The first lunar maps
The rising of a bright Full Moon has inspired poets, songwriters, and torytellers. What they imagined they saw on its surface had a great deal to do with cultural context. The most obvious features visible without optical aid are the vast dark areas known as maria, or seas, which are bordered by lighter areas. Europeans would refer to these features as the man in the Moon, the Cowichan First Nation peoples in Canada’s Pacific Northwest see a toad, and the image of a rabbit is preferred in Asia — to name a few examples. However, these depictions of the Moon’s surface are not the same as maps.
In 1608, news spread quickly through Europe of a new instrument that made distant objects appear closer. The telescope was first sold for military and mercantile purposes, but people soon turned it skyward. In England, Thomas Harriot was one of the first to seriously observe the Moon with such an instrument. He produced the earliest datable map of the Moon on July 26, 1609, though he never published it. Within a few years, Harriot had drawn maps with more details, including the dark seas and prominent craters displayed in correct proportion to each other.
Galileo burst onto the scene in March 1610 with the publication of his Sidereus Nuncius, or Starry Messenger. He realized that the interplay of light and shadow across the Moon indicated a rugged surface, which he captured in his drawings. This upended the wisdom of Aristotle from 2,000 years before, which held the Moon was in the realm of perfection and that there would be nothing to map!
It’s important here to understand the difference between cartography and topography. Harriot was interested in the spatial relationships of lunar features, in part to understand the wobble, or libration, of the Moon. This phenomenon means that over the course of one libration cycle, roughly 18 percent of the lunar farside can be seen creeping around the Moon’s limb. Thus, Harriot’s maps are two-dimensional and tried to capture the physical and spatial relationships of lunar features. Galileo’s sketches were topographic, creating a three-dimensional representation of the Moon that showed how its features varied in height.
Just 37 years after Galileo published his drawings of the Moon, Polish astronomer Johannes Hevelius released his book Selenographia (Pictures of the Moon). Unlike Galileo’s topographic artistry, Selenographia was a first attempt at organized lunar cartography. Hevelius made his money by brewing beer, but became fascinated with astronomy. He built an observatory and many of his own telescopes, including a tubeless instrument 150 feet (46 meters) long.
But why go to the trouble of mapping the Moon — a place no one would ever visit? The answer lies in trade and world power. As ships improved and trade increased, knowing your location at sea became of paramount importance. The problem of finding where you were on Earth was one of keeping accurate time. If you could determine your local time and compare it to the time at a reference location, such as London or Paris, you could find your longitude based on the time difference. But clocks in the 17th century were not reliable enough to keep an accurate reference time over months at sea.
Galileo had suggested using his newly discovered four moons of Jupiter and their regular, repeated motion as a kind of clock in the sky. Others thought the same could be done with lunar eclipses: By observing when the edge of Earth’s shadow crossed a given feature on the Moon, you could use an almanac to compare that time to when this happened back at your reference location, thus giving the difference in time and the longitude. Hevelius hoped his lunar maps might be a suitable reference for this method. Unfortunately, the observations were too difficult to make from the deck of a ship. And in any case, since lunar eclipses are not that frequent, the method would have been of limited use.
The solution to the longitude problem had to wait for a sea-going clock. Nonetheless, Hevelius’ maps continued to set the standard of lunar cartography for a century.