The Central Bureau for Astronomical Telegrams has just sent out a notice about an important event tonight for observers with large telescopes and video-recording capability.

According to Leslie Young (Southwest Research Institute), Pluto may occult a 13.8-magnitude star around 3:22 June 14th Universal Time (11:22 p.m. June 13th EDT) at a low altitude in the sky for the eastern United States and Canada. The star is at right ascension 18h 35m 48.69s, declination –19° 17' '43.6" (equinox 2000.0).

You'll need a large enough telescope to record a half-magnitude light drop, with good time resolution, in the combined 13.3-magnitude image of Pluto and the star.

From the announcement:

"Topocentric predictions for the midtime depend on the combination of Pluto and star positions used, and vary by about 50 seconds for any location. The midtime also depends on the location, affecting the midtime by about 1 minute over the portion of visibility. Adding uncertainty for Pluto's extended atmosphere and the chance of systematic errors in the star's right ascension, the recommended observing time spans 3:20 to 03:37 UT.

"Any location near the entire east coast of North America has a good chance of being in the shadow. Pluto only subtends 0.1 arcsec, so small errors in the star position and Pluto ephemeris can move the predicted shadow path substantially. Predictions tend to be good to about 500 miles, barring systematic errors.

"Multiple [timings from different locations] are critical for reconstructing, after the fact, the geometry of Pluto's passage."

Pluto and the star will be only about 20° high in the southeastern sky as seen from Boston, Philadelphia, and Baltimore; somewhat higher as seen from farther south, lower farther west.

Young describes the scientific motivation for good observations of this event:

"Pluto's thin, nitrogen atmosphere is in vapor-pressure equilibrium with the surface ice, and changes seasonally. We've seen [the atmosphere] double since 1988, and now we measure its pressure once or twice a year. The technique we use is stellar occultation, when a star passes behind Pluto's atmosphere. The atmosphere defocuses the starlight. By the timing of the fading of the star, we measure the pressure and temperature in Pluto's atmosphere at about 10 km resolution."

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