Any location on Earth is described by two numbers--its latitude and itslongitude. If a pilot or a ship's captain wants to specify position on a map, these are the "coordinates" they would use.
Latitude
Imagine the Earth was a transparent sphere (actually the shape is slightly oval; because of the Earth's rotation, its equator bulges out a little). Through the transparent Earth (drawing) we can see its equatorial plane, and its middle the point is O, the center of the Earth.
Measurement:
To specify the latitude of some point P on the surface, draw the radius OP to that point. Then the elevation angle of that pointabove the equator is its latitude λ--northern latitude if north of the equator, southern (or negative) latitude if south of it.
[How can one define the angle between a line and a plane, you may well ask? After all, angles are usually measured between two lines!
Good question. We must use the angle which completes it to 90 degrees, the one between the given line and oneperpendicular to the plane. Here that would be the angle (90°-λ) between OP and the Earth's axis, known as theco-latitude of P.]
Every meridian must cross the equator. Since the equator is a circle, we can divide it--like any circle--into 360 degrees, and the longitude φ of a point is then the marked value of that division where its meridian meets the equator.
Latitude
Imagine the Earth was a transparent sphere (actually the shape is slightly oval; because of the Earth's rotation, its equator bulges out a little). Through the transparent Earth (drawing) we can see its equatorial plane, and its middle the point is O, the center of the Earth.
To specify the latitude of some point P on the surface, draw the radius OP to that point. Then the elevation angle of that pointabove the equator is its latitude λ--northern latitude if north of the equator, southern (or negative) latitude if south of it.
[How can one define the angle between a line and a plane, you may well ask? After all, angles are usually measured between two lines!
Good question. We must use the angle which completes it to 90 degrees, the one between the given line and oneperpendicular to the plane. Here that would be the angle (90°-λ) between OP and the Earth's axis, known as theco-latitude of P.]
On a globe of the Earth, lines of latitude are circles of different size. The longest is the equator, whose latitude is zero, while at the poles--at latitudes 90° north and 90° south (or -90°) the circles shrink to a point.
Longitude
On the globe, lines of constant longitude ("meridians") extend from pole to pole, like the segment boundaries on a peeled orange.
Every meridian must cross the equator. Since the equator is a circle, we can divide it--like any circle--into 360 degrees, and the longitude φ of a point is then the marked value of that division where its meridian meets the equator.Measurement:
What that value is depends of course on where we begin to count--on where zero longitude is. For historical reasons, the meridian passing the old Royal Astronomical Observatory in Greenwich, England, is the one chosen as zero longitude. Located at the eastern edge of London, the British capital, the observatory is now a public museum and a brass band stretching across its yard marks the "prime meridian." Tourists often get photographed as they straddle it--one foot in the eastern hemisphere of the Earth, the other in the western hemisphere.