The slipping equinox

At 18:21 EST, or 15:21 PST on March 20 the Sun crossed the equator heading northwards, marking the spring equinox and the beginning of spring. Last year’s spring equinox was a little under six hours earlier. The trend of six hours earlier a year as we go back in time continues to 2008. In 2007 the equinox was over sixteen hours later. In the years before that it was once again slipping nearly six hours a year. Why does the beginning of spring jump around in this strange way? The explanation comes from the equinox being tied to the Earth’s orbit around the Sun and not to the calendar.

The Earth rotates on its axis once a day, like an orange spinning around a knitting needle that is stuck through it. However that axis is not at right angles to the plane of the Earth’s orbit around the Sun; it is inclined at an angle of about 23 degrees to the vertical. In addition that axis points in the same direction all the time. That is why we have a Pole Star. That means that as the Earth moves around its orbit, it goes through a point where the Earth is leaning towards the Sun, and six months later a point where it is leaning away. Between those extremes, called the summer and winter solstices, there are two intermediate points where the Earth is neither leaning toward nor away from the Sun. These are called the equinoxes, and turn up at particular points in the Earth’s orbit. It is the Earth’s axis leaning towards and away from the Sun that gives us the rhythm of the seasons.

Ever since we tried to set up a calendar that works consistently we have had to deal with the fact that the Earth does not take a whole number of days to orbit the Sun. It takes 365.242 days. That is, the year is a little less than a quarter of a day longer than the standard 365-day calendar. This makes our calendars slip 0.242 days each year compared with the Earthís orbit. We cannot allow this to continue otherwise the dates of festivals related to astronomical events would slip all over the place. For example, Easter is defined as the first Sunday after the first Full Moon after the equinox, and Christmas is a few days after the winter solstice. Christmas is a rededication of an ancient solstice celebration, with a bit of calendar slippage. So every fourth year we add a day to the calendar, in February, before the spring equinox, which sets everything back a day. Then everything starts to slip again. Of course 0.242 is less than a quarter of a day, so we occasionally need finer adjustments.

Actually, the Earth’s axis is not always pointing in the same direction, it executes circular wobbles. Each complete wobble takes 26,000 years. This does not sound  like much, but it has some strange consequences. When early astronomers studied the skies thousands of years ago, the spring equinox point lay in the constellation of Aries, making that constellation the first sign of the Zodiac. However, since then equinox point has slipped into Pisces, giving us a new “first sign”. By 2600 AD, the equinox will have slipped back one more constellation, marking the “Age of Aquarius”.

Mercury lies low in the west after sunset. Jupiter is getting lost in the glare. Saturn rises around 8 p.m; Venus is very low in the dawn twilight. The Moon will reach last quarter on the 26th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

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