2025 Occultations by Major Planets and their Satellites - Updated 2025 April 3
The Occultation by Uranus and its rings on April 8 UT
Some practical advice about methane filters and duration of ring events
These occultations are described and illustrated (with several Occult maps) in this .pdf document. It includes several hot links to external Web pages, but links to internal files, the .xml input files used by the Occult program, are given below. When you open these files with your Web browser, you should get a display that looks like this. The larger files may take a couple of minutes to show this appearance, showing a messed-up display before then. Once the proper view appears, right-click anywhere on the display, then "Save as" to a directory on your computer; the best directory is the \Asteroid\ subdirectory in your Occult\ directory. _ _ _ _ _ _ _ 2025 APRIL 8 OCCULTATION OF 8.9-MAG. SAO 93455 = HIP 16271 BY URANUS As noted in the .pdf document, there will be an important occultation by Uranus visible from central North America on this first anniversary of the 2024 April total solar eclipse, as shown in this Occult map, zoomed in enough to show the US State and Canadian Provincial boundaries. Remember that this occultation will take place at 2h UT, which is the previous evening (April 7) local time across North America. When consulting the Occult map, remember that the nearly vertical lines mark the U.T. of closest approach, or central occultation time, while the possibly observable events will occur 14 to 26 minutes before or after these times when the star and Sun altitudes will be a few deg. different - more about this is shown below. We give two sky-plane views of Uranus, the first zoomed in showing the rings, some of the small inner satellites, and the disks of Uranus and projected Earth (showing that Uranus has nearly 4 times the diameter of Earth); the other view is zoomed out, showing the major satellites. The first view shows how even the largest (epsilon) ring will still be less than 2.0” from the edge of Uranus, too close for reasonable separation under the likely seeing conditions that will occur. Expanded North American Occult SITE file and April 8 (UT) Uranus PREDICTIONS We recently updated the Occult North American .site file to include some of the major observatories, and several smaller ones, to try to include coverage for where many of us actually observe. For example, Sierra Vista AZ is now added. If you have a location that you would like us to add, that's not within 3 miles (5 km) or so of another place that's already there, let us know. Using this expanded file, we’ve generated new lists of contact times (including the ring events), and local central times and circumstances (NAmerLoc). With so many ring events (although for us, only the epsilon ring events are likely to be observed), the contacts list is very wide; when you open it in Notepad or another editor, be sure to select a fixed-space font like Courier New (with it, I suggest using bold font) and use the smallest font size you can read. The contacts list is in alphabetical order by location name (although sometimes I use the observatory name, more usually I use the nearest city instead (for example, Fort Davis TX rather than McDonald Obs, for more general use). I suggest finding your location, then copying its line and moving it to the top, just under the line headings, and add a couple of carriage returns, to separate your line from all the others. If you have word wrap on, which will likely be the only way you can see the last (post reappearance) epsilon ring crossing time, you’ll need to deal with that, to figure out which data line goes to which header line. I checked the contact times in my Occult list for Apache Point Obs. with those computed by Richard French given for that location in the document recently sent to us by Damya Souami, and the epsilon ring times agreed within 0.3 min. while the Uranus D and R times differed by about 0.5 min., the latter likely due to differences in the radius used for the calculations. We think these agreements are good enough. Especially for the D and R, which will each be gradual, lasting at least 3 minutes, and maybe as long as 5 minutes. Dr.Souami's message notes that the professional campaign for this event is organized by William Saunders at NASA - Langley, email william.r.saunders [at] nasa [dot] gov . After you find your nearest location in the contacts list, you can open the NAmerlocs list and search for that location since it is not in alphabetical order, but rather in order of the stations on the sky plane at Uranus, from south to north; hence, the first location in that list is Acapulco, MX. Of course, the contact times are more important than the central time, but only the NAmerlocs file has the star (or Uranus) and the Sun altitude. However, those are for the central time, and the Uranus occultation lasts 28 minutes (within a minute) for all North American locations. Thus, the D is 14 min. before the central time, and the R is 14 min. after it. Add 12 min. to these, that is, 26 minutes, for the offset times for the epsilon ring events. We got the ring crossings, and D and R times, from the contacts list for Fountain Hills, AZ and then used USNO’s Interactive Computer Ephemeris to calculate the altitudes of Uranus and the Sun at those times, and list those below, along with the central data from the NAmerLocs file: Altitudes (deg.) Event U.T. Uranus Sun eps.