4. THE GREAT CRCLE
BOUNDARY BETWEEN
SUNLIT & SHADOWED
HALVES OF THE EARTH
IS
Answers
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.
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The line that separates day and night is called the terminator. It is also referred to as the "grey line" and the "twilight zone." It is a fuzzy line due to our atmosphere bending sunlight. In fact, the atmosphere bends sunlight by half a degree, which is about 37 miles (60 km).
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