Geology Reference
In-Depth Information
oceans stood out starkly red against the black lunar disk. Total lunar eclipses occurred reg-
ularly as well, almost every forty-two hours later, like clockwork. During every full Moon,
when Earth lies right between the Sun and the Moon, Earth's big shadow would have com-
pletelyobscuredthegiantfaceofthebrightshiningMoon.Onceagainthestarsandplanets
would have suddenly popped out against a black sky, as the Moon's volcanoes put on their
ruddy show.
Monster tides were a far more violent consequence of the Moon's initial proximity. Had
both Earth and the Moon been perfectly rigid solid bodies, they would appear today much
as they did 4.5 billion years ago: 15,000 miles apart with rapid rotational and orbital mo-
tions and frequent eclipses. But Earth and the Moon are not rigid. Their rocks can flex and
bend; especially when molten, they swell and recede with the tides. The young Moon, at a
distance of 15,000 miles, exerted tremendous tidal forces on Earth's rocks, even as Earth
exertedanequalandoppositegravitationalforceonthelargelymoltenlunarlandscape.It's
difficulttoimaginetheimmensemagmatidesthatresulted.EveryfewhoursEarth'slargely
molten rocky surface may have bulged a mile or more outward toward the Moon, gener-
ating tremendous internal friction, adding more heat and thus keeping the surface molten
far longer than on an isolated planet. And Earth's gravity returned the favor, bulging the
Earth-facing side of the Moon outward, deforming our satellite out of perfect roundness.
These epic tidal disruptions lie at the heart of why the Moon keeps moving away from
Earth.Howdoesa2,160-mile-wide objectdriftfromamere15,000milesto239,000miles
away? The answer is found in the conservation of angular momentum—the constant sum
ofEarth'srotationalenergyplustheMoon'sorbitalenergy.Thelawsofphysicsdictatethat
whatever angular momentum the Earth-Moon system had at its origin, it must still possess
in large measure today.
Fourandahalfbillionyearsago,agreattidalbulgesweptaroundplanetEartheveryfew
hours. But because Earth's surface revolved around its axis faster (every five hours) than
the Moon orbited around that same axis (every eighty-four hours), the tidal bulge with its
extra mass was always in the lead, constantly pulling on the Moon with the force of grav-
ity, making it go faster and faster with every orbit. The immutable laws of planetary mo-
tion, first proposed about four hundred years ago by the German mathematician Johannes
Kepler, state that the faster a satellite orbits, the farther away it has to be from its central
planet. But if the Moon orbits Earth faster and faster, and thus drifts farther and farther
away, it must also gain angular momentum.
At the same time that Earth's tidal bulge pulled the Moon ahead, the tidally deformed
Moon pulled back on Earth's massive bulge with equal and opposite gravitational force,
thus making Earth rotate more slowly on its axis with every rotation. That's where conser-
vation of angular momentum comes in. The faster the Moon orbited, the farther it had to
be from Earth and the more angular momentum it picked up. To compensate, Earth had to
