I spent last Sunday's lunch explaining to my girlfriend the revolution that being able to compute your longitude was (I could not help myself). There were prizes offered by the trading companies and governments to whomever would devise a reliable way to determine one's longitude.

Conceptually, it's interesting that longitude is mostly useful only if you know your latitude (that's because 1degre of change in longitude does not translate to the same distance traveled whether you're at the equator or at the pole.)