Observe the planet Jupiter with a telescope, and you'll see that it appears flattened. That's not an optical illusion because the planet really is squashed so that it's not perfectly spherical. If you could measure Jupiter, you'd see that its poles are flattened and the part around the equator bulges. Astronomers and geologists call this an equatorial bulge -- a phenomenon that doesn't just exist on Jupiter.
Planetary Bulge Formation
When a planet rotates, locations around its poles move in small circles. Points near the equator must move faster because they have more area to cover during a rotation. This rotation, and resultant centrifugal forces, gives planets bulges around their middles that vary in size depending on a planet's gravity, composition, rotational speed and other factors. The Earth has a small bulge; its circumference from pole to pole is about 40,000 kilometers (24,855 miles), while the circumference around the equator is 40,074 kilometers (24,901 miles). Although scientists believe that Jupiter's core may be solid, that planet consists of mostly gas. Its rapid rotational speed of nine hours and 50 minutes per revolution gives Jupiter a prominent bulge around the equator.
Earth's Equatorial Bulge
Because Earth is also wider at the equator, satellites must adjust their orbits as they circle the planet. As NASA notes, "The equatorial bulge of the Earth and other irregularities cause disturbances of satellite orbits over long periods of time." These disturbances can also change a satellite's orientation as it orbits the planet. In addition, the moon's gravity helps create tidal bulges on Earth. When the moon passes overhead, its gravity pulls ocean water beneath it upward to create a tidal bulge, which increases wave height. Inertia and gravity on the opposite side of the planet create another bulge.
Bulge Sizes Vary
You don't see much of a bulge on the sun because its gravity is so strong. Mercury and Venus don't have significant bulges because they rotate slowly. Saturn, another large gaseous planet, rotates every 10 hours and 39 minutes. Its high rotational speed gives Saturn an equatorial bulge and flattened poles too.
Bulges on Moons and Asteroids
Earth's moon also spins slowly, so you won't find a significant bulge on it. Bulges appear on Jupiter's moons because of the planet's intense gravity. That gravity distorts the face of Jupiter's moon by Io 10 kilometers. Scientists used radar to study asteroid 2005 WK4's size, rotation and other properties. Although the asteroid is between 200 to 300 meters (660 to 980 feet) in diameter, their measurements suggest that the asteroid has a bulge near the equator.
- Isaac Asimov's Guide to Earth and Space; Isaac Asimov
- NASA: The Rotating Earth
- National Oceanic and Atmospheric Administration: A Tour of the Solar System
- NASA: Specific Flight Possibilities
- National Oceanic and Atmospheric Administration: NOAA's National Ocean Service: Diagram of Tidal Bulges
- NASA: Saturn
- Our Worlds: The Magnetism and Thrill of Planetary Exploration; Alan Stern
- NASA: Solar System Exploration - Radar Images of Asteroid 2005 WK4
- Goodshoot/Goodshoot/Getty Images