An occultation is the passing of a Solar System object through the straight line between the observer and another object in the sky, generally a star.
The occultations most easily observed are those produced by the Moon, the planets, and their satellites.
Those produced by asteroids are generally more difficult.
Occultations are the most popular events among amateur astronomers; It is an area where we can contribute scientifically, one in which few professional astronomers can work continuously.
Star occultations are fairly common, this makes then very attractive to amateurs.
The observation of a star occultation by an asteroid can be used to determine the size of the asteroid.
To successfully observe an occultation it generally requires a telescope with enough aperture to clearly show the occulted object. Some occultations are observable through binoculars, or even naked eye.
Observing an occultation requires that the occulted object is visible, but not the occulting object. But the visibility of the occulting object might help in localizing the occulted object before the event (in general, it is not easy to locate a dim star).
A Solar eclipse is an occultation, but a Moon eclipse is not.
A perfect conjunction, or a transit, is also an occultation.
Grazing occultations are of special interest because they let us determine profiles of the surface or atmosphere of the occulting object.
In general, the scientific observation of an occultation implies determining the times of its beginning and ending. Sometimes determining the speeds of increase and decrease of the brightness of the occulted object, even the occurrence of various brightness minima that might indicate the existence of satellites or rings in the occulting object.
To determine the timing of these events, the signal from a "talking clock" is generally used.
It is recorded with the audible signals produced by the observer during the event.
This makes it important to know the response time of the observer that should be somewhat under 0.5 seconds. See Test your Response time!
The international short wave frequencies most used for the transmission of timing signals are: 2.5, 3.33, 5.0, 7.335, 8.638, 10.0, 12.984, 14.670, 15.0, 16.0, 20.0, and 60.0 MHz.
To successfully observe an occultation it is necessary to predict it early enough. The most popular tool is Occult - Free Occultation Calculations software (MS Windows, Dave Herald, IOTA).
This program calculates occultations produced by the Moon, the planets and their satellites, and the asteroids, taking into account the aperture of the telescope in use. It makes maps of the possible observation zones and predicts the expected brightness decreases. It also calculates Solar and Lunar eclipses, and Solar transits.
It is not required to download the Goffin Catalogue, with 4 times more stars than the PPM Catalogue. But one of them is required to calculate occultations of stars by asteroids, planets, and planetary satellites. The Goffin Catalogue functionally substitutes the PPM Catalogue, but it is possible to have both installed.
The most complete and precise star Catalog available for download is the Tycho 2. The much smaller Goffin Catalog and the even smaller PPM Catalog are not as precise as Tycho 2.
The most recent asteroids orbital elements can be downloaded from the Lowell Observatory: Astorb.dat.gz (GZIP) or the Institute of Applied Astronomy, St. Petersburg (IAA): elem?.txz (GZIP), elem?.txt (uncompressed).
Occultations by Asteroids on Stars with Magnitude < 10 for 2004 in Venezuela:
February 03: 3577 Putilin occults PPM 195429 (SAO 138747), at 08:35 UT near Ciudad Bolívar
March 19: 1112 Polonia occults PPM 97574, at 01:12 UT near Maracaibo, Coro and Caracas
May 04: 783 Nora occults PPM 706721, at 06:24 UT near Caracas
June 04: 444 Gyptis occults TYC 0801-00165-1u, at 02:07 UT near Maracaibo and Coro
July 13: 360 Carlova occults TYC 5703-02676-1u, at 03:01 UT near Ciudad Bolívar
September 23: 216 Kleopatra occults PPM 123533, at 06:14 UT near Maracaibo, Caracas, and Cumaná
Event Summary for Longitude -66.83 Latitude 10.5: (Path Distance < 600 Km, Ast. Distance < 0.8", Mag. drop > 0.7, Max dur. > 5 seg, M. Planet Diam. > 25 Km) Occultation & Time M. Planet Occ. Occ. Date U.T. Diameter dur. Star Mag Elon Star Minor Planet Alt Dist y m d h m km " sec. mag drop ° Cat # # Name o " 2004 Feb 03 8 38.7 28 0.01 9.3 7.0 9.3 126 PPM 195429 HIP 60407 3577 Putilin 69 0.2 2004 Mar 19 1 5.8 35 0.02 6.8 9.3 6.1 112 PPM 97574 TYC 1372-02115-1u 1112 Polonia 72 0.2 2004 May 04 6 14.7 40 0.05 11.6 9.5 3.6 143 PPM 706721 783 Nora 71 0.4 2004 Jun 04 2 4.2 159 0.06 5.0 9.7 4.2 56 TYC 0801-00165-1u 444 Gyptis 4 0.0 2004 Jul 13 3 1.3 115 0.07 8.1 9.9 3.5 164 TYC 5703-02676-1u 360 Carlova 63 0.2 2004 Sep 23 6 18.0 135 0.08 5.1 9.5 2.4 74 PPM 123533 TYC 0761-00579-1u 216 Kleopatra 15 0.0
Calculated with Occult V 3.1, Tycho 2 Catalog, with asteroids orbital elements for August '04.
Run it for these dates to see the uncertainty and details for each event.
Given that the geographical uncertainty of asteroid occultations is generally quite large, and their paths quite narrow, an event predicted for near one site might result visible only from another site.
For an updated table of future occultations see Steve Preston's Asteroid Occultation Predictions.
An extra-Solar planet, one that orbits a star different from the Sun, can pass in front of that star slightly diminishing its brightness. A temporal diminishing of the brightness of a non-variable star can then reveal the existence of a planet in orbit around it. This diminishing can be detected with a small telescope and a sensitive CCD camera. Two of these possible transits are expected between December 26 '04 and January 3 '05.
See Amateur Detects Exoplanet Transit (Sep. 2 '04, Sky & Telescope), 'Tis the Season to Find Exoplanets (Dec. 3 '04, Sky & Telescope), Exoplanet Transit Search Observing Program (AAVSO), and Transitsearch.org.
Websites with information on occultations:
American Association of Variable Star Observers (AAVSO)
International Occultation Timing Association (IOTA)
International Occultation Timing Association - European Section (IOTA-ES)
IOTA Asteroidal Occultation, Mid-Atlantic Occultation, Eclipse, and Lunar Impactors Page
Lunar Occultations and Grazes (IOTA)
Occultations by Solar System Objects
Robinson Lunar Observatory
Steve Preston's Astronomy Page Asteroid Occultations, Videography
Steve Preston's Asteroid Occultation Predictions
(With Occelmnt files for Occult including all of the future events)
See also Observatorio ARVAL - Occultation of SAO 164538 by Titania, Sep. 7 '01
This page was updated in: April 25 '06
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Spanish: La Observación de Ocultaciones