Why are large bodies spherical?

2 Answers

Moon formed from the collision n of a mars like body. on Earth.

Explanation:

Material from the top layer of Earth and from the colliding body were thrown out.. The debris joined to form Moon in spherical shape..Gravity pulls equally from center to surface every where.So all large bodies Moons,,Planets, stars all are spherical A sphere is the strongest and most efficient shape for given volume..enter image source here
Space place.NASA.GOV. picture.

Oct 8, 2017

Gravity forces large enough bodies to be spherical.

Explanation:

Small bodies, such as most asteroids and comets, have very weak gravity. This means that gravity isn't strong enough to overcome the rigidity of the materials the body is made from. As a result these bodies can be any shape.

Massive enough bodies have gravity which is strong enough to overcome the rigidity of the materials they are made from. This forces the body to become spherical. This is called hydrostatic equilibrium.

In fact most bodies are not exactly spherical. Most bodies are rotating fast enough to create an equatorial bulge and flattening at the poles. Also, bodies can have topological features such as mountains and valleys.

The International Astronomical Union (IAU) defined three rules which define a planet. The first rule is that it must be in hydrostatic equilibrium and be roughly spherical. The second is that it must orbit the sun and the third is it must have cleared its orbit of other bodies.

So, although the Moon was created from material ejected when a Mars sized planet collided with the Earth, it was massive enough for gravity to make it spherical in shape.