Mon Feb 27, 2006 11:56 am
The total solar eclipse on 29 March will throw a shadow across Africa and Asia. Geoff Elston reveals how to watch it.
A total eclipse of the Sun is a true marvel of nature: once seen it is rarely forgotten. This awe-inspiring experience is happening on 29 March 2006, but you'll have to travel to North Africa, Asia or Russia to see it.
What causes the celestial conjunction that many will travel hundreds of miles to see? It is a fortunate coincidence that while the Moon is some 400 times smaller than the Sun, the Sun is about 400 times further away from us. This means that where the paths of the Sun and Moon appear to cross in the sky, the Moon completely covers the Sun's disk - the period when it's completely covered is called totality. It's during totality that the chromosphere - the lower region of the solar atmosphere, where prominences are visible - and the corona (the Sun's highly tenuous outer atmosphere) become visible.
The track of totality during an eclipse is, in effect, the tip of the Moon's long conical shadow racing across half the Earth. This lunar shadow has two parts: a dark centre called the umbra and a lighter outer region called the penumbra. The shadow's track can be predicted accurately, and if you're in its path when the time is right, you will see the full effect of the eclipse. The length of totality for this eclipse will be a maximum of four minutes and seven seconds, and will be at its best in southern Libya.
Before you even think about viewing the eclipse, you need to make sure you're properly equipped. To protect your eyes, use a solar filter fixed securely over the lens of any optical or photographic equipment right up to the moment of totality. Even if you are just watching the eclipse with the naked eye, a safe solar viewer must be used. Never use sunglasses, CDs, silver foil, or coloured plastic as a makeshift filter. As soon as totality ends you must use a solar filter or viewer again. Without these protective measures, you run the risk of seriously damaging your eyes.
[/b]A solar eclipse begins with first contact, when the dark edge of the Moon appears on the edge of the solar disk. Slowly, the Sun disappears behind the Moon, but it is only when 90 per cent of the Sun is hidden that it becomes noticeably darker and cooler. The dazzling diamond ring effect is seen prior to totality, as is Baily's beads. These bright spots of light along the edge of the Moon are caused by sunlight streaming through the lunar valleys on the Moon's edge. Second contact marks the beginning of totality, and it is only then that it is safe to look at the Sun without a solar filter or viewer. This is when the full beauty of the corona is revealed and the Sun gives tantalizing glimpses of its prominences. The corona's shape and structure vary with sunspot activity. When sunspot activity is at a minimum, the corona can show loops and long streamers, while when sunspot activity is at maximum, it can appear more rounded.
Away from the Sun, the sky often appears dark blue, quite unlike the darkening caused by clouds, as you are engulfed in the ominous lunar shadow. The brighter stars and planets - Mercury, Venus and Mars - become visible to the naked eye, and you may notice a sudden chill in the air. Birds begin to roost and flower petals close. Take in as much of the experience as you can before the brilliant diamond ring reappears, marking third contact, or the end of totality. The corona then disappears as the Sun begins to re-emerge. At this point solar filters and viewers must be used again to protect the eyes. The Moon continues to move across the Sun's disk until fourth contact, or the end of the eclipse, when the Sun is fully visible again.
- Sky at Night team