Why Did Galileo Get Such a Puny Crater?

Cast a glance at the full Moon and you'll see the names of Kepler, Copernicus, and Tycho splashed across the landscape like billboards on a freeway. But where in the heck is Galileo? While not the first person to observe the Moon in a telescope — that honor goes to England's Thomas Harriot — Galileo was the first to record and report lunar craters, spot Jupiter's four larger moons, see the phases of Venus, and discern the starry nature of the Milky Way. Surely he deserves a memento on par with the other greats.

Oh, he's there alright, but I'd guess only a few of us have ever stopped by for a visit. You'll find the undistinguished, 16-kilometer (10-mile) wide Galilaei crater in western Oceanus Procellarum not far from Reiner Gamma, a swirl-shaped magnetic anomaly that played a key role in the story of the crater's obscurity. The area also hosts the Marius Hills, the largest volcanic dome field on the Moon with more than 250 curious, conical mounds. When observed in slanting sunlight near the terminator they resemble an outbreak of hives.

Galileo first appeared on a map drawn by Francesco Maria Grimaldi that was published in the 1651 book Almagesta novum by astronomer Giovanni Battista Riccioli. Riccioli used his former student's map to introduce a system of lunar nomenclature that became the basis for our modern naming system. Craters bore the latinized names of famous scientists and philosophers, while the lunar "seas" received water-related names like Mare Imbrium (Seas of Showers) or states of mind as in Mare Tranquillitatis (Sea of Tranquility). Further, craters in the northern part of the Moon honored notables of the ancient world while Riccioli named those in the south for his contemporaries.

Grimaldi interpreted the prominent, bright oval of Reiner Gamma as a crater — such was the state of equipment at the time — and Riccioli obliged by dubbing it Galilaeus in honor of the great astronomer. In truth, the feature has no depth at all. It's the largest of several lunar swirls, areas where strong, localized magnetic fields — mini-magnetospheres if you will — deflect the solar wind and preserve the bright tone of the lunar regolith.

Galilaeus beamed back at telescopic observers for more than 180 years until the publication of Mappa selenographica, an exquisitely detailed lunar map in four quadrants compiled by German astronomers Wilhelm Beer and Johann Mädler. Placed together the four sections spanned almost a meter across.

Mädler made some changes to Riccioli's naming system. On his map only prominent lunar craters had names; smaller ones near the primaries were designated by letters. For example, Kepler is accompanied by the satellite craters Kepler A, Kepler B, and so forth. Other features such as domes, mountain peaks, or in the case of Reiner Gamma — a bright albedo marking — received Greek letter names. In 19th-century telescopes it was clear that Grimaldi's Galilaeus wasn't a crater. So Mädler chose the largest unnamed crater near the Reiner Gamma oval and named it Galilaei. Pluto is nodding its head knowingly right now.

We should always remember to maintain perspective. Thomas Harriot, long deceased, had to wait until 1970 for his impact namesake on the Moon. And though it's a hefty 56 kilometers (35 miles) across, the crater resides on the lunar far side putting it out of reach of virtually all observers. While Galileo's crater may not be particularly notable in itself, the setting is rich in wonderful observing targets that include volcanic domes, a sinuous rill, veiny wrinkle ridges, and a magnetic enigma.

And don't feel too bad for Galileo. The Italian astronomer scored an even bigger impact on Mars. The Martian Galilaei crater measures 137 kilometers (85 miles) across — even larger than the lunar near side's Tycho!

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Want to learn more about the Moon? The July issue of Sky & Telescope magazine is now out and has several articles focused on the Moon.