When we describe the Earth in spherical coordinates it is called Geographic, or Un-projected. The most common system for locating points on the earth is with "Longitudes" and "Latitudes". Both longitude and latitude are angles measured with the center of the earth as an origin. A longitude is an angle from the prime merdian, measured to the east (longitudes to the west are negative). Latitudes measure an angle up from the equator (latitudes to the south are negative).
Figure 1. With an angle of longitude and an angle of latitude, we can locate any point on the surface of the earth.
You've seen lines running across maps your whole life and may not have noticed them. The lines running North to South are called "Meridians" or "lines of longitude" (Figure 2), while the lines running East to West are called "Parallels" or "lines of latitude" (Figure 3).
Note that the lines are parallel when we look at the earth from the equator. If we look at them from the poles, they appear circular.
The latitude is the angle formed by a line going from the center of the earth to the equator at the point on the equator that is closed to the point of interest and another line that goes from the center of the earth to the parallel that goes through the point of interest.
The longitude is the angle formed by a line that goes through the center of the earth and the equator where the longitude=0 and another line that goes through the center of the earth and a line through the equator and the meridian that goes through the point of interest.
There are different ways of expressing longitude and latitude values.
The system we will work with the most encodes longitude from -180° in the middle of the Pacific to 0° at the Prime Meridian, which runs through Greenwich, England and back to 180° in the middle of the Pacific.
Another common and older way of encoding longitude is from 180° West to 180° East.
The system we will work with most encodes latitude from 90° to -90°.
Another common way of encoding latitude is from 90° N to 90° S.
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