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ent to the study of the crater and will find appropriate place in any comprehensive
discussion of its origin [and] unsettle a conclusion that was beginning to feel se-
cure.” No conclusion, he went on, “is so sure that it can not be called into question
by a newly discovered fact.”
19
ButGilbert neverfoundaplace forthediscussionofthese“eminently pertinent”
ideas.Hisstatureamonggeologists,whichsurvivestothisday,effectivelyputmet-
eorite impact off limits for the next sixty years. Instead of illustrating the value of
multiple working hypotheses and logic, Gilbert showed how wrong assumptions
can lead even a great scientist to discard the correct hypothesis and embrace one
that is not only incorrect but for which there is little or no evidence. There is a les-
son in that all right, but not the one Gilbert intended.
Gaping at the Moon
To make Gilbert's conclusions about Coon Mountain seem even more bizarre, in
the early 1890s he correctly deduced that impact had created the craters of the
Moon. Prompted by his study of Coon Mountain, for eighteen nights in the late
summer and fall of 1892 Gilbert had sat up observing the Moon through the tele-
scope of the Naval Observatory in Washington. He wrote to a friend, “I am a little
has the [Geological] Survey become that one of its most distinguished members
has no better way to employ his time than to sit up all night gaping at the Moon.”
21
But never before had a scientist so skilled at interpreting landforms focused his at-
tention on the Moon.
Gilbert observed that the floors of lunar craters, unlike those of terrestrial volca-
noes, lie below the level of the surrounding plain. Even the summits of the peaks
that often rise from the centers of the larger lunar craters lie below the level of the
plain.Thelargestlunarcratersarevastlylargerthanthelargestterrestrialones.The
lighter-colored “rays” that radiate outward from some craters for scores or hun-
dreds of kilometers, like those we can see from bright Tycho with binoculars, Gil-
bert correctly interpreted as ejecta splashes from impacts. Since terrestrial volca-
noes have none of these features, he concluded that for lunar craters, “the volcanic
theory, as a whole, is therefore rejected.”
22
One fact did puzzle Gilbert: the almost perfectly circular shapes of lunar craters.
Objects flying in from space should strike the surface of the Moon at random,
oblique angles, leaving most lunar craters oval shaped, not circular. While on a
brief leave at Columbia College, Gilbert conducted do-it-yourself cratering experi-
ments, droppingmarbles intoporridgeandlumpsofmudintoamudslurry,rigging
