The best occultation of a star by Pluto ever predicted for North America will occur between 10:40 and 11:00 Universal Time (UT) Sunday morning, March 18th, in northern and possibly central Mexico, the western and central USA, and southwestern Canada. The star is comparable in brightness to Pluto, with mag. V = 14.9 and R = 14.1. Since the motion is relatively slow, useful astrometric observations of the occultation might be made with telescopes of 10-inch aperture or greater, especially with integrating video cameras like the StellaCam II, and I think that the signal-to-noise ratio might be good enough to measure Pluto's atmosphere with telescopes of 16-inch aperture or greater with video (especially integrating) observations, where the integrations might be done over 4, 8, or even 16 frames.

It may be possible to make useful observations from where astronomical twilight is just beginning, in Manitoba, Minnesota, Iowa, Missouri, Arkansas, and Louisiana, and certainly west of there, where the sky will be dark; east of those places, the twilight may be too bright, though Wisconsin, Illinois, and Mississippi have a chance. Low altitude will hamper observations in Washington State and southern British Columbia.

Target Star and Occultation Path

The star is UCAC2 25823784 at J2000 RA 17h 55m 05.70s, Dec -16° 28' 34.4", in northwestern Sagittarius about 2.5° north of M23 and 0.7° south-southwest of 6th-mag. SAO 160915 at J2000 RA 17h 56m 19.1s, Dec. -15° 48' 45". A central occultation would last about 6 minutes.

Maps, detailed finder charts, observing tips, and much other information about this event are on Bruno Sicardy's website at Due to the great distance, there's still a fair amount of uncertainty in where the actual path will be, as can be seen from the different predictions portrayed on Sicardy's website, but at least the uncertainty is relatively small compared to Pluto's diameter so that observers throughout the "lower 48" US states and northern Mexico have a very high chance for an occultation.

With the current information, though, the location of the central line can't be predicted well enough to guarantee a "central flash" that will occur at the center due to focusing by the atmosphere. Observations of the central flash will be especially valuable for studying Pluto's atmosphere, so as many observatories as possible are encouraged to try to record the event so that some of them might record the central flash.

Reporting Your Observations

Let us know your plans to try to observe this occultation (size and location of telescope that will be used, and recording method) to try to coordinate coverage of the event, to try to prevent duplication of fixed- site chords by mobile observers. Now that Pluto and the star are close enough together, accurate astrometric observations with long focal-length telescopes may be possible (especially after the lunar phase decreases enough) to refine the prediction of the central line.

If you obtain any video or CCD images of the March 18th occultation by Pluto, please send an account of your observations, including your observing-site name, longitude, latitude, elevation above sea level, and how these were determined; a description of the telescope used; the observing conditions; the Universal Time range of the observations; the method of recording the observations; and the names of those who participated in the observations, to me at I will forward your report to the astronomers who will be performing the detailed analysis of the observations to obtain information about Pluto's atmosphere. It would be best if you could post your observations (.avi files preferred for video) on a website and also include the URL in your report. This can be done later, or the observations can be written to a CD and mailed to the astronomers who will be analyzing the observations, with details of that to be distributed to successful observers shortly after the event, and which will also be posted on my website for the event at

Please check the online version of this AstroAlert on Sky & Telescope's website for possible minor updates:

Any major updates will be announced via subsequent AstroAlert messages. Good luck, and clear skies!

David W. Dunham
Contributing Editor
Sky & Telescope

Note added March 20th: I know of 19 observatories that recorded or otherwise observed the occultation by Pluto last Sunday morning; information about them, and preliminary results, are at I'll update that website as I learn more.


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