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Today's selection -- from Our Moon by Rebecca Boyle. Our astronauts left mirrors on the Moon:
“While the most notable relics of Apollo — the Moon rocks — are now back down on Earth, the astronauts left a few important things behind. Along with an American flag, a plaque, their footprints, and some trash, Neil Armstrong and Buzz Aldrin left behind a couple of science experiments. One of these was a two-foot-wide panel bedecked with one hundred mirrors, designed to return any light in exactly the direction from which it came.
“Every other Apollo experiment eventually faded into history, from the original rock samples to the seismometers that measure moon-quakes and other geologic activity. But fifty years after the mirrors were delivered, the retroreflector experiment is still going strong. Telescopes in Texas and France continue to use the retroreflectors every day. A telescope on the Calern Plateau in the south of France has been staring at them for half a century. The telescope’s 1.5-meter mirror also contains a laser, which bombards the Moon with ten pulses of photons per second. Only a few will make the trip, and even fewer of those will come all the way home. The round-trip journey takes about 2.4 seconds, and in the echo of light that returns, astronomers can discern the distance between the Moon and Earth to within a few millimeters. Though this seems incredibly precise, it’s somewhat less so than scientists would like, total accuracy being limited by Earth's atmospheric interference.
“This is how astronomers learned that the Moon is spiraling away from Earth at a rate of about 3.8 centimeters, or 1.5 inches, per year. The change in the rate of this separation is roughly equivalent to the rate at which your fingernails grow. About six hundred million years from now, the Moon will be so distant that it will no longer eclipse the Sun.
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| Aldrin next to the Passive Seismic Experiment Package with the Lunar Module Eagle in the background |
“About two billion years from now, when humans will likely no longer exist, the Moon will be too far away to stabilize Earth’s tilt. Earth’s axis will tip toward the Sun, and the unstable hellworld Laskar predicted will come to pass. Earth’s climate will experience regular, possibly violent, shifts. Its tide will falter, and so will its rock tides — the Moon-related stress and strain in the planet’s very innards. If life in any form is still here, the slow retreat of the Moon will very likely pose an existential threat.
“But for now, at least, the Moon will continue to guide our lives. Theia made the Moon and its companion, and may have left parts of itself buried within our world. The Moon’s motherworld may also have donated to us its nitrogen and more crucially its carbon, the elements that enable our existence. After the Moon and Earth coalesced, lunar tides had a profound effect on our planet's geologic and evolutionary history. Extreme Moon-driven tides mixed the primitive oceans like a ladle stirring a pot full of soup, dredging nutrients from the bottom to support the food chain on which our primeval ancestors depended. Without the tides and their effect on ocean currents, nutrients might have remained on the seafloor, never to be used by the vast chain of marine life. Half of the energy required for ocean mixing is provided by the dissipation of tides on the ocean floor. After the first organisms arose, the Moon probably kept time for the symphony of life, and it still may play a role in our own fertility, physiology, and behavior, in ways that surprise many modern scientists and in ways they do not fully understand.
“Large-scale phenomena like the tides are obvious, especially to humans who think in pictures and stories on a scale familiar to us — changing seahorses, a stranded invading military force. But the barely discernible phenomena at play between Earth and the Moon may be just as vital to life’s rhythms, and may have been so from the very beginning. We would not be here without the Moon.”
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author: Rebecca Boyle |
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title: Our Moon: How Earth's Celestial Companion Transformed the Planet, Guided Evolution, and Made Us Who We Are
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publisher: Random House |
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date: |
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page(s): 55-56 |
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