Over The Horizon

Watch the Sky!

Alan Gilmore

Keep an eye on the sky in mid-November, low in the sky before dawn, and you may be fortunate enough to see the remains of a comet send thousands of meteors across the sky.

Meteors are specks of dust shooting into the Earth's atmosphere at great speed, leaving burning trails behind them. Most meteors burn out more than 70 km from the ground. Clumps of dust produce bright "fireballs" that disintegrate in a flash as they hit denser air.

As comets near the sun, the ice in the nucleus evaporates, carrying off dust. Sunlight pushes the fine dust away from the head, forming a tail. Coarser dust stays close to the comet at first, only slowly spreading around its orbit. If we pass near one of these dust streams then the number of meteors increases. The meteors, descending on parallel paths, appear to radiate from one place in the sky; meteor showers are named for the constellation that they radiate from.

In mid November we pass close to the orbit of Comet Tempel-Tuttle. Its meteors radiate from Leo, hence they are called the Leonids. The comet orbits the sun in 33.25 years. So every third of a century we pass through, or near, the denser cloud of dust near the nucleus. The number of Leonids can then increase from a dozen per hour to hundreds. If the Earth crosses the centre of the meteor cloud, then the number can jump to thousands, but only for an hour or two as the meteor shower becomes a "storm".

On the morning of Wednesday November 18, we cross the Leonid stream just 257 days after Comet Tempel-Tuttle passed the same point. So there is a chance that a large number of meteors might be seen, at least from those places where Leo is high in the sky. Unfortunately for New Zealand, the Lion does not rise until an hour before dawn, and so is still low in the northeast when twilight begins. The best time to watch is between am, when the radiant rises, and 5 am when twilight begins. If you're enthusiastic, then the mornings before and after would be worth checking.

There is no guarantee that you'll see anything worth the missed sleep. The meteor storm, if it happens, is not expected until around 9 am, long after sunrise. And there is much disagreement among the experts as to the chances of a good shower. Spectacular showers were seen -- from the northern hemisphere -- in 1799, 1833 and 1966; the latter saw a peak of 150,000 meteors an hour! There were lesser showers in 1866-8 and 1900-1 and nothing at all unusual in the 1930s.

This time round there is more at stake than cosmic fireworks. Hundreds of satellites now orbit the earth, many essential for our daily weather forecasts and communications. A Leonid meteor storm will blast them with dust moving at 70 km per second -- some may find that storm a little hard to weather.

Alan Gilmore works at the Mt John Observatory in Tekapo