TUNGSTEN VALLEY — “The dark side of the moon.”

That catch-phrase for "remote" is burned into popular culture, from everyday conversation to innumerable songs and movies. Scientifically, it’s nonsense, as a perpetually-dark locale.

John Kepler corrected the "dark side" idea way back in 1634 with publication of his story about a trip to the moon, modern history’s earliest sci-fi. Arriving, the traveler sees that every place on the moon goes through a day-night cycle of 30 Earth days.

Kepler was a stickler for accuracy; his tale, featuring roasting-hot days and sub-freezing nights, was factually spot-on. While there is always a side of the moon that’s dark, every place alternately sees daylight for an identical period. At full moon, the near side is lit; at new moon, the far side is bright.

If you visit the single side of the moon always facing the Earth, our planet hangs perpetually motionless in the black sky, passing through full, gibbous, crescent and new phases. On the other side of the moon, you’ll never see the Earth at all. These are the lunar near and far sides (but not the "light" and "dark" sides).

How did this rotational lock come about?

Just after the Earth formed 4.6 billion years ago, and the proto-solar system was swarming with space junk, a body of about one-ninth Earth mass (Mars-sized) sideswiped our planet. The resulting debris cloud, a mélange of the mystery sphere and a goodly portion of the Earth’s mantle, formed a ring around what was left of the Earth.

Fairly quickly (as geology goes), that ring coalesced into our moon, at 1/81 of the Earth’s mass. Lunar minerals returned by astronauts preserve intermingled traces of the ramming-body and the Earth’s mantle.

The infant moon was about 50 Earth radii away, one-sixth closer than now. It spun rapidly; every place on its surface faced the Earth sometime during every month. Earth and moon formed gravitational-tidal bulges toward each other. Tidal deformation radiated energy as heat.

Dissipating its rotational energy, the moon locked into an energetically-favorable spin, reducing its tidal energy loss to zero: One side faced the Earth, as it took the same time to orbit the Earth as to turn once on its own axis.

Down here, we still lose energy from our own tidal bulges in oceans and "solid" crust, but not enough to lock our rotation to the moon. Some tidal energy becomes heat; some goes to the moon.

And what does the moon do with its energy windfall? It gradually climbs. The moon is now 60 Earth radii away, about one-fifth farther than when it formed. Its growing distance, 1.5 inches per year, is measured by laser beams bounced from retroreflectors situated by Apollo astronauts.

Features called rhythmites (some biological, others mechanical) preserve Earth's day length, the number of days per month, and months per year in the geological past. As recently as the Cambrian, a mere half-billion years ago, days were 21 hours long and years were 410 days long, about 14 months. The moon was five percent closer than now, looming magnificently in the sky, driving tides that were 17 percent stronger.

In 50 billion years the Earth’s spin could lock to the moon. But our sun will go red-giant and wipe out the Earth and the moon together in just another couple of billion years.

Sigh.

May Fun Astro-Fact: The moon is a tennis ball, relative to a 12-inch earth globe. Walk 10 paces from the globe with the tennis ball and look back. That’s our earth-moon system to scale.

In May Skies:

The sun begins the month in Aries, entering Taurus on May 18. At mid-month, days and nights are 14 and 10 hours long, respectively.

The moon’s dates are: Full (Flower Moon) May 1; Third Quarter May 8; New May 16; First Quarter May 23; second Full Moon (Blue Moon) May 31.

May Meteor Showers Eta Aquariids (parent, Halley’s Comet ) and Eta Lyrids (parent, comet C/1983 H1, IRAS-Araki-Alcock) peak on May 6 and 9 respectively. Aquariids are visible in Nederland between 3 a.m. to sunrise; Lyrids after 11 p.m.

Best Sky Viewing Nights (Minimal Moon): May 9-22.

Sunset (Mid-Month): Leo with Regulus is prominent, high in the south. Ursa Major (the Big Dipper) is nearly overhead. Follow its arm, arcing to Arcturus and then on to Spica in the southeast. Castor and Pollux are low in the west with Jupiter.

Midnight (Mid-Month): Vega, Deneb and Altair form the Navigator’s Triangle in the east to northeast. Scorpius with red Antares is prominent in the south.

Sunrise (Mid-Month): The Great Square of Pegasus is in the east. Arcturus sets just as the sun rises.

Mercury, moving from Aries to Taurus, is very low in the western sunset at the end of the month.

Venus, moving from Taurus to Gemini, is the Evening Star, low in the WNW at sunset.

Mars, in Pisces, is very low in the east at sunrise. 

Jupiter, in Gemini, is high in the west with Castor and Pollux at sunset, setting at midnight.

Saturn, in Pisces, rises at 4 a.m. just ahead of the sun.

Notable Space Missions: The SMILE mission (Solar-wind Magnetosphere Ionosphere Link Explorer), a joint Europe-China solar-wind research probe, should launch on a Vega-C from Kourou, French Guiana on May 19.

Frank Sanders, a spectrum scientist, takes astronomy-related inquiries at backyardastronomy1@gmail.com.