The third installment of this occasional series helped address the problem of longitude.
Every earthling is a student of the moon. Who hasn’t gazed at its brilliant face, squinted to make out its canyons and craters, and wondered about its dark side? But there’s one person officially credited with founding selenography, the study of the moon’s physical features. That would be Johannes Hevelius, whose most famous lunar map influences astronomy, cartography, and navigation to this day.
Who created this cherub-flocked map?
Born in Poland in 1611, Hevelius spent his adolescence under the tutelage of the famed German astronomer, mathematician, and polymath Peter Krüger. Though Hevelius found success as a brewer by early adulthood, his tutor’s death in 1639 compelled him to commit his life’s work to studying the cosmos, especially the moon. The timing was good, for telescope technology was improving; Galileo had pointed the very first one into the night sky just 30 years prior.
How did he make it?
By then, astronomers and mathematicians had produced many drawings, sketches, and engravings of the moon. These were mainly illustrations of the rough, mountainous surface; they didn’t scientifically depict the moon’s specific features. That’s what Hevelius wanted to do, though the tools available to him weren’t quite powerful enough. So he carved his own lenses, built his own telescopes, and for years observed the moon every clear night he could. Using his personal wealth, Hevelius published his observations and drawings in 1647’s Selenographia, the first scientific treatise dedicated solely to the moon.
Why does the moon appear so oddly … large?
Historian of astronomy Albert Van Helden explains:
In Selenographia he presented engravings of every conceivable phase of the Moon as well as three large plates of the full Moon: one of the ways the full Moon actually appeared through the telescope, one the way a maker of terrestrial maps might represent it (using the conventions of geographers), and one a composite map of all lunar features illuminated (impossibly) from the same side.
It was this last map (pictured above) that proved perhaps most influential. Since Hevelius, virtually all lunar maps have used the cartographic convention of “single illumination,” where the lunar surface is shown equally lit. It was also Hevelius who established the convention of showing the entire surface that is visible from Earth, which is more than one hemisphere. That’s due to libration, the slight oscillations the moon seems to make as it orbits, which Hevelius helped to discover.
In another one of the three maps, Hevelius also offered a system of nomenclature to label the moon’s craters and ridges. It didn’t last long, but it was among the first.
Why map the moon?
Hevelius’s maps helped advance the most vexing problem of exploration in his time: longitude.
During the so-called Age of Discovery, European navigators sailing for weeks without land in sight needed new ways of determining their geographical coordinates. Latitude was easy, but longitude was trickier. Navigators had to figure out the difference between their local time and that of a distant reference point—a frustrating challenge, riddled with inaccuracy on a ship in motion on rocky seas.
Astronomers knew that the moon might be the key. During a lunar eclipse, navigators could note the times when the Earth’s shadow hits a particular feature on the moon’s surface, then compare this time to that of another location using time tables. The time difference directly correlated with the difference in longitude. But to draw up precise tables, astronomers needed a single, good lunar map: “a composite view that pictured the Moon in a way it never appeared in reality but was accurate in its placement of individual features,” Van Helden writes. Hevelius’s map was the first to truly accomplish this aim.
The problem of longitude wasn’t ultimately solved by lunar calculations: chronometers, telegraphy, and later GPS proved more efficient and accurate. But the oldest lunar maps are also the foundations of planetary geology and one of the very first steps towards space exploration.
In fact, Hevelius still appears on modern-day maps of the moon—a large crater on the western edge of the Ocean of Storms is named for him. In so many ways, his impact on the moon is still visible.