Are other bodies in the solar system (such as the Moon and Pluto) spheres, or are they flat disks?
All objects in the solar system are three-dimensional, just like things on the surface of Earth. Furthermore, most bodies larger than hundreds of kilometers across are spherical. They are not perfect spheres, as the radius varies gradually. The typical shape (including those of Earth, Moon, and Pluto) is a oblate spheroid: a squashed sphere.
The simplest evidence for a spherical Moon:
- During solar eclipses, the Sun's shadow is always nearly circular. The only geometric object that can yield a near-circular eclipse in any orientation is a spheroid.
- The terminator of the Moon (the boundary between the day-side and night-side) as viewed from Earth is always arc-shaped. Only spheroids can show such an edge in any orientation.
Theoretical reasons for a spherical Moon: The lowest gravitational potential energy of a system of particles is achieved when they form a sphere as opposed to a disk. However, it is possible for smaller collections of particles to withstand the force of gravity with counteracting forces (mostly electromagnetic forces that give rise to chemical bonds) and aggregate into non-spherical shapes. This is why many smaller objects like asteroids and even Mars's two moons (Phobos and Deimos) are shaped like lumpy boulders.
Modern evidence for a spherical Moon: Data (such as images from orbit, characteristics of the satellite orbits, Moon's gravity field, and images on the Moon's surface) from the lunar missions such as the Apollo, Clementine, Zond, Lunar Prospector, and upcoming data from Kaguya (completed missions are summarized online). The simplest proof from such data is that the Moon looks like a disk when viewed from any point in the orbit - only a spheroid, not a disk, can appear so.
Pluto (first imaged at close range in 2015 by NASA's New Horizons mission) is also a spheroid. Even before New Horizons, we knew that Pluto is certainly massive enough (~0.2% of Earth's mass; the Moon is ~1% of Earth's mass) to be a spheroid due to self-gravitation.
This page was last updated on February 10, 2016.