Al Stahler: Moonshine
Earth, moon and sun rarely line up. Just once a month, the moon lies on a line connecting Earth and sun: It’s then new moon. And just once a month Earth lies on a line connecting sun and moon.
Like Earth, the moon spins beneath the sun in rotisserie-mode, giving the moon both night and day. But the moon spins slowly – a day-night cycle on the moon takes a full month to complete.
That the moon’s day-night cycle takes exactly as long as it takes the moon to circle the Earth is not due to random chance. Sometime in the future, Earth will spin more slowly, and our day-night cycle will also take a full month to complete.
Circling the Earth, the moon covers roughly a million-and-a-half miles. It makes the journey in 27 1/3 days – the length of a month, when seen from outside the solar system.
A million-and-a-half miles in 27 1/3 days equates to 2,300 miles an hour.
That’s fast – why does the moon not fly off into space? What ties the moon to Earth?
Isaac Newton, having observed (perhaps having been bonked on the head by) a falling apple, realized that gravity, which pulls the apple downward, is universal – everything in the universe pulls on everything else. Moon and Earth stay together because they pull on each other, with gravity.
The moon’s pull on the Earth is visible in the tides – the ocean waters rise upward toward the moon.
And not just the waters. Rock can stretch, and the solid rock also rises upward toward the moon … not as much as the water, but the rock does bulge upward some inches, in a “land tide.”
There are no seas on the moon (though the dark parts that paint the Man in the Moon were long-ago named “seas”), but Earth pulls a land tide upward from the moon. More massive than the moon, Earth gravity is stronger, and raises a greater land tide – a mound of rock rising, not inches, but miles upward.
Since the same part of the moon always faces Earth, the land tide on the moon remains in one place, and behaves as a keel – should the moon “try” to turn its face away from Earth, Earth’s pull on the keel would “right” it, forcing the moon to continue pointing the same face toward us.
As the moon pulls upward on the waters it “sees” directly below, Earth fast spin carries that tidal bulge forward. But the moon pulls back on the bulge.
Carrying the tidal bulge against the pull of the moon, Earth’s rotation is slowed. Our days grow longer – by tiny fractions of a second – but those fractions add up.
Even as the moon slows us down, by pulling back on the bulge, the bulge pulls the moon forward – adds some velocity to the moons orbital motion.
When a spacecraft orbiting the Earth needs to move outward, away from Earth, it fires rockets to speed its orbital velocity. Earth’s pull on the moon’s land tide, pulling the moon forward and increasing the moon’s velocity, makes the moon move outward.
The Apollo astronauts planted mirrors on the moon – retroreflectors, that bounce a laser beam back to exactly where it came from. Observatories fire lasers at these retroreflectors, and measure how long it takes for the light to make the round trip.
The moon is moving away from Earth, on average, an inch-and-a-half per year.
The farther a planet lies from the sun, the longer it takes to go around. Mercury completes an orbit every three months; Mars, next out from Earth, takes almost two years; Saturn takes twenty-nine years to go around once.
As the moon recedes from Earth, it takes longer to go around. So, even as Earth’s day is growing longer, so is the month. Some billions of years from now, the moon will be so far from Earth that a month will be twice as long as it is today.
By that time – as Earth’s rotation slows – a complete day/night cycle on Earth will require two of today’s months to complete. A full Earth-day will thus take the same amount of time as the moon takes to go around the Earth.
Even as the moon will always show the same face to Earth, Earth will always show the same face to the moon. The moon will go from new to full, and back to new, but always in the same place in the sky. Earth and moon will orbit the sun – forever – in locked rotation.
As life returns to something resembling normal – we can now legally get a haircut! – please remember to nurture the immune system we’ve inherited from four billion years of evolution: Eat well, move your body, get plenty of sleep.
Al Stahler enjoys sharing science and nature with friends and neighbors on KVMR-FM, and can be reached at email@example.com.
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