The Age of Aquarius

Ken Tapping, January 23rd, 2014

In the sky this week…

  • See Mercury low in the evening twilight. Jupiter still dominates the sky overnight.
  • Mars rises around midnight, Saturn at 3pm and Venus lies low in the dawn sky.
  • The Moon will be New on the 30th.

Thousands of years ago, our astronomical ancestors noticed that although the constellations moved across the sky during the night, and changed with the seasons, their patterns stayed the same. It was as though the stars were fastened to a great, dark, rotating sphere. This became known as the "celestial sphere". However, there were other objects that were clearly not fastened down (or up). The Sun, Moon and planets all move compared with the starry background. Intriguingly though, these other objects did not wander randomly around the sky, they were confined to a narrow strip, now called the ecliptic. This is because the Earth and other planets orbit the Sun in the same plane, and the Moon orbits the Earth almost in that plane too, so what we see is rather like marbles rolling around on a plate, from a viewpoint on one of the marbles. That strip of sky passes through 13 constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio (or Scorpius), Ophiuchus, Sagittarius, Capricorn (or Capricornus), Aquarius and Pisces. There was an aversion to the number thirteen, so Ophiuchus was fired and the twelve remaining constellations became known as the "Signs of the Zodiac".

The ancient picture was that the Sun, Moon and planets moved between the Earth and the celestial sphere, so we would see them with particular zodiacal constellations in the background. For example one might say that Jupiter lies in the constellation of Gemini. Just like the Earth, the celestial sphere has poles, directly above the Earth's poles and called the north and south celestial poles, and midway between them, directly above the Earth's equator, is the celestial equator. Since the Earth is leaning at an angle of about 23 degrees, the ecliptic crosses the celestial equator at two points. The result is that we see the Sun, Moon and planets moving between the northern and southern halves of the sky.

As the Sun moves around the sky each year, following the ecliptic, about 21 March it crosses the celestial equator heading north, marking the spring equinox. On 21 September, give or take a day, it crosses the celestial equator heading south, marking the autumn equinox.

When the idea of the zodiac was first set up, the spring equinox point coincided with the constellation of Aries, so this constellation became the first sign of the zodiac. This was the situation when the science of astronomy and the pseudoscience of astrology went their separate ways. Since then the situation has changed.

If you have ever played with "spinning tops", you will have found that although it is possible to get the top spinning absolutely vertical and unmoving, most of the time the top wobbles, or precesses, where the axis of the top describes a circle. The same thing applies to the Earth. Since the Earth is not absolutely round and there are other objects such as the Sun and Moon pulling at it, the spinning Earth precesses too, with a complete wobble taking 25,800 years. Precession makes the intersections of the celestial equator with the ecliptic change, so that compared with the calendar the zodiac slides backwards one sign every 2150 years or so. So today instead the spring equinox point being in Aries, it has slipped back into Pisces, making that constellation the current first sign of the zodiac. So some 2,000 years ago it was the "Age of Aries". Now it is the "Age of Pisces". In some 2,150 years or so from now, we will have slipped back one more sign, and we will have arrived at the "Age of Aquarius". Of course, by then it's likely few will remember the song. In about 23,650 years, the first sign will be Aries again, and the cycle will start over.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

Date modified: