Observing a solar eclipse
A solar eclipse is nature's most magnificent phenomenon.
The false impression has grown that it is often hazardous to
observe a solar eclipse, whereas, because of educational efforts
on eye safety, actually few reports of eye injury exist and the
incidence of lasting eye damage is even lower.
The total phase of an eclipse is completely safe to watch with
the naked eye. The total brightness of the corona is only that of
the full moon, so is equally safe to watch. The darkness at the
disappearance of the diamond ring effect comes so abruptly
that people have no trouble telling when the safe time begins,
and the diamond ring at the end of totality is so relatively
bright that it is clear when it is time to look away. A glance at
the Sun just before or after totality is not harmful; it is only
staring for an extended time (more than a few seconds with the
naked eye) or looking at the not totally eclipsed Sun through
binoculars or a telescope that can cause harm.
During the partial phases, it is possible to follow what is go-
ing on without any special aid by watching the ground under
a tree. The spaces between the leaves make a pinhole camera,
and project the solar crescent myriad times onto the ground. A
pinhole camera can be made individually by punching a small
hole (approximately 2 – 5 mm or 0.1 – 0.2 in. in diameter) in a
piece of cardboard, and holding it up to the Sun. A crescent
image is projected onto a second piece of cardboard held 20
cm to 1 m (8 to 40 in.) closer to the ground, or onto the ground
itself. An observer looks at the second cardboard, facing away
from the Sun.
For direct observation of the partial phases, a special solar
filter must be used. Fogged and exposed black-and-white (not
color) film that contains silver and is developed to full density
provides suitable diminution of the solar intensity across the
entire spectrum. Inexpensive commercial solar filters made of
aluminized Mylar can also be used. Gelatin "neutral-density"
filters are actually not neutral in the infrared, and so should not
be used. Neutral-density filters made by depositing chromium
or other metals on glass are safe if they are ND4 or ND5, and are
commercially available, as is #14 welder's glass.
Eclipse (continued)
Fig. 6 Paths of all total solar eclipses from 2001 through 2025. The point of greatest eclipse is indicated by an asterisk on each eclipse path.
(Credit: Eclipse map by Michael Zeiler, www.eclipse-maps.com. Eclipse paths by Xavier Jubier, xjubier.free.fr.)
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