Which of the following is an important line of longitude?

The Prime Meridian is an important line of longitude. It is the 0-degree line of longitude that passes through Greenwich, London, and divides the Earth into the Eastern and Western Hemispheres. All other lines of longitude are measured east or west from the Prime Meridian, up to 180 degrees.

The key to this question is understanding the difference between lines of latitude and lines of longitude. Lines of longitude (meridians) run from the North Pole to the South Pole, while lines of latitude (parallels) run horizontally around the Earth.

The other three options are all lines of latitude, not longitude:

• The Arctic Circle is a line of latitude at approximately 66.5 degrees N.
• The Antarctic Circle is a line of latitude at approximately 66.5 degrees S.
• The Tropic of Cancer is a line of latitude at approximately 23.5 degrees N.

Exam tip: If a question asks about longitude, look for meridians (vertical lines). If it asks about latitude, look for parallels (horizontal lines). The Prime Meridian is the only line of longitude among any set of options that includes Arctic Circle, Antarctic Circle, Tropic of Cancer, or Tropic of Capricorn.