Night skies put on a show

I AM often contacted by the media and members of the public about meteor showers.

While most of them are far from spectacular, coming up next week is one of the best ones. It is called the Geminid meteor shower, and keen people who are happy to watch the sky through the small hours of the morning late next week may be rewarded with a view of quite a number of them.

It’s important, however, to understand what astronomers mean by “meteor shower”.

A meteor is the streak of light we see caused by an object burning up in our atmosphere. Most are very small, and with the unaided eye under good conditions, we tend to see about five or six an hour, on average.

A meteor shower occurs when the Earth passes through a stream of debris in space strung out along the orbit of an astronomical body that is shedding material. In most cases, these are comets, although the Geminid meteors have a rather unusual origin: an object called Phaethon that seems to be some sort of “missing link” between the rocky asteroid and the icy comets. It orbits the sun every 1.43 years, and passes closer to the sun than any other asteroid among those that have been named: only 21 million kilometres, which is much closer to our star than the innermost planet, Mercury, ever gets.

Although the word shower is suggestive of a huge number of meteors appearing to “rain” across the night sky, that description is far from the truth.

While it is true that sometimes, but only rarely, we see a meteor storm with thousands every hour, the vast majority of meteor showers result, at best, in one of them being visible about every minute or so. More typically, the rate is about one every few minutes. This is still noticeably more than the average rate of about six an hour.

Because the meteors are in a stream in orbit around the sun, when they collide with our atmosphere and burn up they appear to emanate from a particular direction in space. Astronomers call this the radiant, and for the meteors to be visible at all, the radiant needs to be above the horizon. The paths of the meteors all appear to move in a direction away from the radiant, much as railway tracks appear to radiate outward from a point in the far distance for an observer standing in between the tracks. For the Geminid meteors, the radiant is in the direction of the constellation of Gemini, which rises about midnight and is visible low in the north. The meteors can appear anywhere in the sky, but they appear to be radiating from this point.

The number of meteors one sees is strongly dependent on latitude — that is, how far one is located north or south of the equator — because this determines how high in the sky the radiant can get. It turns out that the northern hemisphere is strongly favoured for most meteor showers. You may see some information suggesting that 120 Geminid meteors per hour can be seen, but this is only from locations in the northern hemisphere where the radiant is high in the sky. For the Geminids as seen from Tasmania, we can expect only about a quarter of that rate.

To get the best view of the Geminid meteors, there are some important considerations.

Firstly, there is something over which you have no control: the moon. A very bright moon makes it difficult to see the fainter meteors, but next week, the light of the moon is not a serious problem: it is a waning crescent rising in the hours before sunrise. Therefore, the conditions next week will be quite good.

However, there are things that you can do to improve your chances. It is important to get out of the light pollution of the city, to let your eyes get used to the darkness, and to be patient. Be prepared to stare at the sky, preferably giving yourself a wide field of view in a northerly or northeasterly direction, and stay up until the twilight becomes too bright.

The best nights are December 13-14 and 14-15: that is, very early on Thursday or Friday morning next week. Happy meteor watching!