Every spot on earth has two numbers: a latitude and a longitude.
Latitude tells you how many degrees north or south a place equator. It is a number between 0 and 90. For example Alexandria in Egypt is 31.25 degrees north.
Longitude tells you how many degrees east or west a place is from London. It is a number between 0 and 180. Alexandria is 30 degrees east.
Therefore we say Alexandria is 31.25 N, 30 E. Sometimes this is just written as “31.25, 30″ where it is understood that the first number is latitude. To show south or west we use negative numbers. For example, New York is at 40.75 N, 74 W or “40.75, -74″.
A degree is a way to measure how far round a circle you are. A circle has 360 degrees. Half way round a circle is 180 degrees. A fourth of the way is 90 degrees. And so on.
The equator is a line that divides the earth into two equal halves, a north and a south.
Longitudes start at the Royal Observatory in Greenwich in London. When the present system was set up, Britain was the top sea-going country so their largest city became the starting point.
In the old days ships set their clocks to the time of the Greenwich Observatory, known as Greenwich Mean Time (GMT). Then, no matter where they went, they could tell what their longitude was by seeing what time was on their clock when the sun reached its highest point in the sky at noon.
Every hour equals 15 degrees of longitude. If noon came before 12:00 GMT, then you were that many degrees of longitude east of London. Otherwise you were to the west.
To find your latitude you measured how many degrees high the north star was – or some other star of known position.
All this measuring was done by an astrolabe, later with a sextant.
The rise of the West since 1500 was made possible in part by knowing how to figure out latitude and longitude: because they always knew where they were, Westerners could cross the ocean and not get lost. That meant they could discover new lands and deliver goods faster and cheaper.
These days you can just use a GPS to find out your latitude and longitude. Invented by the American military, it talks to satellites in orbit around the earth and from that it can figure out where it is. Most ships and aircraft come with one, so do some cars.
The latitude and longitude of every computer on the Internet is also known. So there are websites you can go to to find out the latitude and longitude of the computer that is connecting you to the Internet. This will not be the same as your own latitude and longitude but it will be close.
Latitude and longitude is not as new as you might think. In Roman times Ptolemy recorded the latitude and longitude of 8000 places from Spain to China in his book on geography.
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