (it depends on the Moon's position around Earth), this calendar would be just as strange as a lunisolar one.

Answer 3

the time it takes for the moon to rotate once is equal to the time it takes for the moon to rotate around the earth. That is also the reason why we can only see one side of the moon from earth.

Answer 4

Because of the tides. Tides on the Moon cause friction when it rotates, and acted to slow the Moon's rotation until it didn't rotate relative to us any more. Similarly, Earth's tides cause friction and are slowing down the Earth's rotation relative to the Moon. But the Moon's gravitational effect on Earth is much smaller, so we haven't slowed to a stop yet (and we won't before the Sun dies).

The energy of the Earth's rotation has to go somewhere; it's going into the Moon's orbit, causing the moon to slowly spiral outwards.

Answer 5

It's revolution period is 28 days nd sumthing....wich is approximately same as its revolution period..revolution decides year and rotation a day...so both being same..it's considered that a day is equal to its year on moon...

Answer 6

Because the period of its orbit is (kinda by definition) a 'year' and the period of its rotation is (again, by definition) a 'day'. But its orbital period and rotational period are the same (which is why the same face always faces us) and that means its 'day' and 'year' are the same.

Doug