THE LIMITS OF VISIBILITY OF A PARTIAL ECLIPSE OF THE SUN INFERRED FROM

OBSERVATIONS NEAR THE PARTIAL ECLIPSE LIMIT

By Francis Graham

The solar limb, when viewed through a telescope, either in projection of through a filter, exhibits undulation on its limb which is the result of atmospheric conditions in the Earth's atmosphere called "seeing". It is reasonable to assume that a minimal partial eclipse of the sun would be invisible as the lunar limb is deep within this zone of seeing irregularity.

On April 8, 2005, a minimal partial eclipse of the Sun was viewed from several sites in the greater Pittsburgh tri-State area. Our own site was the Christine Alley Observatory, East Pittsburgh, PA , located at longitude west 79o 51' 59".34, latitude north 40o 23' 42".27, elevation 298 m. where the solar eclipse was viewed by projection in the 16 cm. f/15 Gadela Refractor by Francis Graham, Glenn A. Walsh, William B. Hall, Theresa M. Graham and John Weinhold. (Fig. 1) This event, called "Barely See It III", emphasized the minimal nature of this eclipse. Fortunately it was a perfectly cloudless sky.

The eclipse 1st contact was predicted to be 22:11:35 UT, the maximum eclipse 22:18:09 UT, and the last contact 22:24:53. The maximum magnitude was 0.005.

The actual time the eclipse was first visible was at 22:13:49. The eclipse was barely photographable (Fig. 2, taken at Maximum). The last the eclipse could be seen was 22:23:19, thus, it was visible only 9 min. 30 sec. This differs from its theoretical duration of 13 minutes 18 seconds, so there is the hint that the eclipse would not be visible at all if the duration were less than about 3 minutes.

William B. Hall viewed the Sun without a telescope, but with an aluminized mylar filter. No eclipse was visible, even at maximum.

Several reports from other locations in the tri-state area were examined. Gene Henderson of Henderson Tool Company, near Emsworth, longitude 80o 4' 59" latitude 40o 30' 29", who had a magnitude of 0.002, photographically recorded the most faint hint of an eclipse. A positive report was received by the Amateur Astronomers Association of Pittsburgh of a sighting in New Kensington, where a report to the same group from an observer in Allison Park was negative. At Allison Park the magnitude would have been 0.001 and the duration 3 min. 11 sec.

The theoretical northern limit for the partial phase was in McCandless Township, about 5 miles to the north of Allison Park, at 40o 33' 34" for longitude -80o 00' 00".

Thus the hypothesis that a partial phase of 3 minutes or less would be generally impossible to see seems to be verified. See map, Figure 3.

I thank my colleagues W.B. Hall, G.A. Walsh and J. Weinhold for help, and George Guzik for making reports to the Amateur Astronomers Association of Pittsburgh available. I thank also Theresa Marie Graham for her tireless devotion.

April 22, 2005

Photograph at Maximum Eclipse in East Pittsburgh PA (Moon seen as tiny "nibble" on right side of solar disk)