A great circle is the largest possible circle that can be drawn around a sphere. All spheres have great circles. If you cut a sphere at one of its great circles, you'd cut it exactly in half. A great circle has the same circumference, or outer boundary, and the same center point as its sphere. The geometry of spheres is useful for mapping the Earth and other planets. The Earth is not a perfect sphere, but it maintains the general shape. All the meridians on Earth are great circles. Meridians, including the prime meridian, are the north-south lines we use to help describe exactly where we are on the Earth. All these lines of longitude meet at the poles, cutting the Earth neatly in half. The Equator is another of the Earth's great circles. If you were to cut into the Earth right on its Equator, you'd have two equal halves: the Northern and Southern Hemispheres. The Equator is the only east-west line that is a great circle. All other parallels (lines of latitude) get smaller as you get near the poles. Great circles can be found on spheres as big as planets and as small as oranges. If you cut an orange exactly in half, the line you cut is the orange's great circle. And until you eat one or both halves, you have two equal hemispheres of the same orange. Great circles are also useful in planning routes. The shortest path between two points on the surface of a sphere is always a segment of a great circle. Plotting great circles comes in very handy for airplane pilots trying to fly the shortest distance between two points. For example, if you flew from Atlanta, Georgia, to Athens, Greece, you could fly roughly along the path of one of Earth's great circles, which would be the shortest distance between those two points. When planning routes, however, pilots have to take other factors into account, such as air currents and weather. Great circles are just general paths to follow.

great circle
A great circle is created by any line that slices through the center of a sphere.

Equatorial Bulge
Great circles on Earth are roughly 40,000 kilometers (24,855 miles) all the way around. The Earth isn't a perfect sphere, however. It is an oblate spheroid, meaning it stretches out a little around the Equator in a form called an equatorial bulge. The Equator is around 40,075 kilometers (24,901 miles) in circumference.

air current
Noun

flowing movement of air within a larger body of air.

circumference
Noun

distance around the outside of a circle.

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

Great Circle
Noun

largest circle that can be drawn around a sphere, such as the Equator.

Noun

half of a sphere, or ball-shaped object.

Noun

distance north or south of the Equator, measured in degrees.

Noun

distance east or west of the prime meridian, measured in degrees.

meridian
Noun

line of longitude, dividing the Earth by north-south.

parallel
Noun

line of latitude, dividing the Earth by east-west.

Noun

large, spherical celestial body that regularly rotates around a star.

pole
Noun

extreme north or south point of the Earth's axis.

Noun

imaginary line around the Earth running north-south, 0 degrees longitude.

route
Noun

path or way.

Southern Hemisphere
Noun

half of the Earth between the South Pole and the Equator.

sphere
Noun

round object.

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.