The science that explains why moons and planets tend to be round is primarily related to their formation and the forces acting upon them. The process is governed by the laws of gravity and the physical properties of the celestial bodies.
When a planet or moon forms, it starts as a collection of gas, dust, and other particles in a rotating cloud known as a protoplanetary disk. As these particles come together due to gravitational attraction, they begin to form larger and larger objects called planetesimals. These planetesimals continue to collide and merge, eventually forming planets or moons.
Gravity plays a crucial role in this process. As the planetesimals collide, the force of gravity causes them to compress and collapse under their own weight. This compression leads to a state of hydrostatic equilibrium, where the gravitational force pulling inward is balanced by the outward pressure caused by the material resisting collapse.
Hydrostatic equilibrium tends to favor a spherical shape. This is because a sphere is the most efficient shape for evenly distributing mass, allowing the force of gravity to act uniformly from all directions. If a celestial body were not spherical, with irregularities or bulges, gravity would cause the material to flow and reshape itself over time until it achieved a more balanced, spherical form.
Additionally, the rotational motion of a planet or moon can further influence its shape. The rotation causes a centrifugal force that opposes the gravitational force at the equator, leading to a slight bulging or flattening of the shape around the middle, resulting in an oblate spheroid. However, the effect of rotation is relatively minor compared to the overall gravitational forces shaping the celestial body.
It's important to note that not all moons and planets are perfectly spherical. Factors such as rotational speed, composition, and tidal forces from nearby objects can introduce deviations from a perfect sphere. Some moons, for example, may have irregular shapes or exhibit significant tidal deformation due to the gravitational interactions with their parent planet.
In conclusion, the round shape of moons and planets is a consequence of their formation process, where gravitational forces and hydrostatic equilibrium lead to a tendency for celestial bodies to adopt a spherical shape.