Science: The Stats - The illusion of the ecliptic

As comet Hale-Bopp fades from view, Heather Couper and Nigel Henbest explain why our solar system is in the shape of a disc with the Sun at its centre

For the past month and more, we've perhaps become rather blase about having a beautiful comet gracing the evening sky. Comet Hale-Bopp has lived up to all our expectations, retrieving the reputation of comets and astronomers alike after the fizzlers of Kohoutek and Halley (though, to be fair, no astronomer expected a great show from Halley; it was a victim of media hype). We've been blessed with the comet of a lifetime.

Alas, it's now on its way out. Hale-Bopp has been fading over the past few weeks, and is now heading down south and into the evening twilight glow. It's good news for people in the southern hemisphere, who have missed out on the comet at its best, and will now get to see at least something of Hale-Bopp before it heads back into obscurity in the far reaches of the outer solar system.

As you can see from the chart, early in May Hale-Bopp's track takes it across the imaginary line we've marked as "path of the moon and planets" - technically known as the ecliptic. Comets are one of the few denizens of the solar system that seem to stray from this track through the sky.

If we could strip away the bright daytime sky, we'd see that the sun, too, is always found on the ecliptic. During the course of the year, it seems gradually to trundle around the sky. That's why the names of the constellations along the ecliptic, such as Taurus, Gemini, Cancer, Leo and Virgo, sound familiar. About 5,000 years ago the Sumerians invented these constellations - or "signs" - as markers around the ecliptic, one for each month, just as we have numerals around a clock dial. Because so many of the constellations represent animals, the Greeks called them the zodiac, meaning "band of animals".

The Sumerians also believed that the influence of the sun, moon and planets changed as they moved from one zodiacal sign to the next. The whole basis of astrology is, of course, nonsense - as you could readily see if the stars really were visible during the day. Because the earth's axis swings around gradually in space, the sun's position has changed since astrology was invented. If you are born in early May, astrologers will tell you that your "sign" is Taurus. You'd in fact see the sun in Aries on your birthday.

Why this magnetic attraction for the ecliptic? It's all a matter of geometry. The earth and the other planets circle the sun in a flat disc; seen from outside, the orbits of the planets lie in the same plane. This is a relic of the planets' birth. They condensed from a disc of gas and dust shaped like a CD, with the sun in the hole in the middle. Living within this flat distribution of planets, we see their orbits edge on, so they seem to follow the same line through the sky.

Only in its outer parts did the early constituents of the solar system stray from the thin and narrow. Pluto has an orbit tilted by 17 degrees from the other planets. So this month, for example, Pluto lies in the constellation Ophiuchus, well away from the ecliptic. Comets such as Hale- Bopp start their life further out still, in a vast, spherical cloud around the sun called the Oort Cloud (after the Dutch astronomer Jan Oort who predicted its existence). Hale-Bopp's orbit is tipped at right angles to that of the planets; its swoop over the earth's north pole, before diving down to the ecliptic, was responsible for the grandstand view we've been having.

Diary (all times BST, 24-hour clock)

6 21.47 new moon

14 11.55 moon at first quarter

22 10.14 full moon; Mercury at greatest western elongation

25 Pluto at opposition

29 08.52 moon at last quarter

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