What the Lines on World Globes Mean
A geographic coordinate system allows us to determine the location of any place on earth using numbers and letters. In fact, a location can be specified within a few inches, even if it’s half a world away. This ancient system has been refined over the centuries as exploration has advanced understanding and technology has allowed for more accurate measurements. You don’t need any high-tech gadgetry to use coordinates, though – just grab a map or globe.
The Long and Short of It
The roots of our modern coordinate system are over 2,000 years old; a system based on intersecting lines of longitude and latitude.
Latitude lines are also called parallels because they run parallel to the equator and to each other. A degree of latitude is just a little over 69 miles wide and there are 180 total, from pole to pole.
Because the equator divides the earth into two equal halves or hemispheres, it’s the perfect reference point from which to establish all other latitudes. The equator is thus demarcated as zero degrees (0°) latitude. Each pole is at 90° so the latitude increases as you move away from the equator.
Longitude lines, or meridians, run perpendicular to latitude lines, forming a grid across the planet. If you look at a globe you’ll see that meridians get closer to each other further from the equator, meeting at the poles. That is why, unlike degrees of latitude, degrees of longitude vary in width.
There is no natural, obvious spot for 0° longitude, called the prime meridian, so multiple prime meridians existed until the 19th century. In 1884, the International Meridian Conference voted to adopt the British prime meridian location of Greenwich, England. Now degrees of longitude radiate out from that line until they reach 180° on the opposite side of the world.
Crisscrossing the Globe
The easiest way to understand the relationship between lines of longitude and latitude and the earth is to use a globe as a visual aid. Most globes show parallels and meridians at 15° intervals – wide enough apart not to obscure other important features but close enough to be useful.
When coordinates of a location are given, the latitude is always first followed by an N for north of the equator or an S for south. The longitude is next with an E for east or W for west of the prime meridian. For example, Chicago, IL is 41°N 87°W.
Space and Time
Not only do meridians mark distance but time, too. You’ll count 24 meridian or longitude lines on your globe. It takes 24 hours for the earth to make a full rotation on its axis, so every meridian shown, each 15°, marks an hour in time. The meridian east of a chosen longitude line is an hour later, the meridian west will be an hour earlier.
You can also calculate how many miles per hour the earth is rotating (it varies by place!) when you know the latitude of a location.