Occultation of Stars by Heavenly Bodies
The Study of the Occultation of stars by Asteroids, Planets and Moons.
As heavenly bodies cross over a star, their shadow is cast on the surface of the Earth. By timing the length of the occultation of a star, we can deduce the diameter of the asteroid, and refine its orbital motion. Multiple observations from widely separated sites reveal the outlines of the occultation body. Each site will be timing along a different cross-section or chord of the heavenly body. The International Occultation Timing Association (IOTA) was organized to collate observations of occultation events and publish findings.
Equipment required is modest for most Occultation observations.
A small telescope that can see the background star is adequate. Visual timing can be done by viewing the star and making an audio recording while verbally calling out the start and end of the occultation while also recording the audio of WWV time signals via shortwave radio in the background. More elaborate setups use video cameras and GPS time stamps in the video frames. The most important pieces of information are the observing site’s GPS location and an accurate time base for the observation. Specialized equipment is available for purchase to do video recording with GPS location and time stamps from IOTA. Low cost commercial low light video surveillance cameras are employed that insert as eyepieces into telescopes.
Free software is available for Occultation work.
Software to predict occultations from your GPS site is available at http://www.occultwatcher.net/ . Occult Watcher will link to star maps for the target star and predicted start and end times. Always begin recording 5-minutes before start and keep recording 5-minutes after the end time predictions. Rings, moons, and double asteroids have been serendipitously discovered via multiple occultation of target stars. Even null results are valuable as that will set a boundary for the shadow. http://www.lunar-occultations.com/iota/iotandx.htm is the IOTA main site with links to software and other resources and how-to information.
The broad outline of the Occultation body can be deduced based on timings.
Mountains, valleys and depressions have been discovered along the limbs. For planets, we also get a profile of the atmosphere as the star gradually dims at start and gradually brightens at the end. Video records are images taken at 1/30th of a second interval, which give excellent profiles of the atmospheric extinction by altitude above the planet.
Amateur astronomers are doing real science.
Anybody who is able and willing to record and submit data to IOTA are welcome. It takes dedication to setup and stay vigilant at the eyepiece or computer screen in the late hours when occultations happen. You will be given credit in any papers that are published, and your unique site chord displayed in the dataset. The midnight hour can be confusing as the date changes and most occultations are predicted in Universal time. Calculate the local times and date to be successful.