Occultation Primer

Asteroids occulting stars.  These are the most common events recorded.  (Remember there are thousands of asteroids and millions of stars.)  Thanks to a great variety in the geometry plus the relative sizes and magnitudes, these events are very variable in duration and observability.

The moon occults stars or planets.  A total lunar occultation means the moon totally covers the star or planet.  These are often long events.  A grazing occultation means the object skims one limb (edge) of the moon and will wink in & out as it passes mountains and valleys.

Planets occult stars.  These are not commonly recorded, thanks to the brightness of most of the planets.  (This makes it hard to see the star behind it, unless the star is very bright.)  Since the planets move slowly and are fairly large, these are often long events.

Planet's moons occult stars.  Most often, the events are for Jupiter's four bright moons.  Since the moons appear to move rather swiftly, these are often fast events.

Jupiter's moons in "mutual events".  Every six years, Jupiter's moons' orbits line up with the Earth, so we see one moon cover another or one passes over the shadow spot of another. (We're in that period now - Winter of 2002-3.)

An occultation involves objects many light years apart, one a very small (somewhat) dark disk, one a (somewhat) bright point, and a line of locations on a sphere (Earth) coming into very precise alignment for a few brief seconds.

The star's positions are (practically) fixed and very accurately known. Most are point sources of light which won't show as a disk under high magnification.  This makes an ideal detector:

	Asteroids & airless moons - it will suddenly blink off and back on
	Very close, very large stars can sometimes produce a stepped event where the star gradually disappears rather than winking on or off.
	Planets and moons with an atmosphere will also dim gradually

Planets and larger asteroids will resolve into a disk, even with relatively small scopes.  Smaller asteroids and Jupiter's moons may not resolve into disks with amateur scopes, but are still large enough to occult the star.

The distances can be mind boggling:

	Blocking bodies are relatively near - The moon is light-seconds away, while asteroids and planets can be light- minutes or hours away.
	The stars are dozens to thousands of light years away. (That's why such large objects appear as points of light rather than disks or spheres.)

Like an eclipse, the "shadow" of the star follows a curved path on the surface of the Earth.

	Most occultation paths will have either the start or end point on Earth's surface.
	The path's curve is affected by the blocking object's and Earth's rotation and orbit.
	The size, shape, orbit and spin of the blocking body are used to determine the path's size and shape.  Since many asteroids are not well studied, approximations of these numbers are used, which can mean a 25-50% error in the path location & size!
	Most events move very swiftly across the Earth - The shadow can travel thousands of miles across the Earth in only a few seconds.

To gather useful observations, groups of observers place themselves along the path in a perpendicular slice of that path (see the dots on the map above):

	The prediction gives both a center line (maximum eclipse time) and shadow width (minimum eclipse time or near-miss).
	Ideally, groups would assign people along the center, out to the edge of the shadow, and just outside the predicted path to give full coverage.
	Each person within a group would observe a slightly different profile of the blocking body.  For a 50km-wide path, observers spaced 7-10km apart would give useful profiles.
	Multiple group can help fully define the shape by placing observers in slightly different slices of the asteroid to increase the definition of the profile (see below).
	Even a near miss would help define the maximum size.  Grazing occultations would show the profile in detail.
	Each observation is like slicing a potato into chips.  Stacking the chips then gives us a rough shape for the potato.

Visual observations are the easiest, but least accurate method.  You'll need a telescope able to view the star in question, a short wave radio tuned to 5, 10, 15, or 20 MHz (WWV), a tape recorder, and a finder chart for the event. 

	Finder charts and event info can be found at http://lunar-occultations.com/iota/iotandx.htm
	Tune the radio to WWV and set the volume so the tape recorder can record it and your voice.
	Use the finder chart and set the scope to center on the star.
	When the star winks out, call "out".  When the star reappears, call "in".
	A clicker of some sort would help increase the accuracy of the timing by producing a quicker sound.
	Ensure the timing ticks from WWV are recorded both before and after the event.

Video recording is much more accurate and eliminates human error in calling out the timing.  Unless you're using a large telescope, video setups are not capable of recording very faint events.

	Lunar occultations of bright stars or planets can be recorded with a camcorder zoomed to maximum.  A radio playing WWV will be recorded through the camcorder's microphone.
	Video recording of fainter events requires a low light security camera attached to the eyepiece of a telescope.  A camcorder records the video from the camera and the audio from the radio playing WWV. See http://lunar-occultations.com/iota/video/rnvideosetup.htm for more information on this type of setup.

Objects: 

	The star's position to arc seconds accuracy
	The time to hundredths of a second
	The observer's position in latitude and longitude to within 10

Equipment for visual recording:

	Telescope - 4-8 inches, f/6 or wider
	Tape recorder with microphone
	Short-wave radio tuned to WWV (5, 10, 15, or 20 MHz)
	Charts and information about the occultation

At the most basic, the star helps outline the blocking object.  This will define:

	The orbit, size, and shape of the blocking object
	The mountain and valley profile of the moon (during a grazing occultation)
	Small moonlets of asteroids
	Close companion stars that can't be split any other way
	Redefine the star's position

If the dark object is airless, the occultation appears like switching off a light.

If the dark object has an atmosphere, the occultation is likely to dim or change, then wink out, then reappear dim and then come back to the original magnitude.   Sometimes, scientists can read the change in the light spectrum and determine the gasses in the atmosphere.  Most of the time, this can help define the depth of the atmosphere.

Occultations happen randomly throughout the surface of the Earth.  Some will happen in your neighborhood and your backyard.

Professional astronomers are collected at large observatories (Hawaii, Chile, the Rockies, & sites in Europe & Japan) or at universities.  They or their equipment are likely to be too busy to observe occultations.  Their equipment may not be suitable for occultation observations.

Amateur astronomers are scattered around the world and have equipment suitable to occultation observation. 

Amateurs are also willing to donate time and effort to the project.

The observations return valuable information to expand our scientific knowledge.

This knowledge may help scientists defend the Earth from encroaching asteriods!