Solar Eclipse Observing--the Lunar Shadow and Sky Darkness

Introduction. As the moment of totality approaches, the shadow of the moon sweeps out of the west to immerse observers along its path. This spectacular sight, often compared with that of an approaching thunderstorm, is even visible—indeed, sometimes enhanced—when cloudy weather threatens to obscure totality itself. With the arrival of totality or a particularly deep annularity, the sky darkens appreciably and the brighter stars and planets become visible.

Appearance. Noted portrait painter Howard Russell Butler described the shadow’s appearance through thin cloud cover over the Elkhorn Range and intervening valley at Baker, Oregon for the eclipse of 8 June 1918:

…a greenish pallor overspread the landscape,—but it was not very dark. To the northwest, however, the sky was growing dark. The last half minute seemed long. My eyes were fixed on the sky line. Suddenly the entire range fell to a deep low–valued blue, and simultaneously the lower part of the sky above the range turned to a rich yellow, inclining to orange, streaked with two horizontal blue–gray clouds. Above me the sky darkened rapidly. For an instant the valley retained its light green color and then the shadow seemed to rush toward us and all was engulfed…

Overcast skies disappointed unfortunate observers viewing the 7 March 1970 total eclipse from Florida’s panhandle. As consolation, however, the heavy cloud cover provided an impressive canvas for an especially striking apparition of the rolling, wave–like shadow as it swept out of the Gulf of Mexico and across the Southeastern United States.

The appearance of the shadow varies considerably depending upon the circumstances of the eclipse, atmospheric conditions and the location of the observer with respect to the center line. For instance, a sunrise eclipse will cast a shadow resembling a truncated cone with its narrow end pointing towards the horizon, while observers near the northern or southern limits of the path find themselves surrounded by asymmetrically illuminated horizons.

Even observers several hundred miles outside of the path of totality have reported sightings of the shadow sweeping along the horizon. Certainly, observers who, for one reason or another, find themselves close to the path of totality but unable to travel the remaining distance, would want to attempt shadow sightings along with their observations of the partial eclipse. Brilliant red and orange hues (the sunrise–sunset effect) appear as horizon colors prior to, during and just after totality.

Once the shadow’s leading edge has swept over your site, the sky darkens significantly. Observers who have adapted their vision with dark goggles prior to the onset of totality have reported bright stars and planets becoming visible more than a minute before the thin solar crescent is obscured. 

Similar sightings of celestial objects have been reported in the much brighter skies of annular eclipses since antiquity; observers are encouraged to include such attempted sightings in their observing programs for annular and very deep partial eclipses as well as for total eclipses of the sun.

Procedures. In the 1970’s William H. Glenn, York College of the City University of New York at Jamaica, conducted and compiled extensive observations on the appearance of the shadow. He developed a questionnaire which still provides one of the best sets of guidelines for lunar shadow and sky darkness observations. Among the major shadow and sky aspects to note, Glenn includes:

1. How many seconds before totality was the shadow seen?

2. In what direction did the shadow become visible?

3. How did its darkness, appearance and color compare with the surrounding unshadowed sky?

4. How would you describe its very rapid motion and changing appearance, color and shape?

5. In what directions did you have time to look during as well as immediately before and after totality?

6. How would you describe the appearance and color of the sky in each of these directions as totality progressed?

7. When and how did the shadow disappear?

8. Describe how the light changed as the eclipse progressed (include any times as accurately as possible).

9. What stars and planets were you able to identify?

10. How soon before totality did stars and planets become visible?

11. How long after the end of totality did stars and planets disappear?

12. What kind of weather conditions existed during the eclipse (include the fraction of sky obscured by clouds and the direction of clouds, types of clouds, cloud coloration, sky coloration, haze, etc.)?

13. Did you detect any color on the disk of the moon during totality?

14. How did the darkness and color of the moon’s disk compare to the sky a few degrees away from the eclipsed sun?

Observers should also estimate the degree of darkness using such criteria as the readability of newspaper headlines, ordinary newsprint, the hour or second hand of an ordinary wrist watch, a standard eye chart or some similar method. Measurements made with photoelectric cells or photographic light meters would be of even greater value. Use devices capable of measuring the incident light on a scale that can be readily converted to lumens per square meter (or foot–candles), and make frequent carefully–timed readings of the intensity of illumination at the zenith and at the four compass points along the horizon.

Locating even the brightest stars and planets prior to totality can be difficult unless you know exactly where to look and have taken some steps to dark–adapt your vision in advance. This task has been greatly simplified by widely published charts of the sky around the eclipsed sun. Such charts, which appear in most periodicals and astronomical handbooks for total and annular eclipses and a number of excellent computer programs that produce such charts (see Chapter 2), can be very useful in estimating the faintest stellar magnitude visible. This, in turn, provides yet another strong indicator of sky darkness.

Selecting a star or planet you have identified during totality, and attempting to keep it in sight in the rapidly brightening sky following totality, is an easier chore.

Photography. While capturing the essence of the lunar shadow is a task especially suited to artists and poets, photographers having cameras equipped with fish–eye lenses (a 180–degree field is ideal) have enjoyed considerable success. Mount your camera on a level tripod, point it at the zenith and orient it with respect to the compass points (corrected for any difference between the earth’s magnetic and geographic poles at the observing site), and each frame should capture the entire sky from horizon to horizon, showing the sunrise–sunset effect and darkness as the shadow advances.

Glenn achieved good results by leaving the lens wide open (about f/5.6) and making one–second exposures every 15 seconds from a minute before until a minute after totality on a moderately fast film (around ISO 200). This will yield overexposed images at first, but as the shadow sweeps over, exposures become ideal and produce a sequence of images that is both pleasing and useful. Frames taken during totality frequently show brighter stars and planets as well.

With the compass points penned onto each photograph, it is a simple matter to determine the azimuth of the approaching and receding shadow, and to relate any sunrise–sunset effect noticed to an accurate position on the horizon. The exact time of each exposure can be determined later if a tape recorder is used to simultaneously record the triggering of the camera shutter and WWV or CHU radio time signals (see Chapter 5).

Video cameras with wide–field lenses or attachments should produce impressive results as well, but should be adjusted for full manual operation to prevent them from automatically correcting for the diminishing light by adjusting exposure times or f/–stops or both throughout the eclipse.

Photograph 8-1  A 165º fish-eye lens was used to capture the lunar shadow, easily seen in this 1/8 second exposure on Ektachrome 100 of the 26 February 1979 total solar eclipse north of Winnipeg, Manitoba.  Photograph taken by Mike Reynolds.