Since the meridians meet at the poles, the distance covered by one arc second of longitude changes depending on where on earth you are. At the equator, one arc minute is one nautical mile = 1852m, so 1 second = 30,86m. The parallels (circles of lattitude) are equally spaced, so one sec north or south equals the same distance everywhere.
1 arc second of longitude =
20m in Normandy
27,8m in the Persian Gulf
24m in Nevada