32 August 2014 sky & telescope
History of AstronomyAmerican physicist Herbert Eugene Ives concluded, after
studying craters formed by aerial bombs in tests at Lang-
ley Field, Virginia, that these manmade features closely
resembled those on the Moon.
Unaware of their work, Giff ord, in the splendid isola-
tion of New Zealand, worked on, trying to determine
mathematically the consequences of Bicky’s “pretty fair
bang.” In the early 1920s he arrived at the answer. As one
of his math pupils at Wellington College remembered,
during one of Giff ord’s lectures,
a dry beginning... changed into forty minutes of wide-eyed
interest and learning illustrated by full width chalkboard draw-
ings of a meteor of certain mass, velocity, and approach angle,
causing the release of energy, a nearly round crater, and the
trajectory and mass of the exploded moon material showing
formation of the rim and central pip, all supported by detailed
calculations.
The student was recalling a preview of the classic
paper Giff ord published in the September 1924 New
Zealand Journal of Science and Technology, where it was
guaranteed to have virtually no impact whatever. There he
compared the energies of various explosives used during
the war with those of meteorites moving at various veloci-
ties, and deftly showed that “when a meteorite strikesMETEOR CRATER At the beginning of the 20th century, most scientists thought that Arizona’s Meteor Crater (known then as Coon
Butte) was a volcanic structure. Detailed work by Algernon Charles Giff ord, Daniel Moreau Barringer, Ernst J. Öpik, Ralph Baldwin,
Eugene Shoemaker, and other scientists showed how craters such as this one were formed explosively by infalling meteorites.the main attack was to begin. But due to confusion on
the Allied side, the attack didn’t actually get underway for
another two hours. The blast made a noise said to have
been heard as far away as London, and was witnessed
from the air by 2nd Lieutenant Cecil Arthur Lewis of the
Royal Flying Corps’ No. 3 Squadron, who gave the follow-
ing description:
The whole earth heaved and fl ashed, and a tremendous and
magnifi cent column [of smoke] rose up in the sky... It hung...
in the air, like the silhouette of some great Cyprus tree, then fell
away in a widening cone of dust and debris.
When the dust settled, a crater very much like those
on the Moon, measuring 300 feet across and 70 feet deep,
scarred the landscape. It was the largest man-made crater
produced during World War I, though a mere pinprick by
lunar standards. The experience of the Somme’s scarred
battlefi elds — where shells produced saturation cratering
in No-Man’s Land — gradually inspired new thinking
about the features on Earth’s satellite. Gigantic craters
such as Lochnagar were particularly infl uential.
Also in 1916, Estonian astronomer Ernst J. Öpik
realized that a meteorite crashing into the Moon would
be suffi cient by itself to excavate a circular crater even
without the help of volcanic forces. Three years later,BIGSTOCK PHOTO: ACTIONSPORTS