(Page 6 of 9 pages)
Here
is a drawing of the earth from above, showing the meridians of longitude.
Do you remember that they run from pole to pole? One of these (the prime
meridian) is named 0° longitude.
To the west they are all numbered from
1° West longitude to 180° longitude. To the East of 0°, they
are named from 1° east longitude to 180°.