Final answer:
The starting point for measuring longitude is the prime meridian, which is set at 0 degrees longitude and divides the earth into the Eastern and Western Hemispheres.
Step-by-step explanation:
The starting point for measuring longitude is the prime meridian. Longitude is defined as the number of degrees of arc along the equator between your meridian and the one passing through Greenwich, England, which has been designated as the Prime Meridian. The Prime Meridian sits at 0 degrees longitude and divides the earth into the Eastern and Western Hemispheres. All other meridians (lines of longitude), both east and west of the Prime Meridian, are numbered from 1 to 180 degrees.
It's important not to confuse the Prime Meridian with the International Date Line, which is located at 180 degrees longitude and deals with the change of calendar days, not the measurement of longitude. The Prime Meridian is the globally agreed-upon reference line from which longitude is measured. The International Date Line, on the contrary, does have practical implications for the calendar but is not the starting point for measuring longitude.