The Moon, a celestial GPS

The phenomenon known as a lunar eclipse has been understood since ancient times. As observations by astronomers accumulated over the centuries, the accuracy of prediction reached a point where, in the early Renaissance period, highly precise lunar-eclipse tables could be drawn up.

For astronomers, the phenomenon of the lunar eclipse had become important not only for the sake of observation but also for establishing one’s longitude on the surface of the Earth. Every spot on the Earth can be defined by two coordinates: latitude and longitude. Observation in the northern hemisphere of the height of the North Star above the horizon makes it possible to know one’s latitude. For longitude, the situation is more complex. Without a watch or clock that can keep exact time throughout the journey, only a celestial phenomenon observed simultaneously at two different places made it possible to know the longitude separating those places. And thus the importance of lunar eclipses. By determining the time at the start of an eclipse where you happened to be and by comparing that time with the time at the start of the same eclipse in, say, Paris, you could measure your longitude in relation to that of Paris. That information enabled cartographers of the day to map the Earth’s surface with a high degree of accuracy.

A life-saving lunar eclipse

The navigator Christopher Columbus, on his fourth journey to theAmericas, observed the total eclipse of the Moon on February 29, 1504. Besides allowing him to determine the longitude of theislandofJamaica, the lunar eclipse saved his life. A few days before the lunar event, members of his crew had mutinied and made off with a large part of the food supply. DespiteColumbus’s repeated requests, the local population had no desire to help him replenish his food stores. So he played his last card, informing them that he’d make the moon “disappear” in a few days’ time. And of course, as predicted by the ephemerides (a table showing the positions of a heavenly body on a number of dates in a regular sequence), the total eclipse of the Moon occurred. With the result that the frightened natives honored the explorer’s requests – and Christopher Columbus was saved!