Earth’s Shape, Land, and Water Distribution

Earth is a nearly spherical planet, slightly flattened at the poles, with a surface area of about 510 million km² of which about 71 percent is covered by water and about 29 percent is land. In brief: Earth’s mean radius is about 6,371 km; it is an oblate spheroid, with an equatorial radius about 21 km longer than its polar radius. The land is unevenly distributed: most of it lies in the Northern Hemisphere, and the oceans cover most of the Southern Hemisphere. This distribution of land and water strongly influences climate, ocean currents and human settlement.

Understanding the shape of Earth and where land and water are found is the starting point for all world geography. This guide describes Earth’s shape and size, the distribution of land and water, compares the key measurements in a table, works through calculations of flattening, circumference, rotation speed and area, outlines how Earth’s shape was discovered and corrects common mistakes.

What is planet Earth?

Earth is the third planet from the Sun and the only one known to support life. It formed about 4.5 billion years ago. It rotates on its axis once in about 24 hours, which gives day and night, and orbits the Sun once in about 365.25 days, which gives the year. Its axis is tilted about 23.4° from the perpendicular to its orbital plane, which causes the seasons. Earth has layers (crust, mantle, core), an atmosphere, a magnetic field, liquid water at the surface and one natural satellite, the Moon.

What is the shape of Earth?

Earth is often described as a sphere, but because it spins, it bulges slightly at the equator and is flattened at the poles. Its shape is an oblate spheroid. The equatorial radius is about 6,378 km and the polar radius about 6,357 km, a difference of about 21 km (the flattening is about 1 in 298). The true surface is also irregular: the highest point on land, Mount Everest, is about 8,849 m above sea level, and the deepest known point of the ocean, the Challenger Deep in the Mariana Trench, is roughly 10,900 m below. These differences are tiny compared with the radius, so a globe that is a perfect sphere is a good model. The geoid is the shape that the mean sea level would take under gravity alone, and it differs from the spheroid by up to about 100 m.

How are land and water distributed?

About 71 percent of the surface is covered by water (mostly in the oceans, which hold about 97 percent of all water) and about 29 percent by land. This is why Earth is called the “blue planet”. The distribution is uneven: land makes up roughly 39 percent of the Northern Hemisphere but only about 19 percent of the Southern Hemisphere, so the Northern Hemisphere is called the land hemisphere and the Southern the water hemisphere. Even in the hemisphere chosen to contain the most land (centered near France), land covers only about half of the surface, and in the opposite hemisphere (centered near New Zealand) roughly nine-tenths is water. Most land is in the continents of Asia, Africa, North and South America, Antarctica, Europe and Australia/Oceania, and the oceans are connected to form one world ocean. The distribution affects climate: the oceans moderate temperatures, while large land masses have more extreme seasons (continentality).

Key measurements of Earth

Worked example: size, flattening and speed

1. Difference of radii: 6,378.137 − 6,356.752 = 21.385 km.
2. Flattening: 21.385 ÷ 6,378.137 = 0.003353, which is 1 part in about 298. On a globe 30 cm in diameter, the polar flattening would be only 0.003353 × 15 cm = 0.5 mm.
3. Equatorial circumference: 2π × 6,378.137 = 40,075 km.
4. Speed of rotation at the equator: 40,075 km in 24 hours is 40,075 ÷ 24 = about 1,670 km/h. At 60° N the radius of the parallel is multiplied by cos 60° = 0.5, so the speed is about 835 km/h, and at the poles it is zero.
5. Surface area: 4π × 6,371² = 510.1 million km². Water at 71 percent: 0.71 × 510.1 = 362 million km². Land at 29 percent: 0.29 × 510.1 = 148 million km² (commonly quoted as about 149 million).
6. Relief compared with size: the vertical range from the Challenger Deep to Everest is about 8.85 + 10.9 = 19.8 km, which is 19.8 ÷ 6,371 = 0.3 percent of Earth’s radius, so the surface is smooth on the scale of the whole planet.

How did we learn Earth’s shape?

Greek thinkers argued for a spherical Earth from the 6th to 4th centuries BCE; Aristotle cited the round shadow of Earth on the Moon during lunar eclipses and the changing positions of stars with latitude. Eratosthenes estimated Earth’s circumference in the 3rd century BCE from the shadow angles at two places in Egypt, and the result was within a few percent or tens of percent of the modern value, depending on how his unit (the stadion) is interpreted. In the 17th and 18th centuries, Isaac Newton predicted that a rotating Earth would be flattened, and expeditions to Lapland and Peru in the 1730s measuring the length of a degree of latitude confirmed it. Today satellites measure Earth’s shape and gravity field with great accuracy.

Which mistakes are common?

Mistake 1: Thinking that people once believed that Earth was flat

Educated people knew Earth was a sphere from ancient Greece. The belief in a flat Earth was never the general view of medieval scholars.

Mistake 2: Calling Earth a perfect sphere

It is an oblate spheroid, with a bulge at the equator of about 21 km in radius.

Mistake 3: Believing that the Southern Hemisphere has no land

It has about 19 percent land, including most of South America, Australia and Antarctica, and part of Africa and Asia.

Mistake 4: Confusing the land percentage with the oceans’ share of the water

Oceans cover about 71 percent of the surface and hold about 97 percent of Earth’s water; the rest is mostly ice and groundwater.