Thread: The shape of the Moon.

Why is it our moon is so perfectly round? I have thought of this recently and feel maybe someone would be able to present me with an answer.

The moon has no rotation. Why is that? With no rotation, what forces would cause it to be so nice and round? Why doesn't it rotate? Why no spin of the moon? If the moon was an asteroid that bounced off of the earth wouldn't there be some kind of spin. If the Moon is some piece of this planet knocked off some millenia ago wouldn't there be even the slightest of rotation? It baffles me that our moon is so spherical with no real force but its own gravity. If it was something that is a product of our earth being where it is, e.g. asteroid bounce or chunk of, wouldn't it have retained it's raw form for the most part? Especially with no spin! I can't continue further without more or less repeating myself. Any insight?


I should probably research more but here is where I'm starting.

A sphere is the best shape for gravity where the force is equal in all directions. If a body rotates, then there is a slight bulge at the equator. From memory, the Earth is 7,900 miles diameter at the poles and 7,926 miles at the equator.

The Moon does and doesn't rotate. Let me explain. It is gravitationally locked with the Earth because it is so close, so always presents the same face to us. In travelling around the Earth, different parts of it are lit up by the Sun, so what could be said to be rotation. However if you hold a ball at both arms length and slowly turn 360 degrees, the ball does not spin and always has the same side facing you.

The Moon exerts a force on the Earth, slowing it's rotation slightly. The most obvious demonstration of this force is the tides in the seas.

Welcome to the forum!

The spherical shape of the moon is the result of its formation. The currently widely accepted model includes a hot formation process of the Moon, i.e. it was made of liquid matter - similar to magma on Earth. Every liquid body that is dominated by its own self-gravity (as the Moon was at that time) reaches an equilibrium by establishing a spherical form. The same is true for the Sun or any other star (flattening by rotation neglected).

As to the apparent non-rotation of the Moon: This is a misconception. The Moon is rotating - but with the same rotational period as it takes to revolve around the Earth. As an illustration, you can transform the Earth-Moon system to a moon-centred view, where the Earth revolves around the Moon (in fact, they both revolve around their mutual barycentre). You immediately see that the Moon does rotate.

A different way to illustrate this: The Moon may be facing always the same side to the Earth, but this side is always pointing to different parts of the sky.

The reason for this strange behaviour can be explained with something that is called gravitational breaking or tidal locking. This means: The moon has not always been rotating as it does now. The speed has decreased slowly. Earth and Moon have been exchanging angular momentum until the Moon's rotation and revolution converged.

Originally Posted by Dishmaster

The currently widely accepted model includes a hot formation process of the Moon, i.e. it was made of liquid matter - similar to magma on Earth. Every liquid body that is dominated by its own self-gravity (as the Moon was at that time) reaches an equilibrium by establishing a spherical form.

That explains it well. I'd thought it was just the statistical result of innumerable little bits falling together... but that would tend to form a disk wouldn't it? Is a disk shaped moon plausible?

Originally Posted by Pong

Originally Posted by Dishmaster

The currently widely accepted model includes a hot formation process of the Moon, i.e. it was made of liquid matter - similar to magma on Earth. Every liquid body that is dominated by its own self-gravity (as the Moon was at that time) reaches an equilibrium by establishing a spherical form.

That explains it well. I'd thought it was just the statistical result of innumerable little bits falling together... but that would tend to form a disk wouldn't it? Is a disk shaped moon plausible?

I'd guess that disc shapes (like our solar system and the Milky Way) only happen where larger distances between particles are the norm, because of the inverse square nature of gravity. So the angular momentum lets the matter stretch out perpendicular to the axis of rotation, but is not enough to achieve escape velocity and normally can't be.

The hot formation Dishmaster is talking about is either that one glob of hot material split in two to form our earth-moon system or (the currently accepted model) that a Mars sized object collided with earth and the molten ejecta formed the moon.

Originally Posted by Dishmaster

The spherical shape of the moon is the result of its formation. The currently widely accepted model includes a hot formation process of the Moon, i.e. it was made of liquid matter - similar to magma on Earth.

This is incorrect. (The bolded part.)

The material left in orbit after the collision had time to solidify before accreting to form the moon. During this accretion process the conversion of kinetic to thermal energy generated a magma ocean, but by no means all of the moon was molten. Debate surrounds how deep this magma ocean was, but it likely involved no more than the outer third of the moon.

I think, also, that the presence of masscons cause the moon to deviate significantly from a true sphere.

