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Is the moon spherical?

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 km across are spherical. They are not exact spheres, as the radius varies gradually. The typical shape (including those of Earth, Moon, and Pluto) is a spheroid: An egg-shaped sphere.

The simplest evidence for a spherical Moon: 1. Solar eclipses are always circular. The only geometric object that can yield a circular eclipse in any orientation is a spheroid. 2. 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 asteroids and even Mars's two moons (Phobos and Deimos) are shaped like large boulders.

Modern evidence for a spherical Moon: 1. 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.

Even though Pluto is only about to be imaged at close-range for the first time in human history (the New Horizons mission), it has been imaged from afar and appears to be a spheroid. It is certainly massive enough (~0.2% of Earth’s mass; the Moon is ~1% of Earth’s mass) to be a spheroid due to gravitation.

December 2007, Suniti Karunatillake (more by Suniti Karunatillake) (Like this Answer)

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