Meridian is defined as which of the following?

• A. A Line of Longitude
• B. A Small Circle Formed by a Plane That Does Not Pass Through the Center
• C. A Great Circle Formed by a Plane That Passes Through the Poles
• D. A Great Circle Formed by a Plane That Contains the Earth's Axis and Passes Through the Poles

Answer: (D)

Meridians are the north-south lines on the globe defined by a plane that contains the Earth’s axis. When such a plane intersects the Earth, it cuts a great circle, and the portion from pole to pole along that circle is what we call a meridian. That makes the description “a Great Circle formed by a plane that contains the Earth's axis and passes through the poles” the precise way to define a meridian. In practice we also refer to meridians as lines of longitude, since they locate positions east or west, but the essential geometry is the plane through the axis creating a great circle. The other options don’t capture this great-circle, axis-containing nature: a small circle would come from a plane not through the center, and a plain line of longitude implies only half a circle rather than the full great-circle construct.