Final answer:
Longitude starts at the prime meridian, located at the Royal Observatory in Greenwich, England. It was chosen for its geographical location and historical significance in navigation. Longitudes extend 180° east and west from this line.
Step-by-step explanation:
When using longitude, we start counting from the prime meridian, which is an imaginary line that runs through the Royal Observatory in Greenwich, England. This line, at 0 degrees longitude, divides the Earth into the Eastern and Western Hemispheres. Longitudes are measured from the prime meridian to 180° east and 180° west, with positive longitudes to the east of the prime meridian and negative longitudes to the west.
The choice of Greenwich for the prime meridian was historically significant. It was chosen as the zero point of longitude due to its location between continental Europe and the United States, and for its role in the development of methods to measure longitude at sea. This helped greatly in navigation and charting accurate maps, which was crucial for safe maritime voyages.
Today, at the Royal Observatory in Greenwich, tourists can stand and straddle the exact line where longitude begins, essentially standing in both the Eastern and Western Hemispheres at the same time.