Geology Reference
In-Depth Information
It all boils down to one amazing fact: the Moon formed only 15,000 miles from Earth's
surface—not much farther than a plane flight from Washington, D.C., to Melbourne, Aus-
tralia. Today, by contrast, the Moon is 239,000 miles away. At first blush, it seems utterly
implausible that a giant Moon could just drift away from Earth like that, but measurements
don'tlie. Apolloastronauts left shinymirrorsonthelunarsurface. Laser beams fromEarth
bounce off the mirrors and return to Earth to provide distance measurements accurate to
within a tiny fraction of an inch. Every year since the early 1970s, the Moon has moved
farther away: 3.82 centimeters per year, about one and a half inches per year on average.
That doesn't sound like much, but it adds up over time—to a mile farther away every forty
thousandyearsatpresentrates.Playthetapebackward,anditpointstoaradicallydifferent
situation 4.5 billion years ago.
Foronething,theMoon
looked
totallydifferent.At15,000milesdistant,the2,160-mile-
diameter Moon would have appeared gigantic, like nothing we've ever seen. It spanned
almost 8 degrees of arc in the sky—roughly sixteen times the apparent diameter of the
Sun—and blocked more than 250 times as much of the firmament as the Moon does today.
Andthat'snotall. The early Moonwas aviolent bodyofintense volcanism, quite unlike
the static silvery-gray object we see now. Its surface would have appeared black, with
glowing red magma-filled cracks and volcanic basins easily visible from Earth. The prim-
ordial full Moon was equally dramatic, the surface reflecting hundreds of times more sun-
light than in our modern era. You could easily read a topic under its brilliant illumination,
but astronomical observations would have been futile. Nostars orplanets would have been
visible against the young Moon's intense glare.
Adding to the drama was how fast things moved then. In space, there is no friction, so
spinning objects just keep spinning for billions of years. The total amount of spinning en-
ergyoftheEarth-Moonsystem—itsangularmomentum—ismeasuredbyacombinationof
twofamiliarcircularmotions.FirstisEarth'srotationaboutitsaxis;thefasterEarthrotates,
the more angular momentum it has. The Moon's angular momentum, by contrast, depends
primarily on how far away and how fast it orbits around Earth. Its own rotation is not a
significant part of the equation.
The total angular momentum of Earth's rotation plus the Moon's orbit hasn't changed
significantlyoverthelastseveralbillionyears,buttherelativeimportanceofthosetwomo-
tionshaschangedalot.TodayalmostalltheangularmomentumoftheEarth-Moonsystem
is tied up in the orbiting Moon, with its 239,000-mile distance and twenty-nine-day orbital
period. The more massive central Earth, with its leisurely twenty-four-hour day, has only
a tiny fraction of the Moon's angular momentum. (By the same token, the distant gas gi-
ant planets carry almost all the Solar System'sangular momentum, even though the central
Sun has 99.9 percent of the mass.)