Originally Posted by Dishmaster

As to the apparent non-rotation of the Moon: This is a misconception. The Moon is rotating - but with the same rotational period as it takes to revolve around the Earth. As an illustration, you can transform the Earth-Moon system to a moon-centred view, where the Earth revolves around the Moon (in fact, they both revolve around their mutual barycentre). You immediately see that the Moon does rotate.

Use the example I gave. A ball held in 2 hands at arms length, and then turn 360 degrees slowly. Has that ball spun in your hands or not? Of course the back part of the ball/Moon faces different places as it goes around the person/Earth but the other side always faces the Earth. Well, not quite. There is slight movement, a wobble, which allows us to see a little of each side of the face of the Moon facing us so not perfectly locked. This effect would not be possible if the Moon was merely rotating at the same speed as the Earth. The idea of actual rotation comes from different parts being lit by the Sun as it is in different positions when going around the Earth.

Originally Posted by John Galt

The material left in orbit after the collision had time to solidify before accreting to form the moon.

Is that assuming a rather sparse & cold environment like the solar system is now? I'm thinking if the collision happened while our early system was a disk of hot junk, the orbiting moon components would not cool so rapidly. As well the Earth itself with molten surface and no atmosphere would be pouring up significant heat.

Common sense tells me space is cold so if I take a body like the moon and shatter it into billions of pieces, it should cool to space more rapidly. Greater surface area means greater heat loss right? Yet space is a vacuum so I've actually achieved the quality of fluff insulation.

Originally Posted by John Galt

Originally Posted by Dishmaster

The spherical shape of the moon is the result of its formation. The currently widely accepted model includes a hot formation process of the Moon, i.e. it was made of liquid matter - similar to magma on Earth.

This is incorrect. (The bolded part.)

The material left in orbit after the collision had time to solidify before accreting to form the moon. During this accretion process the conversion of kinetic to thermal energy generated a magma ocean, but by no means all of the moon was molten. Debate surrounds how deep this magma ocean was, but it likely involved no more than the outer third of the moon.

Thanks for pointing this out. The basic explanation does not change, though.

Originally Posted by Cyberia

Originally Posted by Dishmaster

As to the apparent non-rotation of the Moon: This is a misconception. The Moon is rotating - but with the same rotational period as it takes to revolve around the Earth. As an illustration, you can transform the Earth-Moon system to a moon-centred view, where the Earth revolves around the Moon (in fact, they both revolve around their mutual barycentre). You immediately see that the Moon does rotate.

Use the example I gave. A ball held in 2 hands at arms length, and then turn 360 degrees slowly. Has that ball spun in your hands or not? Of course the back part of the ball/Moon faces different places as it goes around the person/Earth but the other side always faces the Earth. Well, not quite. There is slight movement, a wobble, which allows us to see a little of each side of the face of the Moon facing us so not perfectly locked. This effect would not be possible if the Moon was merely rotating at the same speed as the Earth. The idea of actual rotation comes from different parts being lit by the Sun as it is in different positions when going around the Earth.

Well, you are rotating the ball when you are spinning around your own axis. If you reduced the distance between you and the ball to zero, both you and the ball would rotate with the same angular velocity. The situation would be differrent, if you fixed the ball to the ground, e.g. by glueing it to a rod. In your example, you are assuming that you are a reference frame. The reason for the libration of the moon (is this what you are referring to?) is the small eccentricity of the lunar orbit around the Earth. While the Earth and the Moon do not change their angular velocity, the orbital speed changes periodically, being highest at the perigee. This leads to a small periodcal phase shift between the revolution and the rotation of the Moon. But on average, the Moons revolution as well its rotation and the Earth's rotation are tidally locked.

It's shaped like an egg - fat end facing earth (that end is tidally locked)

Other known tidally locked objects in our Solar System:

Locked to the Sun
Mercury (in a 3:2 rotationrbit resonance)

Locked to the Earth:
The Moon

Locked to Mars:
Phobos
Deimos

Locked to Jupiter:
Metis
Adrastea
Amalthea
Thebe
Io
Europa
Ganymede
Callisto

Locked to Saturn:
Pan
Atlas
Prometheus
Pandora
Epimetheus
Janu
Mimas
Enceladus
Telesto
Tethys
Calypso
Dione
Rhea
Titan
Iapetus

Locked to Uranus:
Miranda
Ariel
Umbriel
Titania
Oberon

Locked to Neptune:
Proteus
Triton

Locked to Pluto
Charon (Pluto is itself locked to Charon)
These two are locked to each other they both always see one view of the other.