TUNGSTEN VALLEY - Planets wander nightly among the stars. Mostly, they shift in the same yearly direction as the sun, west to east (called prograde). (The sun flies east to west daily, but its yearly motion shifts opposite to that, one degree per day.)

Every so often, though, each planet stops and loops backward (retrograde) for a month or two before resuming its regular prograde motion.

This month Mars is in full reverse, retrograding through Gemini as it bumps into Castor and Pollux. In late February it will resume regular prograde. It does this every two years.

I first saw Mars go backward when I was in the ninth grade. In the fall of 1975, I was regularly using a new telescope. Jupiter was great, with its Red Spot and moons. And I was tracking the Red Planet nightly as the Viking landers coasted toward it. I grew alarmed as Mars stopped on the eastern edge of Taurus in mid-November and then started moving backward. Was this real, or was I nuts? By mid-December it was in reverse at high speed, like an out-of-control car in a parking lot. It only finally stopped, near Aldebaran, a month later. Then it drove forward again. Whew! (I thought I’d seen Jupiter do the same thing a couple of months earlier, but its retrograde was small and it was in Pisces, where there aren’t any good marker stars.)

Not wanting to discuss this with anyone, I slipped off to the public library. To my relief, Sky & Telescope magazine had an article about Mars’ retrograde behavior. I read it without really fathoming it.

In failing to understand the Mars and Jupiter reversals, I was in good historical company. For over four thousand years, starting with the Sumerians, people had uncomprehendingly tracked thousands of retrograde fly-backs.

The ancient Greeks eventually developed a decent explanatory model, that lasted from 100 BC to about 1600 AD. With the earth motionless at the center of the universe, they surrounded it with planets on geared wheels. Their theoretical cosmic system nearly replicated the planets’ forward-back-forward motions. They even assembled ingenious geared machines, about the size of a shoebox, that replicated the planets’ motions when someone turned a crank on the side.

The earth does move, of course, some conservatives’ beliefs to the contrary notwithstanding. Lacking cosmic gears, why do the planets periodically reverse and go retrograde?

Don’t cheat; try thinking this through for yourself, the way people did when this problem made their heads hurt for four millennia. Here are some clues.

Clue 1: Mars, Jupiter, and Saturn go retrograde every time their direction from us is opposite the sun.

Clue 2: Ride on Nederland’s Carousel of Happiness. Watch something across the way, like the blue house on the west side of the highway.

Clue 3: As you go through a road curve, watch objects on the outside of the curve.

We’ll crack the case of the Great Retrograde Mystery next month. 

In January Skies:

The sun begins the month in Sagittarius, entering Capricorn on January 20. At mid-month, Nederland days and nights are 9.7 and 14.3 hours long, respectively.

The moon’s dates are: First Quarter January 6; Full (Wolf Moon) January 13; Last Quarter January 21; New (Happy Lunar New Year!) January 29.

January Meteors: The Quadrantid shower (parent asteroid 2003 EH1) peaks just before dawn on January 4. The Gamma Ursae Minorid shower (from an unidentified short-period parent) peaks in the pre-dawn hours of January 19.

Best Sky Viewing Nights (Minimal Moon): January 1-5 and 21-31.

Sunset (Mid-Month): The Pleiades, Orion, and Castor and Pollux are rising. Capella is bright-white in the ENE. The Andromeda Galaxy is directly overhead. The squashed “W” of Cassiopeia is a bit northward. The Great Square of Pegasus is just westward of overhead. Deneb is high in the northwest.

Midnight (Mid-Month): Orange Arcturus and brilliant white Spica are rising in the east. Leo, with burnt-orangish Regulus, is mid-height in the eastern sky. Procyon is high in the south, to the left of Orion and well above Sirius. Cassiopeia is low in the northwest.

Sunrise (Mid-Month): Altair is rising in the east. Antares, the Scorpion’s angry eye, is low in the southeast. Vega is high in the northeast, with Deneb below it and to the left. Leo is low in the west while Capella sets in the northwest.

Mercury, in Scutum (the Shield), is barely visible just before sunrise.

Venus, in Aquarius, is the Evening Star in the southwest. It passes (conjuncts with) Saturn on the 18th.

Mars, in Gemini, is up all night opposite the sun, super-bright just below Castor and Pollux. Opposition on January 16 is its closest approach for the next two years.

Jupiter, near Aldebaran in Taurus, is high in the southeast at sunset. It sets at 4 a.m.

Saturn, in Aquarius near Venus at sunset, sets at 11 p.m. Look for its Venus conjunction on the 18th.

Notable Space Missions: Firefly’s Blue Ghost lunar lander mission launches from Cape Canaveral in mid-January or later. Delivering ten science and technology instruments to Mare Crisium for NASA’s Commercial Lunar Payload Services initiative 45 days later, it will operate for a lunar day (14 earth days).

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