The Sandoval Signpost

An Independent Monthly Newsmagazine Serving the Community since 1989


Looking straight up at 9:00 p.m. on February 15. North is at the
top of the chart. Image generated by Cartes du Ciel, Version 3.0.

Save our skies

Be a considerate neighbor:
Reduce nighttime glare.
Shield all your outside
lights downward
(or turn them off completely)
and enjoy the beautiful,
starry night sky.

Night Sky


This month, we will return to the Zodiac and explore one of its members—the Twins. Gemini is above and to the northeast of Orion, and is easily visible in the winter nighttime sky. February is a great time to brave the cold and admire this grouping of stars. This constellation lies right along the Milky Way and the ecliptic, the region in which the Sun and planets roam.

The heads of the Twins are the bright stars Pollux, which is yellowish, and Castor, white and a bit dimmer than Pollux. Pollux is the sixteenth-brightest star in the sky; Castor is the twentieth-brightest. But if you take a closer look at Castor, it is actually a grouping of six stars all bound to one another by gravity, forming one of the most remarkable examples of a multiple-star system in the heavens. Even at fifty light-years away, a modest telescope will show a pair of similar stars only a couple seconds of arc apart. The brighter, Castor A, is mid-second magnitude (1.9), the fainter, Castor B, mid-third (2.9). About an arc minute away to the south lies a third companion, Castor C, at a faint ninth magnitude. The bright pair are in elliptical orbit about each other with a four-hundred-year period, and are now about as close in the sky as they can get. The dim companion is relatively far away from the bright pair, about one thousand times the distance between the Earth and the Sun.

With a larger telescope and spectrographic equipment, astronomers have found that the two bright components, Castor A and Castor B, are both double stars. Castor A’s stars are almost identical to each other. Both are about two solar masses, and circle each other every 9.2 days. That makes them about a tenth of Mercury's distance from the Sun apart. Castor B's twin stars are real speed demons, making their circuit around each other in a mere 2.9 days. And, as a last hurrah for Castor, the faint distant member, Castor C, is also double. It, too, consists of two much cooler low-mass dwarfs. These two are only about two sun diameters apart and orbit each other every twenty hours. It seems ironic that one of the Gemini Twins is itself three sets of twins, or should I say sextuple.

Pollux is the sixteenth-brightest star in the sky and is about thirty-four light-years away from Earth. This star is a typical red giant that is quietly fusing helium into carbon and oxygen in its deep core. Perhaps Pollux's greatest claim to fame is an orbiting planet. This is unusual because it is one of the very few red giants with a planet, and it is the brightest star with a planet. The planet’s mass is at least 2.9 times that of Jupiter and orbits in near-circular orbit at a average distance of 1.69 astronomical units in a 1.6-year orbit. That would place it just outside of Mars’s orbit if the planet were in our solar system. From the planet, Pollux would seem to be 5.7 times bigger than the Sun in our sky.

One other interesting object located in Gemini is its Zeta star. Zeta Gemini is the most distant star in the constellation, about twelve hundred light-years from Earth. It is a Cepheid variable star that pulsates, expanding and contracting with a regular period. Every ten days, Zeta’s brightness changes between its dimmest magnitude, of 4.2, to its brightest magnitude, of 3.6.
According to mythology, the Twins were the sons of Zeus and Leda and brothers of Helen, whose face launched a thousand ships and caused the Trojan War. Ancient mariners regarded Pollux and Castor as the patrons of seafarers, and in Elizabethan times they were also considered the protectors of all at sea. In astrology, Gemini is the third sign of the Zodiac and is associated with storytellers, communicators, and youth.

The middle of January held a surprise visit by Comet McNaught. It turned the corner around the Sun on January 15 inside the orbit of Mercury. The heating from the Sun made the comet burst to life and become visible in the evening sky if you had a clear view of the horizon. Even now, if you were extremely careful, you could use binoculars to see the comet in daylight—that is a bright comet! Warning: never look directly at the Sun with your naked eyes and especially not with binoculars. Unfortunately, as the comet rounds the sun, it will be great viewing for the Southern Hemisphere. All we New Mexicans can do is look at the pictures. A great gallery of pictures can be found at

• Mercury may just peak above the eastern horizon before sunrise the last week of the month.
• Venus will be easy to locate in the west after sunset; it will be the brightest thing in the sky.
• You will need to be an early riser to see Mars late this month. It will rise just ahead of the Sun starting around February 21, but you will need a clear view of the horizon.
• Look for Jupiter to rise after midnight in the east. The Moon will rise with Jupiter about 3:00 a.m. on February 12.
• Saturn rises about 8:00 p.m., with the Moon just above it on February 2. On March 1, you can find the Moon and Saturn almost overlapping about 9:00 p.m.
• The Moon is full on February 2 and new on February 17.

If you have a question or comment for Charlie, you may e-mail him, at


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