Earth is the third planet from the Sun and the only place known to have life. It is 12,742 kilometers (7,918 miles) across on average. That figure is its mean diameter, the width of a perfect sphere with the same volume as Earth. Earth is the largest of the four rocky planets closest to the Sun, and the fifth-largest planet in the Solar System.

Size

Earth is not a perfect sphere. Its spin makes it bulge at the equator, so it is 12,756 kilometers (7,926 miles) wide at the equator but 12,714 kilometers (7,900 miles) from pole to pole. The difference, about 43 kilometers (27 miles), is a third of a percent of Earth's width.

Mountains and ocean trenches make the surface uneven as well. From the highest mountain peak to the bottom of the deepest ocean trench is about 20 kilometers (13 miles), less than 0.2% of Earth's width. Tides, drifting continents and earthquakes change the shape a little all the time.

The science of measuring Earth's size and shape is called geodesy. Earth's size is known very precisely: its average radius, the distance from the center to the surface, is known to within about 10 centimeters (4 inches).

Size comparisons

Venus, the second planet from the Sun, is 12,104 kilometers (7,521 miles) across, about 95% of Earth's width.
Neptune is 49,244 kilometers (30,599 miles) across, nearly four times as wide as Earth.
The Moon is 3,475 kilometers (2,159 miles) across, a little more than a quarter of Earth's width. About 30 Earths lined up side by side would span the average distance from Earth to the Moon, 384,400 kilometers (238,900 miles).
The Sun is about 1.39 million kilometers (865,000 miles) across, 109 times Earth's width.
If Earth were shrunk to the size of a basketball 24 centimeters (9.4 inches) across, Mount Everest, 8,849 meters (29,032 feet) tall, would rise about 0.17 millimeters from its surface. That is less than twice the thickness of a sheet of paper.

Inside Earth

Earth is built in layers. At the center is the inner core, a solid ball of iron and nickel about 1,221 kilometers (759 miles) in radius. Around it lies the outer core, a layer of liquid iron and nickel about 2,300 kilometers (1,400 miles) thick.

Next comes the mantle, the thickest layer, about 2,900 kilometers (1,800 miles) of hot rock with the consistency of caramel. The outermost layer, the crust, is thin by comparison. It averages about 30 kilometers (19 miles) thick under the continents and about 5 kilometers (3 miles) under the ocean floor.

Water and air

Earth is the only planet in the Solar System with liquid water on its surface. Its ocean covers about 71% of the surface, averages about 3.6 kilometers (2.2 miles) deep and holds 97% of Earth's water.

Near the surface, Earth's atmosphere is 78% nitrogen and 21% oxygen, with about 1% of other gases such as argon and carbon dioxide. The atmosphere shields the surface from much of the Sun's harmful radiation. Most meteoroids, small rocks from space, burn up in it before they can reach the ground.

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