Astronomy: Shadow Play

Most Americans have not seen a good solar eclipse since 1954, and after this week they will not see another until 1970. This week's performance, clouds permitting, will entertain most of North America. Saturday's show will start at dawn in Japan's northern island, Hokkaido, where the sun will rise with the moon already squarely in front of it. Then the tip of the moon's black, conical shadow will race northeast, crossing the Bering Sea and coming ashore in Alaska just south of the Yukon. West of Canada's Great Slave Lake, total eclipse will last for nearly 100 sec.

For observers who stay still, 100 sec. is the maximum, but fast modern jet planes can stay with the shadow longer. Lockheed, Douglas and American Airlines plan to contribute airliners whose speed can stretch totality by as much as 44 sec. Astronomer Charles H. Smiley of Brown University will ride an Air Force F-104D at 48,000 ft. to race the shadow across central Canada at 1,160 m.p.h. Since the shadow will be speeding at 2,800 m.p.h., he will lose the race; but he hopes to watch totality for about five minutes.

Bombardment by Radar. Along the curving path of the shadow, which slips between Montreal and Quebec, cuts Maine in two, and grazes the southern tip of Nova Scotia, scientists will deploy their strange instruments. They will photograph the moon-covered sun in every available way, shoot rockets into the shadow. A German group will check Einstein's theory of relativity by photographing stars that appear to be close to the sun to see how much their light is bent by the sun's gravitation. Distant radio telescopes will bombard the moon with radar waves so that observers in the path of totality can see how the waves reflected from the moon respond to the unnatural darkness.

As the shadow sweeps across North America to the populated parts of Canada and Maine, amateur observers will swarm to greet it. The path of totality will cut through Maine in a 60-mile swath where a deep twilight will fall. As seen from Boston, the sun will be 94.4% covered. In New York the crescent will look thicker: 88.7% covered; in St. Louis, 67.1%. In Los Angeles the sun will be dented (26.4% ); in Mexico City barely nicked (7.6%).

Places outside the total shadow will not get dark; even a thin sliver of the sun gives a lot of light, but the birds will feel that darkness is coming and may go to roost for the night. People standing under trees should watch the light that filters through the leaves. Normally it hits the ground as overlapping disks, each a round image of the round sun. But as the moon creeps across the sun, the disks will shrink to crescents.

Baily's Beads. Observers close to the path of totality who stretch a white sheet on the ground may see the mysterious shadow bands, which are somehow caused by irregularities in the earth's atmosphere. They appear as vague lines of light and dark, drifting roughly parallel. An amateur who uses simple apparatus (a yardstick to record their direction and estimate their dimensions) can observe them about as well as professional astronomers.