Moon

[kingkong]

although it's true that we only see 1 face of the moon, we often see more craters on the moon during crescent than full moon. seems that there are alot more craters at the edges of the moon than at the center.

i thought that's because shadow of the craters at the terminator emphasise the surface feature?

[QuantumGravity]

Hi all,

We can see 50% of the moon at any one time, and the moon always appears roughly the same side. We don't see the exact same side due to libration, which i heard/read from somewhere that the libration cycle is decades long. Libration allows around 58& or 59% of the moon to be seen throughout the cycle. Also, the moon's orbit is synchronous, hence we only get to see that one side.

[carlogambino]

Hihi, for those who want to know more about libration and the moon, here's an article from a website:

The Moon's orbit is also tilted to the ecliptic plane and to the Earth's equator by about 5°. As a consequence of these factors, the Moon appears to 'nod' from side to side and up and down during a lunar month, and it is possible to observe about 59% of the Moon's surface over a period of time, although we can only see 50% at any one instant.

The term given to this 'nodding' is "Libration". There are in fact three types of libration involved in the moon's motions. Libration in latitude is due to the Moon's axis being slightly inclined relative to the Earth's. The moon’s poles will appear to be alternately tipped slightly toward and away from the terrestrial observer over a roughly four week cycle.

Diurnal libration is due to the observer being up to four thousand miles to one side of the Earth-Moon axis on the surface of the Earth - a significant proportion of the centre-to-centre distance. The difference in perspective between the rising and setting of the Moon appears as a slight turning of the Moon first to the west and then to the east.

Libration of longitude is an effect of the Moon's varying rate of travel along its slightly elliptical orbit. Its rotation on its own axis is more regular, the difference appearing again as a slight east-west oscillation.

Although the Moon always presents the same face towards the Earth, due to its rotation and revolution being locked to the same period, the combined effect of these different librations allows us over time to see some 59% of its surface.

[weixing]

Hi,
Basically, we see only one side of the moon is due to tidal forces of the Earth. You know that the Moon's tidal forces causes the high and low tides on the Earth. In the same way, the Earth exerts tidal forces on the Moon which are more powerful as the Earth is more massive than the Moon. It turns out that these forces exert torques on the rotating system and tends to slow its rotation till it finally shows the same face towards the other body. Hence, it is the effect of tidal forces of the Earth on the Moon that have caused the Moon to show only one face to the Earth.

Similarly, the tidal forces of the Sun on Mercury have slowed down its rotational period to once in 59 days. The Moon's tidal forces will have the same effect on Earth, so that some day in future (billions of years hence), the Earth may show the same face to the Moon.

Have a nice day.

PS: Above taken from "Ask an Astronomer website"

[shoelevy]

although it's true that we only see 1 face of the moon, we often see more craters on the moon during crescent than full moon. seems that there are alot more craters at the edges of the moon than at the center.

i thought that's because shadow of the craters at the terminator emphasise the surface feature?

king kong, u're right, directoin of approach of sunlight during crescent makes the craters very obvious

[shoelevy]

this thing of moon moving round earth in the same time it rotates on its axis is called captured rotation for your info.
the mechanics is described spot on by weixing

but one must note that this phenomenon is as such only because the moon's mass is not evenly distributed

[Airconvent]

actually, the moon does not really "orbit" the earth perse. The earth and moon are actually a planetary binary pair. Imagine 2 dancers holding hands and swinging in a circular motion with their arms stretched. Because the earth is much more massive than the moon, the axis of rotation is not midway between the earth and moon but lies somewhere just below the surface of the earth. essentially this means that the earth and moon wobble around each other. now back to the dancers, if you dance this way, you will always see the front side of the dance partner and not the back. This is how it is with our moon.

Some prefer to say that the moon's rotation is at the same speed as its orbit.
in anycase, they send probes to the far side of the moon (as the "blind" side is known as) and it is no different from our side. one would expect more craters since it faces "outwards" but oddly this is not so.

rich

[FIRNALDO]

Is it the same as other planets with moons?

[carlogambino]

this thing of moon moving round earth in the same time it rotates on its axis is called captured rotation for your info.

I believe its called synchronous rotation? (e.g The moon is in synchronous rotation about the earth)This also occurs in the pluto-charon pair.

[shoelevy]

its the same................
just like singapore and xin jia puo are the same

this thing of moon moving round earth in the same time it rotates on its axis is called captured rotation for your info.

I believe its called synchronous rotation? (e.g The moon is in synchronous rotation about the earth)This also occurs in the pluto-charon pair.