The most common grid system used to map the earth consists of latitude and longitude lines. Line zero for latitude is known as the equator, and it divides the northern and southern hemispheres. Line zero for longitude is the prime meridian, which runs though Greenwich, England.
Other significant markers of the grid system include the international date line, which is the exact opposite of the prime meridian, and the Tropics of Cancer and Capricorn, each at 23.5 degrees north and south of the equator, respectively. The seasons change very little in the tropics, unlike at the more extreme ends of latitude, where seasonal change is much more prominent.