Earth's Shape

It has been known for centuries that Earth is not a perfect sphere. The diameter of the planet is greater at the equator than it is from pole to pole. This oblateness is the result of Earth’s daily rotation on its axis. Even smaller irregularities in shape have been measured by Earth-orbiting satellites.

Overview

The discovery that Earth is not a perfect sphere dates to the seventeenth century, when measurements of the distance corresponding to one degree of were found to increase systematically from the equator toward both poles. Because of Earth’s oblateness, one degree of latitude has a length of 110.6 kilometers at the equator and 111.7 kilometers at the poles.

Besides rotation, other forces imposed upon Earth affect the shape of the planet, a prime example being Earth tides. The oceans of the world generally have two tidal bulges, or regions where the ocean surface is relatively high, caused primarily by the gravitational attraction of the Moon and, to a lesser extent, by the Sun. When the Sun, Moon, and Earth all are on a straight line (the Sun and Moon either on the same side or opposite sides of the Earth), the oceans display the highest high tides and lowest low tides (spring tides), as the tidal effects of the Sun and Moon reinforce each other. When the line from the Earth to the Moon makes a right angle to the line from the Earth to the Sun, the tidal effects of the Sun and Moon partially cancel out, and the high tides are not very high, and the low tides are not very low (neap tides). Not only does the water of the oceans rise and fall because of tides, but so too does the surface of the “solid” Earth rise and fall very slightly due to the same tidal forces imposed by the Moon and the Sun. This periodically varying distortion is so slight as to render accurate measurements of it quite difficult.

A view of Earth from distance in space would, to the naked eye, suggest that the planet is a perfect sphere, yet measurements reveal that it is not. All planets (including Earth), along with the larger satellites and asteroids, are essentially spheroidal, while smaller objects are not. Sufficiently small solid objects (such as books, boulders, and bones) can maintain any arbitrary shape because of the strength of the material of which they are composed. However, sufficiently large objects (even if solid) have internal forces due to their self-gravity that are strong enough to overwhelm the strength of whatever material composes them, and they pull themselves into spherical shapes. The critical “threshold” size depends on the density and strength of the solid material, but for the three types of solids (ices, minerals, and metals, mainly iron) most common in the solar system, the threshold is approximately the same: on the order of a few hundred kilometers.

Rotation (spinning on an axis) makes a large object depart from being spherical. The Earth’s rotational period, measured in a quasi-inertial frame based on distant stars and galaxies, is 23 hours, 56 minutes, and 4 seconds. As a result of this rotation, a point on the Earth’s equator moves with a speed of 1,674 kilometers per hour, a point at 30 degrees north or south latitude moves 1,450 kilometers per hour, and a point at 60 degrees north or south latitude moves 837 kilometers per hour. The increase in rotational speed toward the equator makes the equator bulge outward, transforming a spherical shape into an oblate spheroid. The Earth’s equatorial diameter is 12,756 kilometers, while its polar diameter is 12,714 kilometers. Its oblateness is defined as the difference in diameters divided by the equatorial diameter, which gives a value of 0.003353 or about one part in 298.257.

A comparison of the planets in the s shows that rotation plays a dominant role in determining oblateness. Mercury and Venus, each with very slow rotation, have no discernible oblateness and are essentially spherical. Jupiter and Saturn the fastest, and both are noticeably oblate, as seen in telescopes and images. However, there are other factors. For example, Saturn rotates slightly more slowly than Jupiter, and Mars rotates slightly more slowly than Earth; yet Saturn is more oblate than Jupiter (about one part in 10 compared to one part in 15), and Mars is more oblate than Earth (about one part in 200 compared to one part in 300). These “discrepancies” probably are due to the distribution of mass throughout the planet and the rigidity of the material composing the different parts of the planet.

