Last month, our resident astronomer Luna answered a fan letter with a puzzle: Although we see the moon going around us in a monthly circle as we stand on the earth, what does the moon’s path through space look like to someone who’s standing on the sun? That is, if someone is floating “still” in space, far from the earth and moon, and they watch the earth-moon pair go past them, what would the moon’s path look like? A set of looping circles, like a Slinky® toy or a Spirograph® sketch? Or a series of semi-circular ins and outs? Or something else?

This month, I interview Luna for her answer.

F: The answer is…?

L: It’s Possibility “C” from last month. The moon doesn’t go in any kind of circle(s) around the earth at all. It follows its own, independent orbit around the sun; a slightly perturbed, single big circular solar orbit!

F: You’re saying, the moon’s monthly circular rotation around the earth, as we stand here and look at it, is an illusion?

L: It’s a total illusion. The moon never loops back toward the earth, ever. Its path is always concave, around the sun. It’s the only moon in the solar system which does that. Its path is a solar orbit, just slightly perturbed by the neighboring earth.

F: What makes our moon so unique?

L: The reason is, the sun’s gravity force on our moon is more than twice as large (2.25 times larger, to be exact) as the force the earth exerts on it. The sun’s gravity totally dominates the moon’s path. (Write to this column’s e-mail for the math.)

F: Which is because?

L: The moon is quite far from earth. The moon’s distance is 30 times the earth’s diameter. That’s way, way out there.

F: Can you help us visualize that distance?

L: Find a standard 12-inch classroom globe. Then get a tennis ball (or for an even closer scale, a 3.25-inch RhinoSkin®Super-Bounce ball). Your sports-ball is the size of our moon compared to the globe. Then, walk 30 feet (10 goodly paces) from the globe with your little moon. Turn around and look back at the globe. That’s how far the moon is from the earth.

F: Where’s the break-even point, where the moon would be equally attracted by the earth and sun?

L: That would be at 20 earth diameters, two-thirds of the moon’s actual distance. With your moon-ball that would be 20 feet (about 7 paces) from the classroom globe.

F: How did this weird situation evolve?

L: The moon was originally formed in a collision of an independent, sun-orbiting, Mars-sized object with our planet, about 4.5 billion years ago. The original earth-planetoid pair had a colossal amount of rotational motion energy (called angular momentum), which the moon retains to this day. It’s still independently orbiting the sun, just like its originating planetoid did.

F: Final thoughts?

L: What looks totally like one thing (a circle, for example) from one point of view can look massively different (maybe no circle at all) from another perspective. Perhaps that’s a good thing to remember about life in general.

In January Skies

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

The moon’s dates are: Last Quarter January 3; New January 11; First Quarter January 17; Full (Wolf Moon) January 25.

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 3-17.

Sunset (Mid-Month)

The Pleiades, Orion and Castor and Pollux are rising. Capella is brightwhite in the ENE. The Andromeda Galaxy is directly overhead, with Cassiopeia (a squashed “W”) 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 midheight 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 and Scorpius’ 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 is visible in the east, just before sunrise, January 8-12.

Venus is moving down, into the sun, rising in the SSE at 5 a.m. Look for its conjunction with the crescent moon on January 8.

Mars is lost in the sun.

Jupiter, in the Ram, is overhead at sunset, setting at 1:30 a.m. It ends its yearly retrograde motion this month, conjuncting with the moon on the 18th.

Saturn, in the Water Carrier, is low in the SW at sunset, setting by 8:30 p.m. Look for its lunar conjunction on the 14th.

Notable Space Missions

Astrobotic’s robotic Peregine lunar lander is scheduled for launch from Cape Canaveral on a United Launch Alliance Vulcan-Centaur booster on or after January 8; its moon landing will be in early 2024. The Japan Space Agency’s Smart Lander for Investigating Moon (SLIM), launched in September, will attempt a lunar landing this month.

Frank Sanders, a spectrum scientist at the U.S. Department of Commerce in Boulder, takes astronomy-related inquiries at backyardastronomy1@gmail.com.