Riccioli's 1651 Moon map.

The Lunar Name Game

Who was Clavius, and why is one of the biggest craters on the Moon named after him?

Not long after Galileo’s first drawings of the Moon appeared in his slim book Sidereus Nuncius (The Starry Messenger), astronomers started mapping the lunar surface and assigning names to the Moon’s various features. The first of these maps appeared in 1645 and was produced by Michael Florent van Langren (also know as Langrenus), a Dutch engineer and mathematician. But we mostly have Giovanni Battista Riccioli to thank for the names that appear on today’s Moon maps.

Riccioli’s 1651 Moon Map: The origin of the names of many lunar features, plus a system for selecting the names for lunar craters, mountains and plains, can all be traced to a Jesuit astron­omer named Giovanni Battista Riccioli and his 17th-century map of the Earth’s satellite. Most of the names on this map remain in use today.

Riccioli was a Jesuit professor at the University of Bologna in Italy who, in 1651, published his richly illustrated work Almagestum Novum, which included his lunar map (above). What Riccioli contributed to lunar nomenclature was a systematic approach to naming features: Craters were named after scientists and various celebrities and mountains for their terrestrial counterparts, while seas (maria) were assigned symbolic, fanciful names. Although other lunar cartographers would add new names and change some, the Riccioli hierarchy has survived to the present day.

The most detailed and accurate prephotographic map of the Moon was Mappa Selenographica by German astronomers Wilhelm Beer and Johann Heinrich Mädler, which appeared in 1837. This map named 427 features, 200 of which originated in  Riccioli’s work. But through the end of the 19th century and into the beginning of the 20th, Moon maps and naming schemes continued to appear. In 1935, the confusion caused the International Astronomical Union (IAU) to insert itself into the fray by producing a Moon map and taking on the role of policing lunar nomenclature. It has done so ever since.

LUNAR CLAVIUS Familiar to backyard observers, the crater named Clavius is one of the highlights of the Moon's southern region. Courtesy LRO/NASA
Familiar to backyard observers, the 245-km-wide crater Clavius is one of the highlights of the Moon’s southern region. (LRO/NASA)

At the moment, there are more than 1,500 named craters on the Moon, roughly half of which are on the near side. A handful are of recent vintage. For example, three small craters in Mare Tranquillitatis are named, appropriately enough, Aldrin, Armstrong and Collins, after the Apollo 11 crew. The crater trio is located a few kilometres north of the landing site where Neil Armstrong and Buzz Aldrin first kicked up lunar soil in 1969. The newest addition came earlier this year when the IAU Working Group for Planetary System Nomenclature named an 8.3-kilometre-wide crater near the lunar south pole Malinkin, after Yegor Ivanovich Malinkin, a Russian meteorite researcher who died in 2008.

Oh, in case you’re still wondering, the crater Clavius is named after the 16th-century German astronomer Christoph Klau Clavius. He was a friend of Riccioli and a fellow Jesuit. Never heard of him, right? Few have. It’s ironic, then, that Galileo, who pioneered telescopic exploration of the Moon, is honoured with a humble 15.5-kilometre-wide divot in Oceanus Procellarum.

No one ever said the lunar name game was fair!