Royal Astronomical Society of Canada
International Occultation Timing Association
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Guide to observing occultations
BY DAVID W. DUNHAM, DAVID HERALD, STEVE PRESTON, AND DEREK BREIT
As major, dwarf, and minor planets, and their moons, move across the sky, they occasionally pass directly between an observer and a distant star, producing an occultation. Astronomers have learned much about solar system bodies by carefully monitoring the changing apparent brightness of stars during the immersion and emersion phases of occultations. If the occulting body does not have an atmosphere, the occultation is virtually instantaneous; if there is an atmosphere, it causes the star’s disappearance and reappearance to occur gradually. If a planet has rings or other debris in its environs, the extent and degree of transparency of this material can be precisely mapped. The rings of Uranus, the ring arcs of Neptune, and the atmosphere of Pluto were all discovered by occultation observations. If an occultation is observed at several distributed sites, the size and shape of the occulting body can be determined more accurately than by other Earth-based techniques.
Amateur astronomers can make important contributions to occultation observing campaigns. This is particularly true for minor planet occultations, for which the paths across Earth are often very narrow and uncertain in location (due mainly to uncertainties in the ephemeris of the minor planet). By recording the times of the star’s disappearance and reappearance as seen from several sites (i.e. by noting the edges of the minor planet’s shadow as it sweeps across Earth), the object’s profile can be directly determined. Often timings of adequate accuracy can be made by visual observers using modest telescopes.
When observing an occultation, it is important that an observer know his or her location to within a fraction of a kilometre. Geographic longitude and latitude as well as the altitude of an observing site can be determined with a GPS receiver, from a high-quality topographic map, or from some map websites. If observations are to be of maximum value, the times of immersion and emersion must be determined as accurately as possible-certainly to better than 0.5 s, and better than 0.2 s for the shortest events (those less than about 10 s in duration). Photoelectric equipment with high-speed digital recording systems is well suited for this work. Attaching a low-light-level video camera to a telescope is a less expensive method for accurate timing. Visual observers using tape recorders and shortwave time-signal receivers can also make useful contributions. Even simple measurements of the duration of an occultation made with an ordinary stopwatch can be of value. CCD observers should be aware that most of these systems are incapable of timing accuracies better than about 2 s; hence visual observation may be better. But many CCD observers use the trick of turning off the telescope clock drive shortly before the predicted time and let the images trail. The occultation will appear as a break in the trail that can be measured to about a tenth of a second if the moment the exposure is started (just after turning off the clock drive) is accurately timed.
Occultation observations are coordinated in North America by the International Occultation Timing Association (IOTA) (www.occultations.org). IOTA member or not, IOTA wants to inform you, and others in your area, able to locate stars to 11th magnitude of prediction updates. IOTA’s free Occult Watcher software (see below) is best for finding events at your location. Otherwise, you can email the longitude and latitude (or location from the nearest town) of convenient observing sites, telescope size(s), and an indication of whether you are mobile to email@example.com. Individuals interested in joining IOTA should refer to OCCULTATIONS BY THE MOON, p. 1xx in this Handbook, for membership information.
More information is in the Solar System Photometry Handbook (Willmann-Bell, Inc., 1983), Sky & Telescope, and papers in IOTA’s Journal of Occultation Astronomy (JOA), Icarus, Minor Planet Bulletin, and other journals.
Observations of occultations by major and minor planets, including negative observations, should be sent to firstname.lastname@example.org for analysis, archiving, and publication by IOTA. When reporting timings, describe your geographic longitude, latitude, and altitude (to the nearest arcsecond and 30 m, respectively), telescope size, timing method, the start and end time of observation, an estimate of the observer’s reaction time (if applicable) and the accuracy of the timing, and whether the reaction time correction has been applied. IOTA’s main Web site at www.occultations.org, especially the Observing tab, has comprehensive up-to-date information on observing and reporting occultations. The Publications tab includes links to IOTA’s now free Journal of Occultation Astronomy (JOA).
Table of Planetary Occultations
The following two-page table (see p. 2xx) of planetary occultations by major, dwarf, and minor planets visible from North America and Hawaii for 2020, is based on predictions by Edwin Goffin, Scott Donnell, Steve Preston, Derek Breit, and David Herald. Preston and Breit assisted Dunham in the selection for the two-page table. Most of the occultations are by asteroids, but there is one occultation by Mars, an unusual event with very slow motion. Of special interest are occultations by distant trans-Neptunian and Centaur objects, many of which have moons and some rings. In general, special astrometric observations with large telescopes are needed to predict these occultations well enough for observing campaigns, and these are accomplished usually only a few weeks before the occultation. The few of these events that can be predicted several months in advance are too faint for most amateur telescopes and so are not in our list. These events are announced on Web sites of the Paris Observatory Lucky Star Project, the Rio astrometry group, and RECON, the network of observatories in the western USA dedicated to these observations. But most observers are informed of them via IOTA’s Occult Watcher software described below, via its Lucky Star and Rio group prediction “feeds”.
The successive columns in the table list: (1) the date and central time of the event; (2) the name of the occulting body; (3) the apparent magnitude of the major or minor planet; (4) the catalogue and number of the occulted star; (5) the star’s apparent visual magnitude; (6) the star’s right ascension and (7) declination; (8) the expected magnitude change from the combined brightness; (9) the predicted maximum duration of the occultation in seconds; and (10) the approximate region from which the occultation is predicted to be visible (locations are listed chronologically from first to last). Due to uncertainties mainly in the ephemerides of the minor planets from which these predictions are derived (most star positions are now accurately determined from the European Space Agency’s Gaia mission), the region of visibility of an occultation is uncertain, by about half a path-width for most of the asteroidal occultations listed. Errors may remain, so those near but outside the paths should try to observe. It’s also useful, especially for the brighter stars that produce high signal-to-noise recordings, to observe even if you are located up to about 10 path-widths from the predicted path, to check for the possibility of an occultation by a previously-unknown satellite of the asteroid. Updated maps and more about these events is at http://www.poyntsource.com/New/RASC_Events.htm.
Note that the times are usually for the geocentric time of closest approach; for any specific location in North America or Hawaii, the event time can be several minutes earlier or later. A few weeks before each event, improved predictions and the latest path maps may be obtained from Steve Preston’s minor planet occultation website: www.asteroidoccultation.com. Much other useful information, including interactive maps to zoom in on the path, circumstances for dozens of stations in and near the path, and lists of stars that can be used to pre-point telescopes to the target stars are at http://www.poyntsource.com/New/Global.htm. “Occult Watcher” finds many other minor planet occultations visible from your site or region; it is a free download from http://www.hristopavlov.net/OccultWatcher/publish.htm. Since Occult Watcher works from an interactive Web site, IOTA uses it to coordinate minor planet occultation observation plans.
Star catalogs are abbreviated as follows: SAO, Smithsonian Astrophysical Observatory; ZC, Robertson Zodiacal Catalog; TYC, Tycho-2; PPM, Roeser Positions and Proper Motions; HIP, HIPPARCOS, and UCACx, xth .U. S. Naval Observatory CCD Astrographic Catalog. For nearly all stars, the stellar data are from Gaia Date Release #2.
Some event stars, marked by * in the main table, have alternative star numbers, shown in the table on the facing page: Planet # is the minor planet number and the alternative star number (Alt. Star #) is from the HIPPARCOS mission catalogs used for the predictions on IOTA’s minor planet occultation Web sites mentioned above. When known, spectral types (Sp.) are listed for the stars in the table.
Noteworthy events (marked in bold type in the main table on the next 2 pages):
Jan. 24: The star, although bright, is not in the SAO catalog because it is a close double, WDS 06116+4843 = STF 845, with 6.2 and 6.9-mag. components 7.5ʺ apart in PA 358˚. It is in the PPM Bright Star Supplement #400085 with an approximate (probably blended, since the mag. is given as 5.8) position that is 3ʺ south of the Gaia position, which is therefore probably for the secondary star. That is consistent with the Gaia magnitude of 7.0. WDS says the secondary star has spectral type A6V. Since Gaia DR2 did not distinguish double star components, the Gaia scan data may need to be consulted, to make sure that the Gaia position is not of the center of light of the system; in that case, the path for an individual component would be thousands of kilometers from this predicted path. Hopefully, observers will be able to resolve the components of this double, but if not, the effective ΔMag. will be only 0.4 rather than 7.9.
Jan. 31: The star is HIP 36376, spectral type A3. Its 8.4-mag. companion 55ʺ away in PA 349° will not be occulted.
Feb. 10: The star is SAO 160479 = HIP 84880, spectral type A2V.
Feb. 11: The star is SAO 93276 = HIP 14439, spectral type K3III.
Feb. 27: The star is HIP 76888, spectral type F7V. 9.4-mag. SAO 159447 only 9.4ʺ away in PA 23° won’t be occulted.
May 11: The star is ZC 1233 = SAO 79995 = HIP 40023, spectral type G8IV. A “gradual” lunar occultation reappearance of the star was observed visually in 1928.
June 18: Antenor is one of the larger Trojan asteroids. Lightcurve observations indicate it may be binary.
July 14: 1978 occultation observations indicate that Herculina may have a large satellite; it remains unconfirmed.
Sep. 10: This is unusual because the shadow will be on the Earth’s surface for over a day. For locations in the southern USA and Mexico where the occultation will occur, the occultation will last up to 3 hours, but less near the northern limit, which will cross northern Fla., northern Texas, and central Calif. Mars’ 20.3ʺ disk will be 94% sunlit, giving a dark sliver only 1.1ʺ at the most, so very good seeing will be needed in order to observe the star so close to the sunlit parts of Mars. Predictions for the times of the occultation for over a thousand cities around the world are posted at . http://www.occultationpages.com/rasc/20200910_Mars.html .
Nov. 19: Observations will have special value since (234) Barbara is probably a contact binary asteroid.
Dec. 13: The star is HIP 44230, spectral type G5. It is double, with component magnitudes of 9.30 and 9.49. The pair has a high eccentricity, 0.78, and a 140-year period. Since periastron was expected on 2019.1 with an error of 0.9 year, the pair’s location in late 2020 is almost indeterminate, with the separation likely to be about 0.06ʺ in position angle anywhere from 250° to 180°. The angular diameter of (593) Titania (it’s the asteroid, NOT the Uranian moon) will be 0.08ʺ, comparable to the star component separation, so step or partial events will be possible. Most occultation events will have an apparent magnitude drop of about 0.7 since one component will remain unocculted for most observers most of the time. New speckle interferometric observations will hopefully be made to obtain better estimates that will allow a calculation of the separate paths of the two components that could be a little different from the path given here. When they become available, updated information will be given at www.asteroidoccultation.com.