Science: Look to the skies - it's eclipse time again

Shadows become unusually crisp,the temperature falls, the sky darkens and turns an eerie silvery-gray, while animals, believing dusk has fallen, begin to howl and wail. That's when the scientists really get interested: it means a solar eclipse is underway. Dan Falk on preparations for the next big one.

In the world of science, few events bring the professional and the amateur together. The botanist peering through a high-powered microscope has entered a world beyond the reach of the hobbyist; a birdwatcher may stop to admire a majestic (but common) blue heron, a sight that elicits only a yawn from the ornithologist. In astronomy, however, the two groups enjoy common ground. And of all the sights the heavens can offer, none holds as much appeal as a total eclipse of the sun.

That's why thousands of astronomy enthusiasts, both professional and amateur, will be heading for the Caribbean this month. The eclipse of 26 February will last more than four minutes, and be one of the best of the decade. The spectacle will be visible across a narrow ribbon of sea and land stretching from Colombia to Antigua.

A solar eclipse occurs when the moon passes between the sun and the earth, blotting out the sun in the middle of the day. Total eclipses occur every few years; however, because they're visible only from a small region of the earth's surface, putting oneself in the "path of totality" often means travelling to some out-of-the-way location.

The last two total eclipses, for example, were visible only from eastern Asia - too far away for many budget-minded amateurs. Prime viewing sites for February's eclipse, in comparison, are relatively accessible. It's also a kind of "sneak preview" for the much-anticipated eclipse of August 1999, which will be visible from south-west England.

The moon's shadow will be just 150km (94 miles) wide as it cuts across Central America and the Caribbean. Only within this narrow path will viewers see the total eclipse; those farther afield will see a partial eclipse. The path of totality crosses southern Panama, northern Colombia and Venezuela, and the islands of Aruba, Curacao, Montserrat, Guadeloupe and Antigua.

"A solar eclipse is one of the most spectacular things in nature," says Francisco Diego, an astrophysicist at University College London and a veteran of nine solar eclipses. Diego, like many observers, finds he keeps coming back for a repeat performance. "Once you see the first one, you want to see more."

Though he shares the passion of the amateurs, Diego will be doing serious research when he observes February's eclipse from Guadeloupe. He'll be taking high-resolution photographs of the corona, the sun's tenuous outer atmosphere. Because the sun's surface is millions of times brighter than the corona, this outer region can only be studied during the fleeting moments of a total eclipse. Diego hopes to plot the precise shape of the corona, and compare his results with measurements made from other locations.

American astronomer Jay Pasachoff, meanwhile, wants to find out why the corona has a temperature of more than one million degrees Celsius, compared with just 6,000 degrees on the surface below. "It's one of the basic unsolved problems in astrophysics," says Pasachoff. One theory is that loops of gas in the corona - held in place by the sun's magnetic field - play a role in the transfer of heat. These loops are believed to oscillate, moving energy from one layer to another. Using an ultra- sensitive electronic camera, he plans to record images of the corona at a rate of 10 pictures per second. The results will show any high-frequency motion in the coronal loops.

John Parkinson, of Sheffield Hallam University, has what sounds like a much simpler task: he wants to measure the size of the sun. The method sounds simple, too. Because the moon's diameter is known to a great degree of precision, you just have to time the duration of totality as accurately as possible, and do some basic trigonometry. "Essentially, you use the moon as a ruler," he says. To perform the calculation, you also have to know your precise location; Parkinson will be using a GPS (global positioning system) when he observes the February eclipse from the island of Curacao.

This will be Parkinson's seventh eclipse, and he notes that getting to the site rather than observing the event is often the greatest challenge. In 1981, he and his team travelled to eastern Siberia. "We lived in a woodcutter's cottage in the middle of the forest for about five days, and we lived on sardines and salami," Parkinson recalls. "We had a great time."

That kind of enthusiasm, rarely heard from the pros, is something keen amateurs display every time they head for an eclipse. For Don Hladiuk, an amateur astronomer living in Calgary, Canada, chasing the moon's shadow has been a habit since 1979, when he saw his first solar eclipse from Manitoba. "I was totally blown away by the beauty of it," Hladiuk says. From that moment, he's been hooked; February's eclipse will be his sixth.

Those who have seen it agree that the sight of the totally-eclipsed sun is one of the most awe-inspiring displays nature can offer. An eclipse begins as the moon appears to take a "bite" out of the solar disk. The bite slowly grows larger, until the sun has been reduced to a thin crescent. Shadows become unusually crisp. Perforations in tree leaves, acting as pinhole cameras, project thousands of images of the sun on the ground. As the moon's shadow sweeps closer, the temperature falls. The sky darkens and turns an eerie silvery-gray. Many animals, believing dusk has fallen, begin to howl and wail. Veterans of eclipse cruises tell of dolphins leaping out of the water as the start of totality draws near. Early man would surely have trembled in fear.

Finally, the sun disappears completely behind the moon, which becomes an incredible "black hole" in the sky, surrounded by the pearly-white glow of the corona. Often, huge red flares - known as solar prominences - can be seen streaming from the sun's surface. Meanwhile, the brighter stars and planets have become visible, while the glow of a "360-degree sunset" colours the horizon on all sides. Midday has become night. As Don Hladiuk puts it, a total eclipse of the sun is "one of those things you have to see before you leave this planet".

Dan Falk is a science journalist based in Toronto, Canada.