James Webb, Florida International University
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Why are the planets round? – Daniel B., La Crosse, Wisconsin
The ancient Greeks proved the Earth to be round over 2,000 years ago and understood its size using simple observations of the Sun.
But how do people know this today? When you drop something, gravity causes it to fall straight toward the center of the Earth, at least until it hits the ground. Gravity is a force that is caused by almost anything that has mass. Mass is a measure of how much matter there is in something. It could be in the form of rocks, water, metal, people – anything. Everything that is material has mass, and therefore everything causes gravity. Gravity always pulls towards the center of mass.
The Earth and all the planets are round because when the planets formed they were made up of molten matter – basically a very hot liquid. As gravity always points to the center of a mass, it squeezed the matter of which the Earth is made equally in all directions and formed a ball. When the Earth cooled and became solid, it was a round ball. If the Earth hadn’t turned, it would have been a perfectly round planet. Scientists call something that is perfectly round in all directions a “sphere”.
The gas cloud that the Earth was made of was spinning slowly in one direction around an axis. The top and bottom of this axis are the north and south poles of the Earth.
Now hold out your right hand. Point your thumb on your right hand upward and curl your fingers in the direction of rotation. Your thumb points to the North Pole. The equator is defined as the plane, halfway between the North and South Poles.
If you’ve ever played on a merry-go-round, you know that the spinning carousel tends to put you off. The faster it spins, the harder it is to stay in place. This tendency to be thrown is called centrifugal force and pushes the mass on the equator outward. This causes the planet to bulge at the equator.
The faster the rotation, the more irregular it becomes. Then when it cools and hardens, it retains that shape. If a molten planet started spinning faster, it would be less round and have a bigger bulge.
The planet Saturn is very flattened – not spherical – because it spins very quickly. Due to gravity, all planets are round, and because they rotate at different speeds, some have equators larger than their poles. Thus, the shape of the planet, the speed and direction of its rotation depend on the initial state of the material from which it is formed.
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James Webb, professor and director, Stocker AstroScience Center for Physics; AstroScience Store Center, Florida International University
This article is republished from The Conversation under a Creative Commons license. Read the original article.