Prime Meridian
The prime meridian is an imaginary line of longitude that divides Earth into two equal parts: the Eastern Hemisphere and the Western Hemisphere. The prime meridian is also used as the basis for the world's time zones. On the opposite side of the Earth at 180 ° longitude lies an imaginary line called the International Dateline. A new day begins at midnight on the International Dateline. It travels around the Earth until 48 hours later it ends back at the International dateline.