Geology Reference
In-Depth Information
But 4.5 billion years ago, things were very different. With the Moon only 15,000 miles
away, everything was turning ridiculously fast, like the ice-skater who's pulled in her arms
to speed up her spin. For one thing, Earth rotated on its axis once every five hours. It still
took a full year (about 8,766 hours) to go around the Sun; that time hasn't changed much
in the history of the Solar System. But there were more than 1,750 short days per year, and
the Sun rose every five hours!
Such an estimate seems bizarre and untestable, but at least a couple of direct measure-
ments confirm this idea of shorter ancient days. Coral reefs are one compelling form of
evidence. Some species of coral display exceedingly fine-scale growth lines that record
both subtle daily and more obvious annual cycles. As expected, modern corals show about
365 daily lines for every year of growth. But ancient fossil corals from the Devonian Peri-
od, about four hundred million years ago, display more than four hundred daily lines per
year, pointing to a faster rotation rate. Days were only about twenty-two hours long back
then, when the Moon was perhaps ten thousand miles closer to Earth.
A second, complementary measurement rests on the euphonic phenomenon of tidal
rhythmites, which are finely layered sediments that reveal the daily, lunar, and yearly
cycles of the tides. Exacting microscopic studies of tidal rhythmites from nine-hundred-
million-year-oldrocksat Big Cottonwood Canyon, Utah, point toaworldwhenEarth days
were only 18.9 hours long, when there may have been 464 days—464 sunrises and sun-
sets—everyyear.ThecalculatedEarth-Moondistanceof218,000milesatthattimeimplies
a recession rate very similar to that of modern times: 3.91 centimeters per year, slightly
more than one and a half inches annually.
Loony World
NodirectevidenceyetdocumentsEarth'stidalcyclesmorethanabillionyearsago,butwe
canbeconfidentthat4.5billionyearsagothingswerealotwilder.NotonlydidEarthhave
five-hour days, but the nearby Moon was much, much faster in its close orbit, as well. The
Moon took only eighty-four hours—three and a half modern days—to go around Earth.
With Earth spinning so fast and the Moon orbiting so fast, the familiar cycle of new Moon,
waxing Moon, full Moon, and waning Moon played out in frenetic fast-forward: every few
five-hour days saw a new lunar phase.
Lots of consequences follow from this truth, some less benign than others. With such a
big lunar obstruction in the sky and such rapid orbital motions, eclipses would have been
frequent events. A total solar eclipse would have occurred every eighty-four hours at vir-
tually every new Moon, when the Moon was positioned between Earth and the Sun. For
somefewminutes, sunlight wouldhavebeencompletely blocked,while thestarsandplan-
ets suddenly popped out against a black sky, and the Moon's fiery volcanoes and magma
