To see a star suddenly drop out of sight, only to reappear several seconds later, is a spectacle so riviting that you never forget it. Such an occultation can happen when a dim asteroid passes directly between you and the star. Events like this are rarer than the lunar occultations (see "Lunar Occultation Highlights for 2004") because the solar system’s major and minor planets have a much smaller angular size than the Moon, and they move more slowly across the sky.
It doesn’t matter if the asteroid is too faint to see — you just need to be able to see the star. Several of the stars occulted by asteroids this year are bright enough for easy viewing in binoculars. The whole idea is to record exactly when the star disappears and reappears.
Amateur astronomers at many separate locations can use simple timing methods to acquire fundamental new data about the size, shape, and atmosphere (if any) of the occulting body. That’s why the International Occultation Timing Association (IOTA) seeks these observations. It is true that large telescopes equipped with adaptive optics can reveal asteroid shapes and the existence of companions, but enthusiastic amateurs can do much the same thing with even greater resolution by observing occultations. The key is coordination — an ellipse can be fit to three or four well-spaced “chords,” but more are better for tracing an irregular asteroid’s profile.
The maps and tables in this article describe many occultations taking place in 2004, with emphasis on those visible from populous areas of North America, Europe, Australia, and New Zealand. Only the best events can be covered here, but many more are accessible with most telescopes. I encourage you to visit the Web sites mentioned later in this article to learn of other events visible from your area.
Please note that the complete version of this Web article, which includes tables with more information about each occultation, appears in the March 2004 issue, page 102, and will eventually be available from Sky & Telescope's
North American Occultations
Predictions of this sort have become more reliable in recent years, since most occulted stars’ positions were measured very accurately in the early 1990s by the European Space Agency’s Hipparcos satellite. Those results were incorporated in the highly refined Tycho-2 Catalogue, published in 2000, which has in turn been improved and greatly expanded with the U. S. Naval Observatory CCD Astrometric Catalog-2 or UCAC-2, released in 2003. The asteroids’ orbits are continually being improved as well.
Each occultation can be thought of as the star casting a “shadow” of the asteroid on the Earth’s surface. This shadow sweeps across the Earth in a few minutes. But the path that this shadow takes can differ by a couple of hundred miles from the nominal path shown here, especially for asteroids with numbers higher than about 500. Fortunately, IOTA’s Steve Preston in Washington and Jan Manek in the Czech Republic can refine a prediction a few weeks before an occultation, using mainly observations from the US Naval Observatory’s Flagstaff Station and the Jet Propulsion Laboratory’s Table Mountain Observatory. With UCAC-2 providing a dense net of faint reference stars, many other observatories — including some operated by advanced amateurs — are also now providing the accurate observations needed for refined predictions.
Notes on Individual Events to June 30
February 28th. Epsilon (e) Sagittarii, also known as Kaus Australis, is the brightest star occulted by an asteroid in 2004, and this is an easy naked-eye event. The star has an 8.4-magnitude companion 2.4" away in position angle 142° (p.a. is measured from celestial north around through east). But the separate occultation of the companion will be impossible to observe with the 2nd-magnitude primary so close.
March 20th. The 9.5-magnitude star SAO 138254 is 11" away in p.a. 127°; it will not be occulted.
April 5th (Guizhou). Nu (n) Virginis is a red-giant star with an angular diameter of 0.006", which corresponds to 11 kilometers at the distance of Guizhou (40 percent of the small asteroid’s diameter). As a result the edge of Guizhou will take 0.8 second to cover the star where the event is central. Projection will produce 14-km-wide partial occultation zones centered on the geometrical path limits, bracketing a 21-km zone of total occultation.
April 8th. The components of ZC 1994, also known as HIP 67953, are separated by 3.5" and easily split telescopically; the p.a. is 98°. If seeing is poor with a small instrument, glare from the unocculted component might make it difficult to time the events of the occulted one. The star is bright enough to be seen with binoculars but not be resolved by them.
April 17th. The star 18 Aquarii is HIP 10566. Although it is a double, the 13th-magnitude companion is 52" away in p.a. 305° and will not be occulted.
May 2nd. Delta (d) Scorpii, also known as Dschubba, has a 4.7-magnitude companion 0.19" away in p.a. 352° that will be occulted in a path south of that shown for the primary. The star is a variable, currently in outburst; the combined magnitude was 1.8 last October.
