what are latitude and longitude coordinates used for

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Latitude is how many degrees you are above (north) or below(south) the equator.
Longitude is how many degrees you are east or west of Greenwich England
For example I am at about 42 degrees, 40 minutes North latitude
and
70 degrees, 40 minutes West longitude

(Lanes Cove, Gloucester, Massachusetts)
Latitude and longitude coordinates are used to find exact locations.

They are like house and apartment addresses -- but used where house or apartment addresses aren't applicable.

Check this map to find the exact coordinates of any place on earth.

http://itouchmap.com/latlong.html
It would take me quite a while to explain how to use a sextant and exact time to determine latitude and longitude but perhaps that is not part of the question?