Methods of Study

Measurements of small departures from an shape became possible with the advent of the and the development of twentieth-century instrumentation. Perturbations in the orbits of Earth-orbiting satellites show that the Earth’s mass is not distributed as it would be if it were a simple oblate spheroid. Some of the earliest data indicated that Earth is slightly pear-shaped, with small bulges of up to about 100 meters near the North Pole and in a band south of the equator. More recently, satellite-mounted radar altimeters have been able to map continuously the topography of the ocean surface to an accuracy of a few centimeters by bouncing radar signals off the water. With wave crests and troughs averaged out, ocean surfaces show small but significant deviations from a smooth oblate spheroid that reflect the topography of the seafloor underneath. Major seamounts and suboceanic ridges are clearly marked by regions of higher ocean surface above. Likewise, the major deep-sea trenches, as are common around the rim of the Pacific Ocean, are marked by troughs in the ocean surface above. This phenomenon is due to small variations in the of gravity. In the case of a seamount or ridge, there is a concentration of mass (the rock composing the feature), so gravity there is a bit stronger and attracts more water over it. In contrast, there is a deficit of mass in a trench, so gravity there is slightly weaker and attracts less water over it.

Context

The passing of geologic time has brought changes in the phenomena that control Earth’s shape. The distance from the Earth to the Moon was less than at present, and Earth rotated faster in the past. These two differences would have produced larger and stronger tides and a larger equatorial bulge. It is interesting to speculate as to what effects those changes might have had on ancient dynamic processes. Today, Earth’s inhabitants suffer little effect from the planet’s distortion. It cannot be observed with the naked eye, and it does not appear to play a role in weather patterns and climate.

However, one practical, though small, consequence of the Earth’s shape is the variation of the effective gravitational acceleration with latitude. Due to Earth’s equatorial bulge, which is the result of Earth’s daily rotation on its axis, the effective gravitational acceleration, and hence the weight of objects, is slightly less at the equator than at either pole. Because of the equatorial bulge, objects at sea level at the equator are about 21 kilometers farther from the center of Earth than if they were at sea level at either pole. This alone reduces the gravitational acceleration at the equator by about 0.15 percent compared to the value at either pole. The effective gravitational acceleration at the equator is further reduced directly by Earth’s rotation since some of the gravitational acceleration that would otherwise exist is used to provide the centripetal acceleration needed to make objects follow the curved paths that keep them in contact with Earth’s surface as Earth rotates. This reduces the effective gravity at the equator by about 0.35 percent. Combining the two effects, the gravitational acceleration varies from 9.832 meters per second squared at either pole to 9.780 meters per second squared at the equator, or about 0.5 percent. As a result, weight at the equator is reduced by about 0.5 percent compared to weight at either pole, so an object weighing 200 pounds at either pole weighs about 1 pound less at the equator.

Global climate change has already had significant effects on Earth, and studies indicate it may have already affected the shape of the planet and its rotation. Scientists have observed an increased speed in glacial movement as the ice has melted more quickly and allowed for glaciers to move. This melting has caused a redistribution of mass from Earth's poles to the equator, making the Earth more oblate, slowing its rotation, and lengthening the day.

Bibliography

Arora, Kusumita, et al., editors. The Encyclopedia of Solid Earth Geophysics. Springer, 2011.

Bhatia, Guneet. "Global Warming Speeds up Movement of Antarctic Glaciers to Alter Shape of the Earth." International Business Times. 6 Oct. 2015, www.ibtimes.com.au/global-warming-speeds-movement-antarctic-glaciers-alter-shape-earth-1471942. Accessed 9 Feb. 2025.

Carrington, Damian. “Climate Crisis Is Making Days Longer, Study Finds.” The Guardian, 15 July 2024, www.theguardian.com/environment/article/2024/jul/15/climate-crisis-making-days-longer-study. Accessed 9 Feb. 2025.

Greenberg, John L. The Problem of the Earth’s Shape from Newton to Clairaut. New York: Cambridge UP, 1995.

Ince, Martin. The Rough Guide to the Earth 1. New York: Rough Guides, 2007.

"Shape of the Earth: The Oblate Spheroid." Earth How, earthhow.com/shape-of-the-earth. Accessed 9 Feb. 2025.

Stacey, Frank D., and Paul M. Davis. Physics of Earth. 4d ed., Cambridge UP, 2008.

Williams, David R. “Earth Fact Sheet.” NASA, 15 Nov. 2025, nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html. Accessed 9 Feb. 2025.