Safe Solar Eclipse Viewing

Typically the first contact, when the Moon begins cutting a notch from the solar disk, occurs about 75 minutes before totality, giving you an extended period during which the movement of the Moon across the face of the Sun may be monitored. How can you view this? Not with the naked eye, and certainly not through any optical device like binoculars or a telescope. At the time of the eclipse in August 1999 over England and much of Europe, the newspapers were full of warnings, telling people of the danger posed to their eyesight. This led one letter writer to the London Times to suggest that "In view of the many warnings regarding adequate safety measures when viewing the eclipse, should we not be sensible and listen to it on the radio?" The following malapropism is especially delightful in that it appeared in the Lady, a hugely staid and strait-laced British magazine: "But few seem to realize that looking at an eclipse with the naked is dangerous . . ." The danger attached to eclipse watching though is a serious matter.

Almost a millennium ago, Al-Biruni, a multitalented Islamic scholar from the lands south of the Aral Sea, warned: "The faculty of sight cannot resist it [looking at the Sun directly], which can inflict a painful injury. If one continues to look at it, one's sight becomes dazzled and dimmed, so it is preferable to look at its image in water and avoid a direct look at it, because the intensity of its rays is thereby reduced. Indeed such observations of solar eclipses in my youth have weakened my eyesight." In the eleventh century Al-Biruni did not have the advantage of either a telescope to project an image safely onto a screen or optical filters. His suggestion was to view the Sun reflected from the surface of water in a bowl, which (depending upon the angles involved) can result in a few percent or less of the sunlight reaching the eye. This was a trick employed far back in antiquity by the Babylonians, Egyptians, Romans, and Greeks alike, the smarter ones using oil or pitch because their high viscosity makes for less rippling. There is no need to resort to such outmoded techniques nowadays; simply use a filter.

What sort of filter is needed? One simple filter is a piece of grossly overexposed black-and-white film that has been fully processed. This leaves it largely opaque and if you peer through this then perhaps just one part in 10,000 of the sunlight makes it to your eye. Note that only certain types of black-and-white film will do: it is the silver granules that block most of the sunlight and the dyes used in color film are not adequate. There are also potential drawbacks with this method, such as the possibility of scratches through the emulsion allowing too much light to strike your eye. Similarly, smoked glass is inadequate and dangerous.

These, then, are cheap but unsatisfactory solutions. Bear in mind the various aphorisms along the lines of "don't spoil the broth for want of a pinch of salt," as this is a case where economy may lead not only to the broth missing its salt, but being poisoned with arsenic to boot. Don't take silly risks for the want of a proper filter. There are many available commercially at little cost, often in the form of goggles with paper frames and flat "lenses." These seem totally black until you look through them at a very bright source, and find that just a tiny fraction of the light penetrates the filter: just enough to enable you to monitor the progress of an eclipse safely.

Amateur astronomers usually have large filters to fix over the openings of their telescopes, allowing direct viewing, but unless you know precisely what you are doing, never put your eye near the ocular of any instrument directed towards the Sun. A projected image may be obtained using a small telescope (as in Figure 1-13), or a pair of binoculars clamped in a stand and with one lens covered,just in case. Often the image is so bright on its screen that it is necessary to stop down the aperture, by covering the top end of the telescope with a card penetrated by a suitably small hole, allowing only a fraction of the impinging sunlight to enter the instrument.

The partial phase of the eclipse can be followed using some sort of pinhole camera, such as a shoebox with one end cut out and a partially translucent paper screen taped in its place and a small hole punched in the opposite end. It would be even simpler to use a small mirror as follows: cut a hole about a quarter-inch across in a sheet of card, fix that over the mirror's surface, and reflect the sunlight coming through the peephole back onto a shadowed wall. This will form an image of the solar disk, the lunar notch enlarging and creeping across it. Breaking a mirror is considered unlucky by the superstitious, but deliberately smashing one may be a good idea for an eclipse because each fragment may be used to reflect the sunlight and produce an image of the partial phase.

Actually, no equipment at all is needed to observe the partial eclipse. I often tell people to think of the surefire cure for seasickness, and also to look at the ground, not the sky. What is the cure for seasickness? Sit under a tree—it always works. If you are positioned under a suitable tree, with dense foliage, and look at the ground, you will see that the tiny gaps between the leaves act as natural pinhole cameras, casting myriad crescent images all around you. An example is shown in Figure 15-1.

To look directly at the Sun during the partial eclipse, on go your eclipse-viewing filters. The only time it is safe to view the Sun without such equipment is during totality, when your goggles or whatever equipment you have been using should be removed, else you will miss seeing the best bits. Apart from the short phase of totality—that precious couple of minutes—you must always have an appropriate filter to protect your eyes, if you want to gaze directly at the Sun.

FIGURE 15-1. The foliage of a tree provides a set of natural pinhole cameras, producing crescent images during the partial phase of a solar eclipse.
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