What is the reference line used to measure degrees of longitude?

The reference line used to measure degrees of longitude is the Prime Meridian. The Prime Meridian is an imaginary line that runs from the North Pole to the South Pole and is defined as being at 0 degrees longitude. It serves as the starting point from which all other lines of longitude are measured, with degrees of longitude increasing up to 180 degrees east and west. This system of measurement helps establish global positioning and facilitates navigation by providing a standard framework for geographic coordinates.

In contrast, the Equator is the reference line for measuring degrees of latitude, dividing the Earth into the Northern and Southern Hemispheres. The International Date Line, while significant for timekeeping, is not a measurement for longitude but rather a demarcation for calendar days. Lastly, the Tropic of Capricorn is another line of latitude and does not serve as a reference for measuring longitude either. Therefore, the Prime Meridian is uniquely the line relevant to longitude measurements.