Two opposite longitudes form

Analyze the question It asks what shape is formed when two "opposite" longitudes (for example, 30°E and 150°W, which are 180° apart) are joined together.

Explain the concept

• Longitudes (meridians) are half-circles that run from the North Pole to the South Pole.
• A meridian and its opposite meridian (the antimeridian, 180° away) together form a complete circle around the Earth.
• That circle passes through both poles and divides the Earth into two equal halves. A circle that divides the Earth into two equal halves is called a Great Circle.

Step-by-step solution

1. Take the Prime Meridian (0°) and its opposite, 180°. Joined, they make a full circle through both poles.
2. Take any other pair, such as 40°E and 140°W. They also form a full circle through the poles.
3. Each of these circles has its center at the center of the Earth, so each is a Great Circle.

Check the other options

• B. The Equator: this is a line of latitude, not longitude. It is a Great Circle, but it is not formed by longitudes.
• C. The International Date Line: this is roughly the 180° meridian, a single longitude, not two opposite ones.
• D. The Arctic Circle: this is a line of latitude at 66½°N and is a small circle, not a Great Circle.

Answer: A. a Great Circle

Tip: Remember "Opposite meridians = Great Circle through the poles." Every Great Circle cuts the Earth into two equal hemispheres, and the Equator is the only line of latitude that is a Great Circle.