Astronomy Answers
From the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the great circle

plural great circles

A great circle is a special circle at the surface of a sphere (for example of a planet or of the sky). You can recognize a great circle by any of the following attributes:

• A great circle divides the surface of a sphere in two equal parts.
• A great circle is the largest circle that fits on the sphere.
• If you keep going straight across a sphere then you go along a great circle.
• A great circle has the same center as the sphere that it lies on.
• The shortest route between two points, measured across the sphere, is part of a great circle.

Some examples of great circles are: the ecliptic, the equator, any meridian.