Occultation of SAO 163583 by Uranus April 10, 1996

by David Dunham, International Occultation Timing Association

On 1977 March 10, the first occultation of a star by Uranus was observed and the rings of the planet were discovered. That event was centered over the Indian Ocean. Since then, there has been only one other occultation of an SAO star by the planet as it moved across star-rich fields of Sagittarius; the small planet slowly covers only a tiny part of the sky. As the planet moves into sparser regions of the sky, there will be only two more occultations of stars brighter than 10th magnitude during the next 40 years. One of them, involving the 9.2-magnitude red (spectral type M0) star SAO 163583 = PPM 237450 = KMU 138, will be visible Wednesday morning, April 10th, from populous parts of western North America (below the horizon in Alaska and Hawaii), the first predicted occultation of a relatively bright star visible from the the continental U.S.A. This could provide your first opportunity to detect the Uranian rings, with telescope apertures as small as 8 inches.

SAO 163583's J2000 position is R.A. 20h 27m 06.7s, Dec. -19 deg. 43' 49", about 1.5 deg. south of Pi Capricorni. Uranus will be 6th magnitude, so it can be located with the chart of western Capricornus showing the planet's path and stars to 7th mag. on p. 70 of the April issue of Sky and Telescope. Another chart is on p. 140 of the RASC Observer's Handbook 1996. The most detailed chart is the one on p. 54 of the 1996 Planetary Occultation Supplement to Occultation Newsletter, Vol. 6, No. 8, for North American Observers distributed with that issue of the newsletter last December. This chart can also soon be viewed and printed from the World Wide Web. The URL is http://www.anomalies.com/iota/splash.htm. Go to the IOTA page, then find the Uranus occultation under either the upcoming events or special notes of interest sections.

For observing the occultation, a red filter, such as one of the Wratten filters, will help decrease Uranus' light relative to the late spectral type star. Use the largest available telescope, and if observing visually, the highest magnification that just barely resolves the seeing disk of the star, so that the star images are focused well. CCD or video recordings, and even visual timings, of the disappearance and reappearance by the Epsilon ring (and any fluctuation of the star's light while behind the ring, if the star remains faintly visible) could be useful, when combined with many other timings of the events. At the distance of Uranus, the star, whose angular diameter may be as large as 0.001", will subtend about 15 km. It will take about 1 second for the edge of the ring to cover and uncover the star's disk, so it will be possible from the observations to determine the star's diameter. The ring's width varies around the planet, I believe in the range of 100 to 200 km, so the occultation by the ring should last several seconds. Because the ring plane is tilted some towards the Earth, the star will be only about 0.7" from the edge of Uranus' disk when the Epsilon ring occults it, so good seeing and red filtering to decrease the planet's light relative to that of the star may be needed to see the ring occultation events.

The other rings, all interior to the Epsilon ring, are all smaller and less opaque, so the star will not dim as much, and for shorter periods of time, when covered by them. The disappearance crossings of all of the other rings will occur within 8 minutes after the disappearance Epsilon ring crossing, and the reappearances will take place during the 8 minutes before the reappearance Epsilon ring crossing. To obtain approximate predicted times of the occultations by the other rings, first calculate the predicted central time of the occultation, which is halfway from Epsilon ring disappearance to the Epsilon ring reappearance. Multiply half of the Epsilon ring R - D time difference by the radius ratio for each ring, to tell how many minutes from the central occultation that the ring D will occur before the central time, and the R after it. The radii of the rings, and the radius ratio (relative to Epsilon), for the rings are given below:

Ring      mean radius, km   radius ratio

Epsilon        51180           1.000
Delta          48330           0.944
Gamma          47660           0.931
Eta            47210           0.922
Beta           45700           0.893
Alpha          44750           0.874
4              42600           0.832
5              42270           0.826
6              41870           0.818
No predictions are available for possible occultations by the satellites of Uranus. Observations of such events could be useful for improving the orbits of those objects. The mean radius of Miranda's orbit is 129,390 km, for a radius ratio of 2.53. So an occultation by that satellite could take place as much as an hour before the Epsilon D, or after its R. The ten new satellites discovered by Voyager have mean radii ranging from 49,770 km to 86,010 km, or radius ratios from 0.972 to 1.682. So observation for about half an hour before the Epsilon D, and a similar time after its R, might be useful to check for occultations by these objects, whose sizes range from 13 km to 77 km in diameter. Those with larger telescopes will be able to see the major satellites, Ariel, Umbriel, Titania, and Oberon, and can see if the star will pass near them, although an occultation is rather unlikely. The mean orbital radius of the outer satellite, Oberon, is 583,520 km, for a radius ratio of 11.4, so the star will cross its orbit about 8 hours before the Epsilon ring. A CCD image showing the star and the four major satellites would be interesting.

Predictions of the disappearances and reappearances of the star by Uranus and by the Epsilon ring, computed by Doug Mink, Harvard Center for Astrophysics, are given for many North American cities below. The altitudes of Uranus ("Ura") and the Sun in degrees at the event time are given following the Universal Time. Since the star position can be in error by 0.25" or more, and the motion is only 3.47"/hour, the times can be in error by 5 minutes or more. I would recommend recording at least ten minutes before the Epsilon ring D, and preferably half an hour or more before to try to catch an occultation by one of the new satellites. Then observe for a similar amount of time after the Epsilon ring R. Due to the error in the time, you don't need to interpolate between cities to your location; just find the city closest to you, and use those predicted times. The cities are listed in alphabetical order by State (the postal abbreviations are used), and then by city within the State. Times are also given at the end for five Mexican and two Central American cities. Times are not given for any Canadian cities, since the file of cities that I sent to Mink were selected for the May 8th lunar occultation of Comet Hale-Bopp, and the northern limit for that event passes just south of the U.S.-Canadian border. Although the Uranus events might be seen from southwestern Canadian locations, the altitude will be quite low so that observation will be very difficult there.

David Dunham, IOTA, e-mail david_dunham@jhuapl.edu and dunham@erols.com
1996 April 5