Longitude says how far east or west you are of the prime meridian, the north–south line through the Royal Observatory at Greenwich, in London. It is the angle at the Earth's axis between that meridian and yours, from 0° to 180° east or west. A boat in the middle of the North Pacific might be at W 140°00.0′: 140 degrees west of Greenwich.

Predict Two boats are on the same parallel at 60° N, one degree of longitude apart. How far apart are they?

Drag the dot east, west, north or south. Watch the size of a minute of longitude change.

Your longitude: W 040°00.0'
Your latitude: N 40°00.0'
One minute of longitude here: 0.77 nm
One minute of latitude: 1.00 nm everywhere

Meridians meet at the poles, so a minute of longitude shrinks as you go north or south. Distances are measured with latitude.

Meridians meet at the poles, so a minute of longitude is a mile only at the equator; at 60° north it is half a mile. That is why distances are always measured on the latitude scale, on a chart and in this course.

Longitude, and why distance uses latitude