# What is the circumference of the Earth?

The circumference of the Earth at the equator is 24,901.55 miles (40,066.59 kilometers). Due to the irregular, ellipsoid shape of the Earth, a line of longitude wrapped around the Earth going through the north and south poles is 24,859.82 miles (40,000 kilometers). Therefore, the Earth is a little bit (about 41 miles [66 kilometers]) wider than it is high. The diameter of the Earth is 7,926.41 miles (12,753.59 kilometers).

# Is the Earth a perfect sphere?

No, the Earth is a bit wider than it is "high." The shape is often called a geoid (Earthlike) or an ellipsoid. The rotation of the Earth causes a slight bulge towards the equator. The circumference of the Earth at the equator is 24,901.55 miles (40,066.59 kilometers), which is about 41 miles (66 kilometers) greater than the circumference through the poles (24,859.82 miles [40,000 kilometers). If you were standing on the moon, looking back home, it would be virtually impossible to see the bulge and the Earth would appear to be a perfect sphere (which it practically is).

# What is a hemisphere?

A hemisphere is half of the Earth. The Earth can actually be divided into hemispheres in two ways: by the equator, and by the Prime Meridian (through Greenwich, England) at 0 degrees longitude and another meridian at 180 degrees longitude (near the location of the International Dateline in the western Pacific Ocean). The equator divides the Earth into northern and southern hemispheres.

The Earth is not a perfect sphere but rather an ellipsoid. As the Earth spins, centrifugal forces cause the planet to bulge slightly around the middle.

There are seasonal differences between the northern and southern hemispheres but there is no such difference between the eastern and western hemispheres. Zero and 180 degrees longitude divide the Earth into the eastern (most of Europe, Africa, Australia, and Asia) and western (the Americas) hemispheres.

# What are the Arctic and Antarctic Circles?

The circles are imaginary lines that surround the north and south poles at 66.5 degrees latitude. The Arctic Circle is a line of latitude at 66.5 degrees north of the equator and the Antarctic Circle is a line of latitude at 66.5 degrees south. Areas north of the Arctic Circle are dark for 24 hours near December 21 and areas south of the Antarctic Circle are dark for 24 hours near June 21. Almost all of the continent of Antarctica is located to the south of the Antarctic Circle.

# If the Earth is so large, why did Columbus think that India was close enough to reach by sailing west from Europe?

The Greek geographer Posidonus did not believe Eratosthenes' earlier calculation, so he performed his own measurement of the Earth's circumference and arrived at the figure of 18,000 miles (28,962 kilometers). Columbus used the circumference estimated by Posidonus when he argued his plan before the Spanish court. The 7,000 mile (11,263 kilometer) difference between the actual circumference and the one Columbus used led him to believe he could reach India rather quickly by sailing west from Europe.

# How fast does the Earth spin?

It depends on where you are on the planet. If you were standing on the north pole or close to it, you would be moving at a very slow rate of speed—nearly zero miles per hour. (n the other hand, those who live at the equator (and therefore have to move about 24,900 miles [40,000 kilometers] in a 24-hour period) zoom at about 1,038 miles (1,670 kilometers) per hour. Those in the mid-latitudes, as in the United States, breeze along from about 700 to 900 miles (1,126 to 1,448 kilometers) per hour.