Totality Approaches

As the obscuration of the Sun increases the sky darkens, although it never gets as black as dead of night. That would be too humdrum. The sky qualities during an eclipse are much more intriguing and unusual than this.

First we should think about what can be seen because the sky is dark. Many people seem to believe that no stars exist during the day, but they are there, simply drowned by the bright blue sky. If you don't believe me, arrange to use a telescope one clear day and be sure to avoid pointing it at the Sun. The stars are there and of course with the naked eye the Moon is also often visible. Similarly, if you know where to look then Venus can be viewed unaided during daytime, although because of its orbit it's always quite near the Sun, which is why one sees it best either soon after sunset or just before sunrise.

Similarly Mercury always stays close to the Sun, and many people only consciously spot that planet during an eclipse. I write "consciously" because it is often seen and yet not recognized by the viewers. Many have sat and watched the Sun go down in the west over a placid ocean, and then wondered about a bright, slightly reddish "star" just above the horizon. If you've done that, chances are you've seen Mercury. My favorite memory of the type is having sat in a Jacuzzi at a splendid house on Malibu Beach with a movie producer friend, and after the sky had darkened still more we could see Comet Hale—Bopp blazing across the firmament.

The other planets though also move across the sky on paths close to the ecliptic. Depending upon the particular eclipse, it's likely that you'll have Mars, Jupiter, or Saturn providing a celestial jewel or two to glitter and attract your attention. The bright stars will also be out to dazzle you as the sky darkens, the specific array depending upon the season. Maybe it will be Castor and Pollux, the Gemini twins, accompanied by such stellar beasts as Sirius, Procyon, and Capella. But all of these are available at some time of the year during clear nights. If you're blessed with cloud-free skies for an eclipse, it is the special phenomena that should occupy your attention.

Let us imagine that totality is now imminent, a few minutes to go. The temperature is dropping perceptibly, and many watchers start to shiver (so take a sweater). An effect often glimpsed just fleetingly is the shadow-band phenomenon.Turbulence in the Earth's atmosphere causes differential refractive effects (bending of the paths taken by light), which is why the stars twinkle, as discussed in Chapter 12. The planets, however, look bigger because they are much closer to us and so do not twinkle. This is an easy way to differentiate Mars or Saturn from the stars at night. The Sun is normally much too large to twinkle, but as totality approaches only a slender crescent of the solar disk is left, making the equivalent of twinkling possible, except that here we have a very bright

FIGURE 15-4. The shadow band phenomenon sketched, with some imagination, after an eclipse in Spain about a century ago.

source. If the conditions are right then you may see wavy bands of light flickering quickly over the terrain; their viewing is easier if you have something like a large white sheet spread over the ground. These shadow bands are similar to the patterns seen on the bottom of a swimming pool, except with much less contrast (they vary in intensity by a few percent at most). Photographs of these bands have proven elusive, with few clear examples. A sketch, drawn with very considerable artistic license, is shown in Figure 15-4.

The Moon's shadow traverses the Earth at about 1,600 miles an hour. During the partial stage the increasing penumbral penetration is not noticeable on a minute-to-minute basis, but as the umbra approaches things start to happen fast. The complete lunar shadow can be seen zooming towards you from the west like a vast storm bearing down at supersonic speed. An elevated viewing location with a clear horizon to the west has much to recommend it, such that the rapidly encroaching shadow may be seen in these last 10 to 20 seconds before totality.

There are other aspects of the shadow to note. Totality only

takes place within the narrow band that you have sought out, and a few tens of miles to the north or the south there is incomplete blanking of the Sun. You can see the sky that far away—looking beyond the edge of the shadow—and it will appear the same orange as twilight, eventually all around the horizon.

Now to the Sun and Moon themselves. In the last quarter-minute Baily's beads appear around the lunar limb, the final few specks of light passing between the mountains of the Moon, these seeming to shift around the periphery of the disk until only one is left: the diamond ring effect. A few more seconds and it is gone. That's second contact. Totality is with you.

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