Sky and Telescope - May 2015 - 32

Naming the Craters

"THE MONSTROUS BEAST OF WAR," detail from a typical ﬂysheet of the time, shows the Thirty Years' War laying
waste to the land, pillaging churches and towns, and slaughtering their inhabitants. Were Riccioli's grim waxing-gibbous
names a reﬂection?

Riccioli was using hellish surroundings to launder the
Copernicans, perhaps he'd be less likely to get called out
for memorializing them so prominently.

Riccioli a Closet Copernican?
Next up in the Tycho-Copernicus-Kepler progression
was Galileo. But try to spot Galileo's pinprick of a crater!
It's invisible in most binoculars unless you catch it, and
its later-named satellite Galilaei A, as twin sparks of light
on the terminator.
So what happened? Don't blame Riccioli; only later
was Galileo relocated to the insigniﬁcant crater that
bears the name today. (Beer and Mädler's 1836 map
seems to be the culprit.) Riccioli gave Galileo's name to
the last bright splash left in VIII Octans: the ﬂat albedo
feature Reiner Gamma, which Grimaldi had mistakenly
drawn as a largish crater.
Riccioli published his Moon map in his great encyclopedic work the Almagestum Novum, a book "no serious
seventeenth century astronomer could do without," as
John Flamsteed, England's ﬁrst Astronomer Royal, put
it around the 17th century's end. As its frontispiece illustrates dramatically (see previous page), Riccioli held in
public that the Copernicans got it wrong. He discusses
77 objections to a moving Earth. Most were variations
on "where's the big wind?", but a couple he thought
were legitimate. Where were the Coriolis eﬀects if Earth
was spinning? Falling objects and ﬂying cannonballs
didn't seem to be deﬂected. And if the Earth circled
the Sun without the stars showing parallax motion, the
stars would have to be extremely far away. But the small
telescopes of his day showed them as little disks (now
known to be illusory diﬀraction disks), which at such a
distance would make the stars huge beyond belief.
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May 2015 sky & telescope

As for the ﬁrst objection, and as the Jesuit in Riccioli might have suspected, absence of evidence was not
evidence of absence. Coriolis eﬀects are signiﬁcant only
on large scales, because Earth is large; they would be
demonstrated in the 19th century. So would the stars'
parallaxes. Against these objections was the Copernican system's seductive simplicity. So did Riccioli have
his doubts? We'll never know, but it's telling that he
gave Aristarchus, the Greek Copernicus, such a blazing
beacon unifying the ancient and modern octants, and
Copernicus himself a crater second only to Tycho.
The practical purpose for mapping the Moon - so
that terminator timings could be used to determine
longitude on ocean voyages - turned out to be impractical. But that didn't stop Riccioli's map from becoming
a hit with astronomers from Rome to Greenwich. Who
could argue with a Hall of Fame, especially if it included
them? And hats oﬀ to the way Riccioli handled the
Copernicans. But the map's enduring appeal was poetic.
The Moon, its gaze ﬁ xed upon Earth, had seen a lot of
history. Riccioli made it bear witness to all those centuries of observation, speculation and calculation.
As for Galileo getting short-changed, there's a postscript. In the far-oﬀ future, a spacecraft orbiting the
Moon would discover something unusual - that the
Reiner Gamma marking is a magnetic anomaly whose
mini-magnetosphere may be shielding it from the darkening eﬀect of the solar wind. Left where Riccioli put
him, Galileo would have kept his name in lights, and in
binoculars, for millennia to come. ✦
In all his years in advertising, Andrew Livingston has never
seen such an action-packed layout as Riccioli's map of the
Moon. As for Galileo, when binoculars show the Sun rising
on his crater, he gets an honorary visit with a 13-inch Dob.

Further Reading
* Antonín Rükl's indispensable Lunar Atlas (edited by
S&T's Gary Seronik) is a labor of love deserving a crater of its own. It includes a wealth of lunar information
as well as a who's who on our side of the Moon.
* Ewan Whitaker, Mapping and Naming the Moon.
This is the Almagest on the topic, by someone who
had a hand in directing the International Astronomical
Union's lunar nomenclature and in preparing the map
used by the Apollo missions.
* C. M. Graney, Teaching Galileo? Get To Know Riccioli!, available at arxiv.org/abs/1107.3483.
* Also by Graney: Giovanni Battista Riccioli's SeventySeven Arguments Against the Motion of the Earth,
arxiv.org/abs/1011.3778.

http://www.arxiv.org/abs/1107.3483 http://www.arxiv.org/abs/1011.3778

Sky and Telescope - May 2015

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