Earth: The Home Planet
For centuries, astronomers and geologists proposed theories to explain how Earth originated. Scientists are now fairly certain about Earth's origins. They have reviewed information returned by both piloted and robotic visits to other worlds of the solar system in recent decades. Experts are now certain that Earth and its planetary neighbors formed from a spinning disk of debris left over from the birth of the Sun.
Three planets in the solar system are smaller than planet Earth. Four are considerably larger. In terms of mass, therefore, Earth is not an outstanding member of the vast solar family. But it is the home planet and the site of all life. Earth is the vantage point from which the universe is viewed.
The Shape of Earth
For centuries, Earth was thought to be flat. But early scientists used their keen observations to correct this mistake about Earth's shape. For example, they noticed that distant ships seemed to drop below the horizon. Travelers to faraway lands saw that the position of stars changed relative to the horizon. And early astronomers watched the movement of a lunar eclipse. As a lunar eclipse advanced, Earth cast a curved shadow on the Moon. This meant that Earth was a sphere.
Geodesy is the study and measurement of Earth's shape. Modern geodetic scientists have the advantage of photographs. Photographs of Earth have been taken from rockets, satellites, and other spacecraft far above the surface. The photos reveal that the planet is nearly round.
But Earth is not a perfect sphere. Instead, it is slightly flattened at the poles. This shape is probably caused by the planet's force of rotation. The rotational force deforms the somewhat plastic Earth into a form that is in balance with the forces of rotation and gravity. Earth's diameter is 7,900 miles (12,700 kilometers) from pole to pole; it is 7,920 miles (12,750 kilometers) around the equator. Recent measurements also indicate that Earth is slightly more flattened at the South Pole than at the North Pole, which makes it slightly pear-shaped.
