Earth

Humans have known that Earth is round for more than 2,000 years! The ancient Greeks measured shadows during the summer solstice and also calculated Earth's circumference. They used positions of stars and constellations to estimate distances on Earth. They could even see the planet's round shadow on the moon during a lunar eclipse. (We still can see this during lunar eclipses.)

Today, scientists use geodesy, which is the science of measuring Earth's shape, gravity, and rotation. Geodesy provides accurate measurements that show Earth is round. With GPS and other satellites, scientists can measure Earth's size and shape within a centimeter. Pictures from space also show Earth is round like the moon.

Even though our planet is a sphere, it is not a perfect sphere. Because of the force caused when Earth rotates, the North and South Poles are slightly flat. Earth's rotation, wobbly motion, and other forces are making the planet change shape very slowly, but it is still round.

Earth