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Monday, January 17, 2011


Find Your Zodiacal Constellation – A Teachable Moment.

Left panel, northern horizon as seen from Adelaide at 22:00 daylight saving time on Monday January 17 showing the stars that form some of the zodiacal constellations (similar views will be seen elsewhere in the southern hemisphere at the equivalent local time. Right panel, stars with constellation figures overlain (click to embiggen).

One of the reasons that the recent zodiac kerfuffle became a news item is that, amongst other things, most people don’t know that the star sign in their daily newspaper horoscope is not actually associated with the constellation of the same name (and hasn’t really been so since around 1600) and most people can’t find their zodiacal constellation in the sky.

But this is a teachable moment. Most people do know their star sign, and although astrology is rubbish it would be fun to try and find your star sign in the sky. Finding a number of the zodiacal constellations can be difficult as they are dim and hard to see in our light polluted skies, but it’s worth the effort.

If you go outside tonight at 10pm local daylight saving time, Southern hemisphereians can see five of the 12 Zodiacal constellations (Pisces, Aries, Taurus, Gemini and Cancer). Tonight (January 17), if you look due north you will see a bright red star which forms part of a V shaped group of stars. This is the head of the constellation Taurus, the bull. To the right you will see the Moon. It is between the two stars that from the tips of the bulls horns. To the left you will see a small pretty cluster of stars, the Pleiades (well worth viewing in binoculars). The diagram above shows what you should see, and has the constellation figure shown as well on the right (for northern hemispherians, look south, the constellations will appear upside down to the depictions shown here).

Diagram showing the zodiacal signs in relationship to the constellations, The yellow band is the zodiacal sign location, and the red block is the zodical sign of Taurus in the newpaper horoscopes (click to embiggen).

That’s the constellation of Taurus, but not the zodiacal sign of Taurus. The horoscopes you read in the papers start the astrological zodiacal signs from the position of the sun at the Vernal Equinox (in the northern hemisphere). Due to the precession of the equinoxes, as the Earths axis of rotation changes its direction like a spinning top, the location of the Vernal Equinox has shifted by 23 degrees over the last 2000 years. As well, the astrological zodiacal signs are not associated with the boundaries of the constellations, but are slabs of sky 30 degrees long by 14 degrees wide[1].

So the zodiacal sign of Taurus (indicated by symbolic bulls horns (), now covers the constellation of Aries (and a bit of the tail of Cetus, the whale, and the left leg of the bull). In fact the three brightest stars of Aries lie entirely outside the chunk of sky that makes up the zodiacal sign of Taurus. In the diagram the zodiacal constellation band is shown in yellow, and the zodiacal sign of Taurus in red.

The constellation of Taurus is covered by the zodiacal sign of Gemini (symbolized by two parallel lines () . The Constellation of Gemini is to the right of the constellation of Taurus in the north-east, the bright stars Castor and Pollux (the twins) being easily visible (tomorrow night, January 17, the Moon will be in Gemini, and on the 19th it will be in cancer). The zodiacal sign of Cancer (symbolized by stylised crab claws ()) covers the constellation of Gemini (excluding the star Castor), bits of the constellations Orion, Auriga and a sliver of Cancer.

And so it goes; the constellations that symbolize the zodiacal signs are no-longer associated with associated with those signs. Not that it matters, the properties of the zodiacal signs are purely imaginary constructs, but it is worthy of reflection that the zodiacal sign of the mighty, charging bull is now represented by an unprepossessing collection of faint stars (as is spectacular Scorpio).

Still, getting out in the fresh air and seeing the constellation that gave your zodiacal sign its name over 2000 years ago is a good excuse for some sky watching.

[1] There are 4 different major forms of astrology, some set the zodiacal signs by the vernal equinox, some don’t, the newspaper ones do.

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