Terrestrial Latitude and Longitude

Before you try to read the sky, you need to understand how your position on Earth affects what you can observe above you. Earth is always moving, so latitude, longitude, seasons, and time of day (or night) collectively determine which part of the sky you'll see at a given time.


Going back to Earth, what do latitude and longitude mean? Earth isn't really flat like a map, of course—it's a sphere, and we live on the surface. Latitude is actually the angle between a point on the equator, a point at the center of Earth, and a point at your location (let's consider Boston, in this example). Why does the equator come into this idea?

For a coordinate system to make sense, it needs an origin. Coordinates start at the zero point. On Earth, we've defined the equator as the zero point for latitude. If you live on the equator, your latitude is 0 degrees. If you live north of the equator, say in North America, then your latitude is north, or positive; therefore Boston's latitude is 42 degrees north. If you were to draw a line from Boston straight in to the center of Earth, and then draw a line from the center to the equator, the resulting angle would be 42 degrees.

If you live in the Southern Hemisphere, below the equator (say in South America), then your latitude is negative. Such latitude is sometimes referred to as south latitude. Just as 42 degrees north and +42 degrees both refer to Boston's latitude, if you live in Santiago, Chile, your latitude would be either 33 degrees south latitude, or −33 degrees. The maximum possible latitude would be either 90 degrees north or south. Ninety degrees north is the North Pole; if you draw an angle from the equator to the center of Earth to the North Pole, you get a right angle that has 90 degrees in it. Ninety degrees south is the South Pole.

FIGURE 5-1:Latitude ranges from 0 to 90 degrees


What about longitude? That concept is a little more complicated. The equator is a very obvious origin (zero line) to use when we're talking about latitude, but not for longitude! On Earth, the zero line for longitude is at the British Royal Greenwich Observatory, in Greenwich, England. This line is called the prime meridian, and was established there at an international conference in 1884.

Start learning coordinates by figuring out where you are!

San Francisco, CA 37° 37’ N 122° 23’ W
New York City, NY 40° 39’ N 73° 47’ W
Sydney, Australia 33° 52’ S 151° 12’ E
Paris, France 48° 49’ N 2° 29’ E

Degrees of longitude proceed east and west of Greenwich, England, until they meet up at 180 degrees east and 180 degrees west out in the Pacific Ocean. The line where they meet is called the International Date Line, where it's a different day on either side of the line.

To determine your longitude, take an angle between Greenwich, England, the center of Earth, and your location. For example, Washington, D.C., is at a longitude of 77 degrees west. The angle between the prime meridian, the center of Earth, and Washington, D.C., is 77 degrees.

Since a whole degree of latitude or longitude is quite large (a degree of latitude is about 69 miles, or 111 kilometers, long), degrees are subdivided into minutes and then into seconds. Each degree has 60 minutes, and each minute has 60 seconds (just like a clock). Then, each second can be divided even further into tenths, hundredths, or more.

The exact coordinates of the U.S. Capitol in Washington, D.C., are expressed as 38°53'23" N, 77°00'27" W. What these numbers mean is that the U.S. Capitol building is 38 degrees, 53 minutes, and 23 seconds north of the equator, and that it is 77 degrees, 0 minutes, and 27 seconds west of Greenwich, England.

FIGURE 5-2:Globes display latitude and longitude

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