Neptune was discovered more than 150 years ago, but astronomers had never been able to get a firm handle on the length of the Neptunian day. Determining the length of a day is generally pretty basic. Pick a feature on the surface of a planet, note its position, wait until the feature returns to that position, and the waiting period is the length of the day. If a planet's surface isn't visible, astronomers use atmospheric features to get a rougher value.
Neptune was a challenge because its beautiful blue atmosphere is bland. It has no features to really track. Using images from space probes in various wavelengths, however, astronomers have finally determined Neptune's day is two minutes and one second short of sixteen hours long.