Occultation TimingAn occultation is when one celestial body passes in front of an other and blocks its view. Several types of occultations of interest to astronomers that amateurs can help out in are lunar and planetary (including minor planets or asteroids) occultations. Lunar eclipses, solar eclipses and jovian satellite eclipses are also considered occultations but most of the amateur work is done with the first two.
Lunar occultations happen when the moon passes in front of a star, major planet or minor planet. Grazes happen when a star passes behind hills and valleys close to the moon's north or south pole. When this happens the star seems to wink on and off several times. The 'graze zones' are a mile or two from the edge of the lunar occultation path. Amateurs can observe and time lunar occultations to help astronomers refine their knowledge of the lunar topography and help map the lunar limb profile as well as helping with lunar orbit dynamics and stellar positions.
Planetary occultations happen when a planet or asteroid passes in front of a star. Amateurs can observe and time asteroidal occultations to help astronomers determine the size and shape of the asteroid.
Jovian satellite eclipses are when one of Jupiter's moons blocks the sunlight onto another or passes in front of another. Observing and timing these events provides information of the satellite's position with respect to Jupiter. This has helped NASA plan exploratory spacecraft missions.
This is a relatively easy task for amateurs to do. You just have to be in the right place at the right time. Depending on the occultation the observation can be made with the unaided eye, binoculars or telescopes of all sizes. Camcorders and/or audio tape recorders are also used. An inexpensive CCD security video camera (PC-23C from Supercircuits inc. for about $79) is gaining popularity for this research. An accurate time base is also required to make measurements within tenths of a second. A short wave radio capable of receiving WWV or CHU will accomplish this. A phone connection via AT&T to the US Naval Observatory's master clock will also work. AT&T is required to prevent the call from being routed via satellite. Satellite communications (other phone companies and GPS receivers) will add an unacceptable delay to the time signal.
Accurate position is required to within 15 meters. This includes latitude, longitude and elevation. GPS receivers and/or USGS topographical maps can provide this information. Maps with the occultation paths visible at specific locations on Earth and stellar finder charts for the occultations can be found from the magazines and occultation groups listed below. These groups have detailed information on predictions, measurement techniques and how to submit data.
To make an observation you can watch it visually with or without optical aid and talk into an audio tape recorder (noting events as they happen) while the time signal is playing in the background. With a video camcorder or camera just hold it (or mount it) to the telescope with the microphone picking up the time signal in the background. The PC-23C camera has a built in microphone.
For more information contact:
WCAC Occultation Timing SIG - The Wilderness Center Astronomy Club's Occultation Timing Special Interest Group.
Astronomy Magazine - search on occultation. Predictions and upcoming events.
IOTA - International Occultation Timing Association. Encourages and facilitates the observation of occultations and eclipses. President David Dunham maintains email lists for potential observers and collects data.
Sky and Telescope's Occultation Page - How and why to make observations, observing tips, occultation highlights and predictions.
RASNZ Occultation Section - The Royal Astronomical Society of New Zealand. Good information for beginners and advanced observers on predictions and how to collect and contribute data.
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Last updated January 11, 2009, by Brian Gray