The 13th ‘sign’ of the zodiac
RECENTLY, we have seen the story that a new constellation called Ophiuchus, The Serpent Bearer, has been “discovered”, and that this implies major changes for the way we think about the constellations of the zodiac. However, nothing has changed at all. The Sun and planets are still moving against the backdrop of the same constellations, as they have done for a very long time.
I have received questions about how this affects the “star signs”, but I must say firstly that astronomy and astrology are, of course, two different things. Astronomy is a science, whereas astrology is based on age-old beliefs that people and events can be affected by the positions of Solar System bodies against the starry backdrop.
Astrology has no scientific basis at all, but the recent headlines do offer me a good opportunity to clear up some misconceptions.
Constellations are not actual physical groupings of stars. They are merely directions in the sky in which stars are said to form patterns resembling objects, characters and creasizes, tures. Our modern “official” star charts show the sky divided into 88 constellations. The Southern Cross is a fine example, but it looks like a cross only because of the particular directions in which we see each of its stars.
Quite apart from the main 88 constellations, people from different cultures, including Australian Aboriginal people, have imagined other patterns that are important to them.
The stars are all at different distances from us, so if we were to move even a fraction of the distance across our galaxy they would form quite different patterns, because we would be looking at them from a different location. Not only that, but most constellations look nothing like what they are named after. Indeed, many of them were named without regard to any pattern that the stars made. Therefore, constellations do not get “discovered”.
One of the headlines that I saw a few days ago read: ‘ZODIAC CALENDAR OVERHAULED AS NASA DISCOVERS NEW STAR’. This even ignores the fact that all of the fuss is about a constellation, not a star.
The 12 “star signs” of the zodiac do exist as actual constellations in the sky. These constellations have different but the astrological zodiac does not even pretend that they do.
The one-month periods that astrologers use are convenient 30-degree divisions, based on a circle having 360 degrees and the year having 12 months (12 x 30 = 360). The actual borders between the constellations were originally quite vague, but they were defined exactly in 1930 by the International Astronomical Union. Many of them have very odd shapes.
The apparent path of the Sun, called the ecliptic, actually passes through 13 constellations, with part of the 13 one — called Ophiuchus — being tucked in to the north of Scorpius and to the west of Sagittarius.
The Sun spends longer in Ophiuchus than it does in Scorpius. Its time within the boundaries of Scorpius is just a few days.
If the Sun’s light did not prevent us from seeing other stars in the daytime, it would be far more clear that the line of the ecliptic misses most of the main pattern that resembles a scorpion. This can be easily demonstrated in a planetarium. And quite apart from that, the direction of the Earth’s axis gradually changes over time, meaning that, for example, right now the Sun appears within the constellation of Cancer, not Leo.
The fact that astronomers still refer to constellation names is sometimes mistakenly taken to mean that astronomy and astrology are the same. They are not, although there was less of a division between the two a very long time ago, before the world took a more scientific approach to astronomy.
Astronomers have no scientific need for the constellations, but still refer to them, and note their borders on charts, for traditional reasons. They have inspired beautiful artwork on celestial maps, and watching for the more familiar patterns helps people to find their way around the sky.
You could start this evening by watching for The Scorpion, high in the east in our Tasmanian sky.
Martin George is manager of the Launceston Planetarium (QVMAG).