Ken Tapping, March 7, 2007

For the next couple of weeks, assuming we get any clear, dark nights, it will be worth having a good look at the western sky after sunset, as soon as it's dark. What you will be looking for is a glimpse of some of the stuff left over from the birth of our Solar System. There is a fair amount still out there.

About 4.5 billion years ago a huge cloud of dust and gas collapsed. From a shapeless cloud it became a rotating disc. The centre condensed to form a star – the Sun. Some of the rest condensed into smaller objects that orbited around the new Sun: the planets. In the outer parts of the Solar System, the dust condensed into a vast collection of smaller objects that never became planets, and pervading the rest of the Solar System is a vast amount of dust that never became part of anything.

Compared with even house dust moving in the air currents, these interplanetary dust clouds are very rarefied. However space is on the large side, and even a very rarefied cloud adds up when you are looking through millions of kilometres or more of it. This is why over the next couple of weeks we will have a chance to see some of the oldest and most important stuff in the Solar System.

Because all the bodies in the Solar System formed from that primordial disc, all the planets are still moving in the same plane, like marbles rolling around a plate, along with what's left of that dust. From our viewing point on one of the planets embedded in the disc, we see all the other planets always moving in a single strip of sky. This strip passes in front of 13 constellations, so that we see for example Saturn moving in front of the constellation of Leo. These constellations form the constellations of the Zodiac, which are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius and Pisces. We have an aversion to 13, so our ancestors quietly forgot about Ophiuchus and left us with the 12 constellations of the Zodiac that we are familiar with today.

At this time of the year the path of the Zodiac, which we non-astrologers call the ecliptic, rises up from the western horizon at a steep angle. With some luck, together with a clear and dark evening, as soon as it is reasonably dark, look into the west where the Sun went down. You should be able to see a sort of pyramidal glow extending from the sunset point up into the sky. It looks rather like a faint and uniform piece of the Milky Way. It isn't; it is the zodiacal light – the sunlight scattered in your direction by countless dust particles spread along the ecliptic. This scattering is strongest when the light is deviated by only slightly from the original direction – forward scatter, and when it is sent back in the original direction – backscatter. The forward scatter gives us the glow in the west, close to the line of sight to the hidden Sun, and the backscatter can cause a fainter loom in the eastern sky, opposite the Sun. This is called the gegenschein (counter glow), which is harder to see. It is well worth the effort to see the zodiacal light. Its appearance is quite magical, especially when we consider what we are really looking at.

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Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca