This is because the Earth is in constant rotation, moving 15 each hour. So, each 4 seconds error in the time will result in a position error of 1 nautical mile.
Longitude is the coordinate affected by time errors.
In fact, if you don't have the correct time, you can't calculate your longitude by celestial navigation. The traditional way to calculate the time using celestial navigation is by performing a Lunar Calculation . This uses the Moon fast celestial sphere movement as a clock. More details click here . In the old days of mechanical watches, keeping the time was a real nightmare.