The Earth Is Round, and Is Also a Shifting, Squashed Spheroid

Key points to remember about the roundness of the Earth

• The Earth is round, and we know this from detailed measurements and geometric inferences based on laser-equipped satellites.
• The Earth is often described as a circle or an oval, but it is neither. As the planet spins at more than 1,000 miles per hour at the equator, it develops a slight bulge around its middle. The result is what geodesists call an oblate spheroid: a sphere gently flattened at the poles like a Frisbee.
• Just like its exterior, the Earth’s interior is also not uniform. Rising plumes from the hot mantle shift mass and density within the planet, subtly changing the gravitational pull between places.

What is the shape of the Earth? This seemingly simple question has intrigued researchers for centuries, with theories ranging from flat to cylindrical. Philosophers like Aristotle were among the first to give weight to our modern conception of the sphere.

Observations of constellations, horizons, and eclipses have all led to this complete conclusion, but these findings do not mean that our planet is perfectly smooth, or even perfectly spherical. It turns out that Earth might throw a few more upsets at us than Greek scholars predicted.

What proves that the Earth is round?

Some of the most powerful evidence of our planet’s globosity comes from satellites armed with lasers. Through detailed measurements and clever geometric deductions, they can record the size and curvature of the Earth with millimeter precision, definitively demonstrating that it is round, according to Solid Earth JGR.

Other phenomena are perceptible from the ground. “I like the ship-horizon effect,” says Dr. Frank Flechtner, geophysicist at the GFZ Helmholtz Geoscience Center. As a ship sails away, it slowly disappears behind the curving ocean.

The sky provides further proof. “During a lunar eclipse, you can see the curved shadow of the Earth,” says Dr Attreyee Ghosh, a geophysicist at the Indian Institute of Science. “The Sun and the Moon are spheres. Why should Earth be an exception?”

Dr. John Vidale, professor of earth sciences at the University of Southern California, points out something more mundane.

“The times of day vary simultaneously in different cities around the world,” he says. “Such an observation makes perfect sense for a round Earth and is much more difficult to explain without it.”

Learn more: When will the Earth’s magnetic poles shift? Probably not anytime soon – here’s how we know

Is the Earth more round or oval?

With the roundness of the Earth now established, a more complex question follows: how round?

In common parlance, the Earth is often described as a circle or an oval, but it is neither. As the planet spins at more than 1,000 miles per hour at the equator, it develops a slight bulge around its middle. The result is what geodesists call an oblate spheroid: a sphere gently flattened at the poles like a Frisbee.

“If it didn’t rotate, it would almost be a perfect sphere,” says Dr Jon Kirby, associate professor of geodesy at Curtin University. “It’s centrifugal force that causes the bulge at the equator.”

Kirby explains using the analogy of a roundabout in a children’s playground. “If you sit in the center, you’ll spin, but you won’t get thrown out,” he says. “As you move further away, you feel a force pushing you back, and this force grows so that when you are right on the edge, you hold on for dear life.”

This same phenomenon occurs on our planet. Masses of rock and water near the equator are farther from the axis of rotation and experience greater outward acceleration, causing an Earth-sized bulge about 21 kilometers from the center of the planet.

Compared to the entire planet, this bulge is tiny, equivalent to placing a single human hair on a tennis ball. Still, that’s enough to place Ecuador’s volcanoes closer to space than the summit of Mount Everest, although the latter rises higher above sea level, according to the National Oceanic and Atmospheric Administration (NOAA).

Because the Earth’s rotation applies a force that throws objects outward, the apparent weight of anything along the equator is slightly decreased. The force of gravity also decreases with distance, so being further from the center of the Earth weakens the attraction.

Learn more: A strange zigzag solar phenomenon has just appeared near Earth

What shapes the gravitational field?

Just like its exterior, the Earth’s interior is also not uniform. Rising plumes from the hot mantle shift mass and density within the planet, subtly changing the gravitational pull between places.

Geodesists describe this gravity-defined shape as the geoid, the surface Earth’s oceans would form if they were perfectly calm, according to NOAA.

“These variations are not something you could see with the naked eye,” says Ghosh. “They reflect how mass is distributed deep within the Earth, not the shape of the surface.”

One of the most striking examples is the geoid depression in the Indian Ocean, where sea levels are a hundred meters lower than expected. In a 2023 study published in Geophysical research lettersGhosh and his colleague attributed the anomaly to low-density material in the upper mantle. Strangely, the ocean does not pool as high above the Geoid Low, because the additional water would “spill” into the surrounding high-gravity regions that hold it more tightly.

The shape of the Earth’s geoid, however, is not fixed. Since 2002, scientists have tracked changes using the GRACE and GRACE Follow-on (GRACE-FO) satellite missions, which measure tiny variations in Earth’s gravity field, according to NASA.

“We can measure the amount of mass moving in the Earth system every month,” explains Flechtner, who is also project manager for GRACE-FO.

As ice caps melt, water moves from the poles to the oceans. Like a figure skater stretching his arms, the Earth’s rotation slows down because of this mass shift. In a 2024 study published in Proceedings of the National Academy of Sciencesresearchers found that accumulation of water at the equator was on track to slow Earth’s day by one to two milliseconds this century.

This affects the shape of our planet. “Slowing down decreases the centrifugal forces, which are the main reason for the flattening,” explains Dr. Benedikt Soja, professor of space geodesy at ETH Zurich and author of the paper. “So the Earth also becomes less flattened.”

The horizon may look like a static line, but beneath that calm curve lies a restless planet. Our world is constantly being sculpted by rotation, gravity, deep movements and now by climate change which is rewriting its dimensions quickly enough to be measured from space.

Learn more: Earth particles travel to the Moon by hitchhiking along Earth’s magnetic field lines

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