-ring-1 2:04.9 32.2 -3.4 D 2:17.0 29.7 -5.9 Center 2:31.0 27 -9 R 2:45.0 23.9 -11.5 eps.-ring-2 2:57.0 31.4 -13.9 You can see the significant differences from the altitude values for the central times; similar differences can be expected for other locations, although the differences should be a little less for the more northern locations. METHANE-BAND FILTERS to increase contrast for your observation Another aspect of this event is the use of a methane-band filter, to greatly darken Uranus relative to the star. We asked IOTA/ES about sources for these filters, and Oliver Kloes responded, with this message giving recommendations. We have added some remarks there; we are rather sure the narrower-band filter will cut out too much of the star’s light for all but the largest telescopes. On March 29, Sam Deen provided advice on the contrast and overall dimmings that might be expected with these filters in this message. He also notes that the occultation by the epsilon ring for this event will last from 0.7s to 3.2s, depending on where around the ring, relative to its pericenter, the event occurs, but since that precesses, we can't predict that. To see the ring event with a methane filter, you'll need to integrate no more than those amounts, but of course, you need to integrate enough to have a signal of the star, and the stronger, the better, to detect any variations. The other rings are narrower and with less opacity than epsilon, so they will all be more difficult. I think the last time a star this bright was occulted by Uranus was in March 1977 with the occultation of 8.8-mag. SAO 158687, observations of which led to the discovery of the rings. The successful observations of that event that detected the rings were all made with telescopes larger than 14 in. And that event, occurring near opposition, was high in a dark sky, without the low altitude and/or evening twilight challenges that this year’s event has. So we don’t have much experience with methane-band filters and scopes in the size range we typically use for events like this. Hopefully, some of us will gain some information about what might work for these events in the future. If you already have, or obtain, a methane-band filter, be sure to test it on either the target star, or a similar one in the Pleiades, which is not far from Uranus and has many brighter stars to help you set gain levels. A star that might be good for a first test is 8.3-mag. SAO 76188, spectral type G0, at J2000 RA 03h 47m 16.6s, Dec +24d 07' 42", which is close to Alcyone. The Uranus target star, SAO 93455, has spectral type F5, reasonably close to SAO 76188's. SAO 93455 is listed as double in the WDS, with the companion 7" away in PA 283, but at mag. 13 according to WDS, it is much fainter than the target, but possibly similar in brightness to the major Uranian satellites. _ _ _ _ _ _ _ For other occultations by major planets and their satellites during the rest of 2025, read the .pdf document with link near the top of this page. _ _ _ _ _ _ _ OCCULT PREDICTION INPUT FILES FOR 2025 Using the input files described below, you can use the Occult4 program to compute your own predictions using the files and instructions given here. Worldwide major planet and satellites events, 826 events A list of all of these events is here. Worldwide asteroid events from Steve Preston, including the major planet and satellite events - for the asteroidals, including many NEA and TNO events prediction maps and data are available at https://www.asteroidoccultation.com/. Most of them are available now only as Occult input files, Occult Watcher, described in the .pdf document, is the best way to find out about UPDATED upcoming events near your location. You should learn about and obtain the software at this link. First versions of the pages below will be posted in a few weeks, except for the already-posted brighter main-belt (MB) occultations. That page has most of the Occult input files for the year for the other classes of occultations that aren't posted. Brighter stars occulted by Main-Belt Asteroids The other pages below are not yet posted. The text associated with them for the Handbook, written in August 2024, is here. The maps and tables mentioned there are in this document. Near-Earth asteroids (NEAs) Selected Special Main-Belt Asteroids Trojan Asteroids (Lucy targets are emphasized) Distant objects (Comet SW1, Centaurs, TNOs, and some satellites) ______ David and Joan Dunham, 2025 March 29, updated April 3 e-mail: dunham@starpower.net cell phone: 301-526-5590