The Bank of Time - Four Revolutions in the Earth Sciences - page 17

Kelvin admired. 23 After Kelvin wrote to George, father Charles expressed delight
at the budding association between his son and his long-time nemesis.
George Darwin made two calculations that bore on the age of the Earth. The
first concerned the length of the day. As the Earth and the Moon have been braking
each other, working backward there must have been a time when the length of a
day, determined by the speed of the Earth's rotation, and the length of a month,
determined by the Moon's orbit, were equal. At that time, Darwin calculated, the
Moon would have been within 6,000 miles of the Earth. This must have been close
to the time the Moon and the Earth separated: “something over 54 million years”
ago. 24 In a second calculation, Darwin estimated that in something less than 57
million years, the pull of the Moon would have produced the current tilt of the
Earth's axis. He could not resist adding, “It is particularly important to notice that
all the changes might have taken place in 57 million years; and this is far within
the time which physicists admit that the Earth and moon may have existed.” 25
Later George Darwin cautioned, “The actual period, of course, must have been
much greater,” saying that his calculation “is only a wild speculation, incapable of
verification.” 26 But scientists ignored his cautions and regarded his estimate of 54
to 57 million years for the age of the Earth as yet another confirmation, one by a
rigorous and independent method, of Kelvin's result.
As scientists were coming up with one variation after another on the hourglass
theme, citing one another as authorities, Kelvin continued to move the target. Each
time he redid the calculation, using improved data for heat flow and conductivity,
the age of the Earth dropped with “almost harmonic regularity.” From the 400 mil-
lion years of his 1862 calculation, the permissible lifespan of our planet descended
to 100 million in 1868, to 50 million in 1876, to a possible low of 20 million in
1881, finally settling at 24 million years in 1897. 27
The24-million-yearestimatehadcomefromthelaboratoryofanAmericangeo-
logist named Clarence King (1842-1901), the founding director of the U.S. Geolo-
gical Survey and one of the oddest figures in the history of American science. 28
Kelvin had assumed that the molten Earth had initially been at a temperature of
7,000°F. King started there, but he had more current information about the melt-
ing point of rocks and the distribution of temperature within the Earth. Using those
data, King found “no warrant for extending the earth's age beyond 24 millions of
years.” 29 He ended by saying that “the concordance of results between the ages of
the sun and earth throws the burden of proof upon those who hold to a vaguely
vast age, derived from sedimentary geology.” 30 But for Archibald Geikie, the “as-
tronomers” had finally gone too far.