Eye Astronomy #7: Three Out of Four Ain’t Bad
By: Dale E. Lehman
Even if you know nothing about astronomy, you probably know about the band of sky called the zodiac, famous as the province of astrologers. We in the astronomical community take a dim view of astrology, of course. It’s particularly annoying when people confuse the two, as happened last year when an event’s organizers identified us as the Harford County Astrological Society. Oh, boy.
Still, we get the last laugh. Astrologers use the zodiac to cast horoscopes because of an astronomical fact: the planets follow orbits that all lie in about the same plane in space. As a result, the sun, moon, and planets all traverse the same narrow band of sky as they move against the background of stars. That band is the zodiac.
The zodiac contains twelve constellations. According to how the ancients divided the sky, the sun moved through those twelve over the course of the year: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius, and Pisces. Modern constellations are drawn differently, so the ecliptic, the path of the sun through the sky, now crosses thirteen constellations, the intruder being Ophiuchus, a little of which lies between Scorpius and Sagittarius.
But let’s focus on just four of the zodiacal constellations. These are all pretty well up in the night sky right now, mid-February.
Taurus, the bull, lies above Orion’s head. The bright red star Aldebaran marks the creature’s eye, while his horns extend toward the bottom of Auriga, the charioteer. (Auriga is home to another bright star, Capella, the sixth brightest star in the sky.) Within Taurus hides one of the most fascinating astronomical objects, the Crab Nebula, the remnant of a supernova that lit up the sky in A.D. 1054 and was recorded by Chinese astronomers. Unfortunately, the Crab Nebula is way too faint to see with your eyes. As compensation, Jupiter is in Taurus right now, outshining all the stars.
Moving east from Taurus, we come to Gemini, the twins, marked by the bright stars Castor and Pollux. Although they’re usually written in that order, Pollux is the brighter of the pair. Mars currently sits in the midst of Gemini, bright and orange-red. If you connect Mars and Jupiter with an imaginary line, that line will be roughly mark the plane of the Earth’s orbit. Of every planet’s orbit, in fact. It can also guide you to other constellations of the zodiac.
As we continue east, we arrive at Cancer, the crab. Well, maybe. Don’t take it personally if you pass right over it and bump into Leo instead. Cancer is home to zero bright stars. Zero. Its brightest is Tarf at magnitude 3.5, which is faint enough to be a challenge to find under many suburban skies. Finding three constellations out of these four ain’t bad.
Cancer does have one gem, though, visible to the eye under dark skies and with binoculars otherwise: the Beehive Cluster, also known as Praesepe, the Manger. Only about 577 light years away, the Beehive is a collection of stars that were all born together and haven’t yet drifted apart. It’s home to over 1,000 stars.
Finally, climbing up the sky in the east we reach Leo, the lion. Leo is one of those constellations that looks like what it’s supposed to be. Big and reasonably bright, it sports an obvious body and a sickle of stars for its head. Lying beneath the bowl of the Big Dipper, it’s marked by the bright star Regulus.
As well as being part of the zodiac, Leo occupies an interesting position in the galaxy as seen from Earth. The plane of the Milky Way—the thickest part of the galaxy—runs up along the top of Canis Major and between Orion and Gemini. As we move eastward from Gemini, we look up and out of the galaxy. With less interference from the dust in the spiral arms, we can begin to see other galaxies. The region from Leo to Virgo bristles with galaxies, but of course you’d need a telescope to see even the brightest of them.
Still, much of the joy of astronomy comes not only from seeing but from understanding what you’re seeing. Even when you can’t spot objects like the Crab Nebula, the Beehive, or the galaxies hidden within Leo, you know they’re up there, and that knowledge can increase your awe at the grandeur of the universe.