May 11th. This asteroid, 1172 Aeneas, belongs to the Trojan family that orbits with Jupiter.
June 17th (1995 XJ1). Beta1 (b)1 Capricorni, also known as Dabih Major, has a 4.8-magnitude companion 0.08" away in p.a. 210° that will be occulted in a path a little north of that shown for the primary.
Australia/New Zealand Occultations
Notes on Individual Events to December 31
August 21st. Alpha2 (a)2 Librae, also known as Zubenelgenubi, may be a very close binary based on lunar-occultation and spectroscopic observations.
September 5th (Camilla). Although the star is rather faint, IOTA strongly encourages observations of this occultation. Adaptive-optics work with large telescopes shows that relatively large 107 Camilla has a sizable satellite, and William Merline (Southwest Research Institute) plans to update the satellite’s orbit so that its occultation path can be predicted and that event observed. We want to show that occultations by asteroidal satellites can be confirmed.
September 6th. Eta (h) Ophiuchi, also called Sabik, is a close double whose companion lies 0.59" away in p.a. 239°. Nephthys may have a satellite, according to adaptive-optics observations.
September 12th. SAO 36280 is a close double with components of magnitude 7.2 and 9.3 separated by 0.6" in p.a. 108°. This separation is too close to resolve. The path shown on the North America map is that for the primary star. The path for the secondary is 220 km to the north, nominally passing over Massachusetts and central Ohio.
September 20th (Chryseïs). The star 131 Tauri is HIP 27316, a close double separated by 0.16" in p.a. 196°.
October 2nd. Henk Bulder in the Netherlands saw a gradual disappearance of SAO 97890 during a lunar occultation on April 25, 1996, indicating that the star may be a close double.
December 14th (Budovicium). The star 51 Tauri is HIP 20087, a close double separated by 0.12" in p.a. 201°.
Occultations of Stars by Major Planets
Generally more difficult than occultations by asteroids are those by the major planets, since their brilliance overwhelms most stars. The table below lists events of this type predicted by Edwin Goffin and David Herald for 2004. For the occultations by Mercury and Venus, the disappearance generally occurs on the dark side of the planet.
|Occultations of Stars by Major Planets in 2004|
|7.9||7m||China, SE Asia|
|7.9||9m||Central, SW Asia|
|9.5||3m||East N. America, Bermuda|
|7.9||54m||NW S. America, Caribbean|
|7.4||2m||West N. America|
|6.6||1.4m||South Atlantic Ocean|
|8.0||12m||Portugal, NW Africa|
|8.5||5m||East Canada, East S. America|
The star 40 Cancri is in the Beehive Cluster. This occultation occurs at sunset for the Falkland Islands and Tierra del Fuego.
Herald has also predicted a 188-second-long occultation of the 8.2-magnitude star SAO 79556 by Titan at 7.7h UT on August 24th. During this event Titan will be 1' north-northeast of the planet. The region of visibility, including the central line from which a central flash might be observed, is shown on the world map of asteroidal occultations on page 1. The Titan occultation will be difficult, requiring a sensitive detector, but observations in an infrared methane band can increase the signal-to-noise ratio. (In 2001, occultations of two 14th-magnitude stars by Titan were recorded this way with the 200-inch reflector on Palomar Mountain.)
For More Information
The Celestial Calendar section of Sky & Telescope is a good place to check for finder charts of the best occultations in a given month. Those for most other events can be found on IOTA’s Web site and on Steve Preston’s Web site. Use a finderscope of 50-millimeter aperture or more. On your main telescope, a low-power, wide-angle eyepiece helps. I can’t stress enough that you need to allow a generous amount of time, at least a half hour if a bright star is not nearby, for locating the target star.
Finder charts, detailed maps, observing news, and information on events world-wide are carried in the Occultation Newsletter, published by IOTA. For local data about all events possibly visible from your loca-tion, send your longitude and latitude, $1.00, and a large, self-addressed envelope to Jim Hart at 2616 Monte Cresta, Belmont, CA 94002-1214, or obtain the information free by e-mail request to firstname.lastname@example.org.
Please send observation reports to Jan Manek at Stefanik Observatory, Petrin 205, 11846 Praha 1, Czech Republic, or by e-mail to email@example.com. A report form is available from IOTA's Web site or from Manek by e-mail. Remember that if you are near a path and watch the target star yet see no event, your report is still important to us. In a few months, all known past asteroidal occultation observations will be available on IOTA’s Web site.