
In a nutshell, the occultation initially predicted for European observers on the night of July 12 did not occur. But a second event on July 1920, not mentioned in Dunham’s article because it was thought to be a near miss, probably will produce an occultation for observers in a wide swath across South America.
The Big One

This event can be timed visually with a 20-centimeter (8-inch) telescope that clearly and steadily shows the star. Better yet, Dunham and Wolfgang Beisker of the International Occultation Timing Association (IOTA) suggest videotaping the event with a sensitive camera like the PC-164C (available for about $130 from Supercircuits) on at least a 25-cm telescope. Pluto is known to be 2,300 to 2,400 km in diameter, and careful timings could help to refine this value.
This occultation also presents a rare opportunity to probe Pluto’s atmosphere, believed to be mostly nitrogen and perhaps 1 percent methane. But light curves obtained by the Kuiper Airborne Observatory during the 1988 occultation left planetary scientists with a puzzle. Either Pluto’s atmosphere has a strong temperature gradient near the surface, possibly due to radiative heating by the methane, or there is a thick aerosol layer. If an observer can record the event at five images per second in two or more widely separated wavelengths (including near infrared), we should find out which model is correct.
Another question is whether Pluto’s atmosphere has cooled and shrunk in the last 14 years. The planet reached the perihelion point of its orbit in 1989 and is now heading back into deep space. It already receives 6 percent less sunlight than in 1989, enough to make the ground temperature decrease by ½° Celsius and shrink the atmosphere significantly.
An observer precisely centered in the occultation path may be able to record a brightening of the light curve for several seconds in the middle of the event. This "central flash," due to starlight focused by the lower layers of Pluto’s atmosphere, is expected to go over northern Chile, where dry and clear skies prevail. It could yield precious data on Pluto’s atmosphere, including any departure from spherical shape by as little as a few kilometers.