Eclipse: The science and history of nature's most spectacular phenomenon. J. McEvoy P.

6 minutes 26 seconds, close to the theoretical maximum for a solar eclipse. The Moon’s 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 Moon’s disk just touches the Sun’s. 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 humanity’s 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 Moon’s 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 Baily’s 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.

       Baily’s 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 Sun’s atmosphere, the solar prominences.

       The diamond ring

      I look away at my fellow sky-gazers along the beach. It is like a scene from Spielberg’s 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 Moon’s 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 Baily’s 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 Moon’s shadow. The travel, the hassles, the expense have all been worth it, viewing one of the greatest eclipses of the twentieth century.

       UNDERSTANDING AN ECLIPSE

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

      Johannes Kepler, 1605

      THE 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 Sun’s 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 Sun’s 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.

      APPARENT MOTION OF THE SUN AND THE MOON: THE CELESTIAL SPHERE

      Suppose a sphere that contains the whole universe is drawn with the Earth as its centre, as shown in Figure 1.1. The outer shell is an imaginary infinitely large dome onto which the positions of the stars and all other celestial bodies are projected. At any one time, we can see only half of this sphere from our position on the Earth. The celestial sphere works as a model because we are interested only in the directions of celestial bodies, not their distances from the Earth. The celestial equator is a projection of the Earth’s equator onto the sphere. Directly above the Earth’s north geographical pole is the north pole star, Polaris, marking the position of the north celestial pole. This star appears stationary in the sky as the other stars appear to revolve around it because the axis of the Earth’s rotation passes through it. (It is actually nearly a degree from the north celestial pole itself, but this is close enough for it to appear more or less fixed.) Instead of the Earth’s rotating in one direction, the sphere is imagined to turn in the opposite direction once every 24 hours so that all the stars complete one cycle every day. This simulates what we see from the apparently stationary Earth.

      The Sun moves around the celestial sphere on a path called the ecliptic, describing a complete 360° circuit at a rate of approximately 1° per day in its annual cycle of 365 days. The Moon’s path on the celestial sphere differs distinctly from the Sun’s. First, the Moon moves more swiftly than the Sun, completing a circuit of the celestial sphere in 29.5 days as seen from the Earth. Second, the Moon’s orbit has a different orientation from the Sun’s, intersecting the ecliptic at an angle of about 5°, as shown in Figure 1.1. The intersections of the paths of the Sun and the Moon defines two points on the ecliptic called the nodes of the Moon’s orbit. The nodes, denoted by the letter N in Figure 1.1, are crucial to the study of eclipses. The arrows showing the direction of the orbital motion of the Moon indicate that one node, N₁, is the descending node, where the Moon crosses the celestial equator from north to south. The other node, N₂, is the ascending node, where the Moon crosses the celestial equator from south to north.

       Figure 1.1. The celestial sphere, showing the paths of the Sun and Moon and the position of the lunar nodes.

      To study eclipses, it is necessary to consider the motion of the Sun and the Moon simultaneously. The Sun advances about 1° per day along the ecliptic, and the Moon moves in the same direction at about 12° per day along its orbital path. As the Moon completes a circuit of the celestial sphere about twelve times faster than the Sun, the Moon is always catching the Sun up and