Bailys Beads Seen In 1780

One may or may not consider another outcome of the eclipse expedition to Maine in 1780 to be an embarrassment. In the opening chapter we met Francis Baily, the British astronomer who in 1836 gave a description of the luminous phenomenon seen just as totality begins and ends, universally known as "Baily's beads." It so happened that Samuel Williams noticed these beads of light during the 1780 eclipse. If his account had been better known, then people would nowadays talk about spotting "Williams's beads," and he would not have been relegated to the fringes of science history. Those few tens of miles made all the difference.

In fact there is more to it than that. Paradoxically, the mis taken location of the Harvard expedition made it more likely that they would witness the beads than if they had been right in the middle of the eclipse track. Ever since Halley made eclipse track prediction possible, astronomers have glibly assumed that the best place to be is right on the central line, but in recent years it has slowly dawned on eclipse watchers that this is not the case at all.

Imagine that a particular eclipse track is precisely 100 miles wide, and the maximum eclipse duration is to be 3 minutes. There is 50 miles from the center to the edge and beyond that edge there will be no totality. If you are positioned 10 miles from the central line, the duration of totality falls by only a few percent, and so there is no need to worry about one's precise position. If, though, you are located 47 or 48 miles from the central line (2 or 3 miles from the edge) then totality will last for only one minute, rather than three.

That's an assessment of the quantity of totality; a quite different thing is the quality. It happens that total solar eclipses are rather more spectacular if they are viewed from near the periphery of the path. Because they depend upon the light just reaching your eyes around the crinkly edge of the Moon, both the diamond ring effect and Baily's beads may last for about ten times longer if you are near the fringe of the track. Indeed, the beads often seem to run quickly around one edge of the Moon in that position. The elusive shadow bands discussed in Chapter 15 may last for up to five times longer and be easier to see. But more important still is your chance to see the chromosphere, the most colorful feature of an eclipse.

For decades astronomers labored to get good photographs of the chromosphere, and in particular its spectrum. Close to the central line, the opportunity to obtain such observations lasts for only a few seconds and because of that it is often termed the "flash spectrum" (see Figure 5-7). If only they had positioned themselves well away from the central line, the astronomers involved would have stood a far better chance. The reason is this: If you are located near the very edge of the eclipse track the apparent disks of the Sun and the Moon glide along the same tangential line. In consequence the bright red chromosphere may be visible for up to a minute and a half, allowing ample time for its spectrum to be captured. Similarly, giant prominences can be seen for longer, jutting up above the solar surface.

Although they experienced no actual totality, Samuel Williams's party was close enough to the edge of the track that only a very thin layer of the Sun was visible. This was slender enough that it appeared to become broken up into separate patches by the mountainous limb of the Moon, and so they gave an account of the phenomenon that later became known as Baily's beads. They were aided by the fact that they were viewing by telescope: it is easier (but dangerous, unless you know precisely what you are doing) to see the beads with a telescope, rather than the naked eye.

It is worth noting in passing that Francis Baily could perhaps have been aware of the discussion of the eclipse by Williams. In 1796 a youthful Baily spent some time in America, traveling west from the east coast and then down the Mississippi by boat to New Orleans. From there he walked much of the two thousand miles back to New York. His published account of that trip is a classic of the era. In those days, however, his knowledge of astronomy was still modest. It was only after his return to England, and his amassing of a small fortune in business, that he was able to devote his time to the study of celestial phenomena and assist in the founding of the Royal Astronomical Society.

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