Ken Tapping, October 5, 2005

For the next couple of weeks, we'll have the chance to see something intriguing, elusive and quite beautiful – the Zodiacal Light. It's quite easy to see. In the west, 30-60 minutes after sunset twilight, or in the east, 30-60 minutes before dawn twilight, you should see a ghostly glowing cone streaming up the sky, looking rather like the Milky Way. Through binoculars the glow will be brighter but still just a glow, whereas the Milky Way can be seen to consist of countless stars. Obviously, this strange glow, known as the Zodiacal Light, is something completely different.

Our Solar System formed about five billion years ago from the collapse of a huge cloud of gas and dust. We see lots of these clouds scattered around our galaxy and other galaxies, and other "solar systems" in the process of forming. The core of the cloud condensed to form the Sun. The impact of the infalling material compressed the embryo Sun to the point where it became hot enough for nuclear fusion to start, and the new star began to shine. The rest of the collapsing cloud material formed a huge disc about the young Sun. Some of that material condensed into lumps too small to become stars. They became the planets. Most of the rest of the material was blown away by the solar wind.

The remainder was left as a sparse but dusty disc around the Sun. However, without being topped up with new material, that disc would not last long, no longer than a few thousand years. We see one way the material is lost every night, when dust particles burn up in our atmosphere as meteors or shooting stars. There is a reservoir of left over construction material out in the remote outer reaches of the Solar System, much of it in the form of lumps up to a few kilometres across. Every year many of these get deflected into new orbits that take them into the inner Solar System, where we see them as comets. What we call a comet is the Sun interacting with the dust and gas coming off the lump as it is evaporated by the Sun's heat. Eventually, much of that material ends up in the dusty disc giving us the Zodiacal Light.

Since the Earth and other planets all formed from that rotating disc of gas, we see the planets all moving around in the belt of sky in the plane of the disc. This belt is called the ecliptic, and happens to pass through twelve (actually thirteen) constellations, known as the Zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, (Ophiuchus), Sagittarius, Capricornus, Aquarius, Pisces. Obviously the dust disc lies in this belt too, so we see it as a ghostly glow extending along the Zodiac, hence the name Zodiacal Light.

The reason we cannot see the Zodiacal Light throughout the year is that it is faint, and we need to have the ecliptic coming up from the horizon as steeply as possible. This happens in February-March and September-October. To see it best, get right away from city lights. Fortunately we get lots of crystal-clear nights this time of year, ideal conditions for seeing this elusive spectacle.

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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