Now is the Dawning...

… of the Age of Aquarius. Though the song may have been a bit premature, it is true that somewhere in the next 150 years, the Age of Pisces will end and Aquarius will take over. With threats of rising sea levels, the easily persuaded may take that as some sort of sign. Of course, Aquarius is a sign – of the zodiac. But its significance as the source of an age is astronomical, not astrological.

The constellations that lie along the path the sun takes are of various sizes and so occupy different chunks of the zodiacal circle. The Aquarius referred to here is the astronomical division of the ecliptic into 12 identical portions of 30 degrees each and given the name of the nearest constellation, whatever its size.

So why is the Age of Aquarius dawning? It's all to do with precession – that tip in the earth's axis that makes it spin like a top whose spindle has been given a nudge. Try it with a top and you'll see the top of the shaft actually traces out a slow circle in the opposite direction to the way you set it spinning. The earth does that too, with the axis through the North Pole at present pointing to the Pole Star, but it won't be for that much longer.

Precession is best, if only very slowly, observed at the time of the spring equinox. Just before the sun rises at 6am, due East, there is a constellation in the sky above it that effectively leads the sun up to start a new day. For the last couple of thousand years, Pisces has had that honour but soon the age of the fish will end and Aquarius will take over.

Since one complete lap of the zodiac takes about 26,200 years, in roughly one lifetime (just over 74 years), the sun will have precessed by one degree, which may be a cause for special birthday celebrations at 75 – or a new government-imposed retirement age?