Solar Eclipse Observing--The Sky at Totality

Introduction. Shortly before second contact, the sky darkens appreciably and the brighter stars and planets become visible to the unaided eye. With the onset of totality, stars fainter than the second magnitude may be seen.

Searches for faint comets, asteroids or infra—Mercurian planets have been carried out during total eclipses for more than a century, but usually such searches, to be fruitful, require sophisticated cameras which produce images for later analyses.

Total eclipses even provided the first experimental support for Einstein’s General Theory of Relativity. The theory predicted that light from a distant star would be bent by the gravitational field of the sun (due to the curvature of space itself) and the star would appear deflected slightly from its actual position in space. Just such a deflection was detected in a star close to the sun’s limb during the total eclipse of 29 May 1919, only four years after Einstein presented his theory.

Observers who are able to tear themselves away from the beauty of the eclipse itself, may find some interesting and worthwhile targets among the celestial objects which would otherwise be hidden from view by the sun for another month or two.

Popular astronomical magazines, handbooks, almanacs and computer planetarium programs usually provide diagrams of the sky during totality, including the brighter objects likely to be visible.

Some observers want to dark—adapt their eyes well in advance of totality, and do so by wearing dark glasses or a single eye patch during the partial phases.

Planets. The inner planets, Mercury and Venus, are rarely far from the sun, and are usually observed close to the horizon through turbulent layers of atmosphere and in twilight. During totality, however, both planets may be viewed high in the sky. Planetary enthusiasts might take a few minutes from a particularly long eclipse to observe and sketch telescopically the dusky markings which Mercury and Venus exhibit.

Outer planets Mars, Jupiter and Saturn, when near solar conjunction, may offer out—of—season targets for enterprising planetary astronomers wishing to see what features they are exhibiting in advance of their return to morning skies from the sun’s glare.

Variable stars. Variable star enthusiasts may wish to check on the brighter irregular or eruptive stars on their programs. Certainly, any novae discovered shortly before becoming lost in the sun’s glare would warrant examination during totality.

Comets. Observations of bright comets in close proximity to the sun might be conducted. Others may wish to search for previously undiscovered comets which would otherwise remain lost in the sun’s glare for weeks more. Dividing the sky around the sun into sections assigned to many observers would be more likely to achieve success than would one observer attempting to sweep a large area of sky during the brief period of totality. Use wide—field telescopes, and avoid areas known to have a lot of bright nebulae or galaxies.

Meteors and fireballs. An alert observer who spots a bright meteor or fireball might make an effort to record its time, magnitude, duration and direction of flight. For very bright objects, plotting the path on a star chart or "Sky at Totality" illustration would be useful. Note any bursts (and associated sounds, if any) and any lasting trains left behind by its passage.

Artificial satellites. The brighter satellites should be readily visible during totality. They are inarguably of less relevance than planets, variable stars or comets, which may be obscured by the sun’s glare for many weeks around conjunction. Still, the widely available computer software and modem—accessible elements warrants running predictions for the eclipse site and an effort to observe them. This task is ideal for people with few other projects to conduct.

Faintest star. One project which relates to the sky’s overall darkness during totality is to determine what the faintest star visible to the unaided eye is. This may also be attempted using binoculars and telescopes of various apertures. The best approach is to select a star which you know will be bright enough to locate easily and which also is surrounded by fainter stars offering a broad range of magnitudes. Different observers might focus on several different star fields at increasing distances from the eclipsed sun to determine how sky darkness varies as the shadow’s edge is approached. Observers who have not dark—adapted at least one eye in advance should not expect their faintest star observations to have any value.

Zodiacal light. The zodiacal light is a faint glow that travels along the ecliptic preceding and following the sun by up to several hours, and appears to be caused by sunlight reflecting off interplanetary dust particles in the inner solar system. In the evening and morning hours, it may appear as bright as the Milky Way at times when it stands nearly vertically off the horizons. For the western evening sky, this occurs in February and March for mid—northern latitude3, while the eastern morning sky is favored in September and October, providing there is no moonlight to interfere.

The sun’s brightness makes it difficult to study the size and density of these dust particles closer than about 20 degrees (80 solar radii) from the sun. However, some studies of the zodiacal light have been conducted closer to the sun during eclipses using cameras carried aloft by aircraft and balloons. University of Minnesota scientist Dr. Edward P. Ney led several expeditions to areas where the eclipsed sun would be ten degrees below the horizon, thereby enhancing the zodiacal light.

The zodiacal light is probably too faint to be observed visually during totality, except from very high elevations. However, eclipse observers with a very low sun angle, or who find themselves just outside of totality for an eclipse occuring before sunrise or after sunset at their locations, might wish to attempt observations of this nature.

Gegenschein. This "counterglow" phenomenon is related to the zodiacal light but appears on the ecliptic directly opposite the sun. It appears as a very faint elliptical patch of light. Observers who find themselves directly opposite some point along the path of totality might monitor the gegenschein for several nights on either side of the eclipse date and several times centered on the time of totality in their opposite hemisphere for any changes in its appearance.