Latitude and Longitude
Latitude and longitude are the measurements used to give the position of someone or something on the surface of the earth. Latitude is the distance north or south. Longitude is the position east or west. The units of measurement of both latitude and longitude are degrees, because the earth is circular of course.

Latitude is measured in degrees north or degrees south of the equator, which is the line around the exact middle of the earth. The equator divides the earth in two halves: the top half is called the northern hemisphere, the bottom half is called the southern hemisphere. Hemisphere means half a sphere. The North Pole is at a latitude of 90 degrees north of the equator, whereas the south pole is said to be at a latitude of 90 degrees south of the equator.

Longitude is measured in degrees east or west of something called the Prime Meridian. This is the line going from the North Pole to the South Pole and running through the middle of the Greenwich Observatory in London. Actually any line at all going from the North Pole to the South Pole could be chosen as a starting point for measuring longitude. In the 19th century the French chose their own line which ran through the old French Royal Observatory in Paris. But this just caused confusion and so now everyone in the world agrees to measure longitude from the line going through Greenwich. The reason Greenwich was chosen is because for many hundreds of years Greenwich was the most important astronomical observatory in the world.

Latitude and longitude are the most fundament mathematical measurements used by geographers, navigators and cartographers (people who make maps). The exact position of any place in the world can be given in terms of its latitude and longitude. The latitude of London is 51.5 degrees north and 0 degrees east or west. New York is 41 degrees north, 74 degrees west. Sydney is 34 degrees south, 152 east.