J. P. McEvoy (Joe McEvoy) - Eclipse - The Science and History of Nature’s Most Spectacular Phenomenon

For Emily, Muirenn, Joel

and the baby arriving with the eclipse

in August 1999

Stretching over 1,300 km south of the California state line between San Diego and Tijuana is a peninsula of mountains, deserts and plains ending at one of the most beautiful beaches in the world. Fine golden sand for miles and miles slopes into the azure Sea of Corts and the Pacific Ocean. The peninsula, called Baja California, is actually part of Mexico. In the heat of the summer of 1991 I arrived, planning to view my first total eclipse of the Sun.

The morning of 11 August is bright and clear. Amid the palm trees and cactus plants all along the beach, tripods are being set up in the sand, an army of straw hats and Bermuda shorts appear as far as the eye can see. Everyone is buoyant. Not a single cloud in the sky, though still a few hours to go. Totality would be unusually long today at Los Cabos, 6 minutes 26 seconds, close to the theoretical maximum for a solar eclipse. The Moons shadow, when it reaches Baja, will be 260 km wide, moving along the beach at a speed of about 40 km per minute. As the bell in the small church tolls 10:00 a.m., the crowd makes final adjustments to telescopes and cameras. The long wait is over. Twenty-three minutes to go.

First contact occurs at 10:23:17 as the Moons disk just touches the Suns. The sky continues to be cloudless and no one is thinking of the weather. The show has begun.

In earlier times humanity held its breath during this solar disappearing act, offering sacrifices to appease the evil spirits who might destroy humanitys source of heat and life itself. Slowly the Moon cuts deeper and deeper into the Suns image and it is now obvious that the two disks have the same diameter, a remarkable coincidence. The light fades imperceptibly.

Two small Japanese girls watch the progress of the eclipse through special Mylar sunglasses, while their mother watches anxiously. The Moons shadow is now sweeping across the globe towards us on the beach at Los Cabos at twice the speed of Concorde. In an orbit above the earth, a weather satellite photographs the shadow of the Moon every half-hour during its journey.

As second contact approaches, the Sun has been reduced to a thin crescent and now breaks up into a string of bright beads. These are known as Bailys beads, caused by the streaming of the last rays of sunlight between mountains at the edge of the Moon. One by one the beads disappear until only one is left, radiating brightly from a single point on the edge of the eclipse, like a diamond ring.

Bailys beads

My watch reads 11:47:40 and the miracle happens: second contact. The diamond ring disappears and a delicate pearly white halo springs into view around the eclipsed Sun. This is the corona. I have 6 minutes and 26 seconds. I look at the sky map and locate four planets in the noonday sky, lined up just to the east of the eclipsed Sun. Mercury and Jupiter are the closest, then Mars and Venus. The twin stars Castor and Pollux are clear and bright in the darkened sky, quite near the Sun. Sirius is just due south of the Sun. Through my telescope I see massive pink gaseous formations floating in the Suns atmosphere, the solar prominences.

The diamond ring

I look away at my fellow sky-gazers along the beach. It is like a scene from Spielbergs Close Encounters of the Third Kind. Hundreds stand transfixed, motionless, staring directly up into the sky. No goggles or Mylar glasses are needed now. There is not a sound even birds have stopped chirping. In the distance, I see what appears to be a sunset in all directions, 360 around the horizon. This is the illuminated Earth outside the canopy of darkness under the Moons shadow.

I check my stopwatch as third contact approaches. Then at 11:54:06, the corona disappears. In its place, the diamond ring effect and Bailys beads repeat in reverse order. Cheers of excitement ripple through the crowd. A sliver of sunlight is now visible, and safety viewing devices are taken up to guard against the invisible ultraviolet waves. The Darkness at Noon is over.

The corona

One hour, fourteen minutes and forty seconds later, fourth contact occurs at 13:18:46. The Moon moves away from the Sun and the full disk returns. Everyone seems satisfied. The eclipse-chasers of the world have had their day in the Moons shadow. The travel, the hassles, the expense have all been worth it, viewing one of the greatest eclipses of the twentieth century.

A solar eclipse is a gift to us from the Creator.

Johannes Kepler, 1605

T HE SYSTEMATIC UNDERSTANDING of the motion of heavenly bodies was one of the earliest problems confronting humankind. The development of conceptual models to reproduce this motion is one of the great stories of the history of science.

Even the most casual observer knows that the Sun and the Moon are continuously changing position in the sky. And surely all would agree that the Suns motion appears to be regular. But other observations are more puzzling. Many people are surprised to see the Moon high in the daytime sky. Why are bright wandering stars, the planets, often seen close to the Moon or the setting Sun? Why does the pole star, signposted by the stars of the Plough, never change position? What is the significance of the constellations along the Suns path?

How can one make sense of all this? The best way is to use a model of the sky called the celestial sphere, an imaginary surface upon which may be represented the motions of the Sun, Moon, stars and planets as seen from the Earth.