A meridian is an imaginary line running from the North Pole to the South Pole on Earth's surface. These lines help us measure longitude, which tells us how far east or west a place is from a starting point called the Prime Meridian.

Picture an orange with lines drawn from top to bottom: each line is like a meridian. The Prime Meridian runs through Greenwich, England, and is marked as 0 degrees longitude. From there, meridians are numbered up to 180 degrees east and 180 degrees west. When you look at a globe, those vertical lines running through the poles are meridians.

Meridians matter because they help us pinpoint exact locations on Earth and establish time zones. Every meridian represents a slightly different time of day, since the sun reaches each one at a different moment as Earth rotates. This is why New York and Los Angeles have different times: they sit on different meridians.

Sailors once used meridians and careful observations of the sun to figure out their position at sea, a skill that required both mathematics and precise instruments.