Ophiuchus Part 2 — Are You A Serpentarius?

Should Ophiuchus become a member of the zodiac? The sun passes through Ophiuchus for nearly three weeks every starting every November. Stellarium with additions by the author

In ancient times, Ophiuchus, the constellation we met yesterday, also went by Serpentarius. That name rings true since he crosses the sky with a large snake coiled around his legs. After the constellation boundaries were formalized in 1930, a section of the sun’s yearly path that once led seamlessly between the zodiacal constellations Scorpius and Sagittarius, now passed through southern Ophiuchus. The sun enters that constellation on Nov. 30 and departs it for Sagittarius on Dec. 18, a span of nearly 3 weeks. That’s almost long enough to make an argument for Ophiuchus as the 13th sign of the zodiac.

The 12 traditional zodiac signs represent the constellations the sun passes through during Earth’s yearly orbit. Wikimedia Commons

Your astrological sign is determined by which of the 12 constellations the sun was in on the date of your birth. So if you were born at the end of July, you’d be a Leo. Unfortunately, the sun isn’t actually in Leo on that date … anymore.  A couple thousand years ago, when Rome was a world power, the sun shone from Leo on July 31 and passed through Sagittarius from late November to late December. Not anymore. Because of precession, it’s now in Sagittarius from December 17th to January 20th. All the other signs are likewise off about a month. So if you’re a Leo like me, your 21st century sign is Cancer instead.

The slow, cyclic wobble of Earth’s axis called precession resembles the motion of a top slowing down and with the axis making one wobble or “circle in the sky” every 26,000 years. The wobble causes the pole star to change and the sun to move westward along the zodiac. Earth and Planetary Magnetism Group ETH-Zurich

Precession is the slow wobble of Earth’s axis over a period of 26,000 years caused by the combined gravitational tugs of the sun and moon. As a spinning top slows down, you’ve probably noticed that the axis of the top describes a little circle (wobble) in the air before eventually tipping over. The Earth’s axis traces out a similar circle in the sky. Since the pole star is determined by where our axis points, it follows that the pole star will shift position and change in sync with that wobble. Right now, Polaris in the Little Dipper occupies the hallowed spot at which our axis points, but in 14,000 A.D., brilliant Vega will displace Polaris as the pole star. Due to the cyclic nature of precession, Polaris will return as the North Star again in 28,000 A.D.

The same wobble also causes the sun to drift westward along the zodiac 1.4° (about three sun diameters or an index finger held at arm’s length) per century. In 2,000 years, that adds up to 28° or about one zodiac constellation width. Astrology practitioners stick with the sun’s position two millennia ago, causing a disconnect between the Sun’s true position and one’s birth sign. In the meantime, if you feel your zodiacal rights are being compromised, and you’d like to identify as a “Serpentarius,” go for it. We live only once.

Current astrologers would argue that zodiac signs are not so much constellations as a cyclic series of changes in a person’s life. That’s fine, but since the essence of astrology concerns the movements and positions of the stars, sun and planets and their supposed effects on people’s lives, astrologers would seem to want it both ways — is it about the stars or isn’t it? For an interesting essay on the topic, click here.

The area covered by Ophiuchus is shown in white. Several globular clusters are marked on the map as circles with crosses in them. M10 and M12 are the easiest and brightest. IAU

Ophiuchus lies along the fringe of the rich summertime Milky Way and is home to several nice star clusters you can see as little blobs of fuzzy light in binoculars or as rich heaps of close-packed stars in a telescope. Several are marked on the map, but the brightest and easiest are the two labeled M10 and M12. Each is a globular cluster, so called because they look like “globes” or balls of stars. Most are spherical and contain tens of thousands of stars.

M10 and M12 are the jewels of Ophiuchus. Both are globular clusters that orbit in a halo around the core of the Milky Way galaxy 14,300 light years (M10) and 15,700 light years (M12) from Earth. Globulars really come into their own in 8-inch and larger telescopes. Hunter Wilson

2 Responses

    1. astrobob

      Hi David,

      Thanks so much – very happy to hear you like the book! I will contact you via e-mail. Thanks for dropping by.

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