Friday, January 13, 2012

Chasing Shadows

Three years from now, I shall be off to the Faroe Islands. (They lie in the North Atlantic, midway between Iceland, Scotland and Norway.) This is why.

Ask any Englishman the significance of 11th August, 1999, and I doubt he would know. If he were to google it, though, all would be revealed. At 10:02 a.m., God hit the dimmer switch.

To use grown-up terminology, (part of) England experienced a total solar eclipse for the first time since 1927. Most English people over the age of 25 will remember where they were at the time. I was working in the northwest of the country, which was set to experience a partial eclipse of 92% magnitude (Figure 35.1). Without a second thought, I decided to down tools and go outside in order to savour one of the most awesome, and rarest, natural phenomena. Wearing a borrowed pair of arc welding goggles, I stared at the sun (!) and waited for the moon to cross my line of sight.

Figure 35.1: Path of total solar eclipse across Europe in 1999

Copyright © 1997 Fred Espenak

I was lucky. The morning clouds followed the script and dispersed. I lay on a park bench until, at precisely 11:16 a.m., all but 8% of the sun was eclipsed by the passing moon (Figure 35.2). Never before had I experienced nightfall during daytime. The ‘accelerated dusk’ was wonderfully surreal, as was the equally rapid re-emergence of full sunlight, by 12:34 p.m. Other parts of the UK were not so fortunate. In Cardiff, 150 miles due south, a clear sky had been spoiled by a single passing cloud at the crucial moment.

Figure 35.2: Heavy partial eclipse, almost exactly as I viewed it

Copyright © 1999 BBC

Almost as breathtaking as the spectacle was the proof that the science of astronomy had been deadly accurate with its calculations. Mathematicians from many years ago had succeeded where psychics and astrologers had always failed: they had foretold the distant future – to within a fraction of a second. The mathematical basis for predicting eclipses is extremely sophisticated. American astrophysicist Fred Espenak, of NASA’s Goddard Space Flight Center, is a renowned expert and has at his disposal state-of-the-art infrared spectrometers. Only a fool would bet against him.

It is natural that such a magical event attracts fanatics. So-called ‘eclipse chasers’ ascertain geographical co-ordinates of forthcoming eclipses. This allows them, by means of supersonic aircraft, to follow the precise path of the shadow, thus prolonging the experience. In 1973, Concorde 001 took off from the Canary Islands and ‘chased’ an eclipse across North Africa for a record-breaking 74 minutes. Much scientific knowledge was gained.

Given that both the Earth and moon rotate in an anticlockwise direction, the speed of the shadow’s movement can be calculated by subtracting the earth’s rotational speed from the orbital speed of the moon. It must be remembered, though, that the earth’s rotational speed varies across its surface. The shadow moves relatively slowly at the equator (where rotational speed is greatest) and faster at the poles (rotational speed is zero). The moon travels at approximately 1.03 km/s, whereas the earth’s surface rotates at an average of 0.46 km/s. This means that the shadow travels at around 2,000 km/h. Concorde had a top speed of 2,100 km/h, and so was ideal for ‘chasing’.

On average, there is a total solar eclipse, somewhere on Earth, about once every eighteen months. Of course, 70% of the Earth’s surface is covered by water, so most places are not easily accessible. As England is a relatively small country, the probability of the moon casting over it a shadow of only a few hundred square kilometres is remote – hence my plan to visit the Faroes in 2015, where, on Friday, 20th March, at 9:40 a.m., a total eclipse will last almost three minutes (Figure 35.3).


Figure 35.3: Graphic showing path of totality (black) for the 2015 solar eclipse

Copyright © 2000 A.T. Sinclair

The next time a part of England experiences a full solar eclipse will not be until 23rd September, 2090. I shall be 124 years old. I just hope my eyesight holds out.

Copyright © 2012 Paul Spradbery

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