The Leonid meteors in 2002

Meteors or shooting stars are likely to be seen on any clear night. One's chances are improved by the absence of the Moon and by being well away from the glare of artificial lighting. The appearance of such meteors is generally random occasioned as they are by particles pulled into the Earth's atmosphere by our planet's strong gravitational pull. Ignition is caused by friction between the incoming, fast moving particle and the Earth's atmosphere.

At certain times of the year, however, there are circumstances when Earth encounters the debris from comets and the incidence of meteors is then much enhanced. In such cases the geometry of the Earth's motion combined with the motion (orbital movement) of the debris produces a perspective effect to observers on the Earth's surface causing the meteors to appear from a given area of space marked by a given constellation.

Thus we have meteor displays associated with the constellations Lyra (April), Perseus (August), Gemini (December) and so on.

iFig.2 A photograph of Comet Ikeya-Zhang; Date 2002 March 21.90 400mm f/5.6 lens using Fujicolour 400 ASA; Exposure 4.5 minutes manual guiding; The length of the comet's tail is 4.5 arc degrees. Note: there was a 7 day old Moon contributing to sky glow.

The comet Temple-Tuttle provides the material we associate with the Leonid (November) meteors. The comet orbits the Sun in 33 years. It was last at its closest to the Sun (and hence to the Earth) in February 1998. The comet itself was nowhere near as spectacular as the great Hale-Bopp and a good deal less prominent than the comet Ikeya-Zhang C2002/C1 (fig. 2), which visited our skies this March.

A comet is by no means a compact body such as a planet or minor planet (asteroid). Material associated with Temple-Tuttle is dispersed over a range of its path such that we may expect to encounter this material at times close to perihelion (the comet's closest approach to the Sun). Thus on the 16-17th November 1998 observers in Britain and other northern hemisphere sites were rewarded with a fine display of very bright meteors and fireballs. (The brightest witnessed by the writer on the morning of the 17th lit the entire sky briefly, being over fifty times brighter than the planet Venus.) Last year similar "storm" displays were observed from North America.

The time of maximum activity is impossible to predict with certainty; likewise the duration. The only thing to do is to be prepared and to keep watch throughout the likely period (weather permitting of course).

This year maximum activity has been assessed for around 04 hrs GMT on the morning of the 19th November.

Fig 1: Sky showing position of radiant 2002 November. 19 at 04 00 GMT. Click the image for an enlargement.

By then the radiant (see fig. 1) will be well above the horizon indicated by the circle in the constellation Leo, a little above the star Regulus. Unfortunately the Moon will be close to full (in constellation Aries).

It is best to commence observing as soon as it gets dark on evening of the 18th. Depending upon one's dedication to the task, checking the sky throughout the course of the night has to be recommended. Having said this, it is advisable to keep a look out from time to time on the previous two nights - this I would suggest the more since the circumstances for observing a good display of the Leonids is not likely to occur again until the year 2098 (the apparitions in 2031 and 2065 are not expected to be at all favourable).

Twilight

For convenience twilight is divided into three categories.

• Civil twilight - the Sun* up to 6 arc degrees below the horizon.
• Nautical twilight - the Sun between 6 and 12 arc degrees below the horizon.
• Astronomical twilight - the Sun between 12 and 18 arc degrees below the horizon.

Below 18º the Sun's light gives no perceptible light to the otherwise dark, night sky.

We are very aware in Orkney and Shetland of the light nights through summer for astronomical twilight lasts all night from late April through to late August. However, what is not so readily appreciated is the fact that our autumn and late winter months are by no means as dark as is generally portrayed. Thus, compared to the latitude of London, our nights are longer only between November 2nd (in 2002) and February 9th. (These dates are replicated approximately year to year.) This is because at high latitudes the Sun's diurnal track makes a shallower angle to the horizon than it does at lower latitudes. Those who have spent any time close to the equator will be aware of how quickly the light fades during an evening twilight.

Another factor tending to lighten the northern night skies is the presence of auroral glow. So far this autumn there has been an auroral glow, if not an active aurora, on most nights.

A full feature on time measurement, including equinoxes and solstices, will appear next year.

* Refers to the geometric centre of the Sun's disc and not the limb.

Next feature: Saturn at opposition, December 17.
Aspects of Saturn.

JV Nov 02