SkyandTelescope.com August 2014 33a surface with a velocity of many miles per second it
becomes an explosive compared with whose violence that
of dynamite is insignifi cant.” He demonstrated this by the
following table:
Energy of explosives in Calories per gram
Tri-nitro-toluene (T.N.T.) 924
Dynamite 1,100
Nitroglycerin 1,478Energy per gram of a meteorite moving with
a velocity of 1 mile per second 310
3 " 2,779
5 " 7,745
10 " 30,980
20 " 123,900
40 " 494,700From an analysis of meteor shower orbits as well as
the few cases of authenticated observed meteorite falls
to date, he showed that cosmic velocities in the range of
10 to 45 miles per second (16 to 72 km per second) would hardly be exceptional. What would result from the impact
of a meteorite moving with such velocity on the lunar
surface? Regardless of the impact angle, the resulting
explosion would form a circular crater. Furthermore, Gif-
ford derived profi les for lunar craters such as Copernicus
and Theophilus — and even accounted for the existence
of their central peaks.
After retiring from teaching, Giff ord lived on in idyl-
lic retirement at Silverstream, outside Wellington, until
his death in 1948 at age 86. Only a year later, American
planetary scientist Ralph Baldwin published his classic
account of lunar impact theory, The Face of the Moon.
Drawing on the experience of World War II and hitherto
censored U.S. Army and Navy data on the diameters and
depths of craters taken from the fi ring records of hun-
dreds of shells, Baldwin showed that the profi les of these
manmade craters fi tted precisely on a line with those of
lunar features. Finally, in the 1950s, Eugene M. Shoe-
maker of the U.S. Geological Survey defi nitively showed,
from his studies of atomic bomb test craters and of the
geological structure of Meteor Crater, that lunar craters
were impact features.
Although Shoemaker deserves to be remembered
as the founder of the modern paradigm for the impact
features on the Moon and other solar system bodies, it
detracts nothing from him to remember, in this centen-
nial of the onset of the Great War, that the key result —
the explosive nature of the impact — was fi rst worked out
in detail by Charlie Giff ord of New Zealand. ✦
Contributing editor William Sheehan spent a delightful year,
1999–2000, working in beautiful Aeoteoroa (New Zealand),
while fi lling the margins of his time observing Southern stars
and researching the contributions of “Uncle Charlie.”TELLTALE GRAPH Drawing upon data on bomb craters and
explosion pits from World War II, Ralph Baldwin produced this
graph showing empirically the relationship between diameter and
depth of terrestrial explosion craters, meteoritic craters, and lunar
craters that Giff ord had surmised from his 1924 calculations.
Adapted from Baldwin’s The Face of the Moon, 1949, page 132.
THE LEGACY CONTINUES The Giff ord Observatory on Mount
Victoria, near Wellington, N.Z., was named after the astronomer
whose work anticipated later research proving the impact origin of
lunar craters. The Giff ord Observatory Trust and Wellington Astro-
nomical Society operate the observatory, which is frequently used
by students. Its largest telescope is a 130-mm Zeiss refractor.UNIVERSITY OF CHICAGO PRESSPAUL MOSS / WIKIMEDIA COMMONS