2006/09/05

What is an occultation?

Very simply, an occultation is an eclipse involving a moon, planet, or asteroid in our solar system passing in front of a star in our galaxy. The circular shadow of the asteroid usually makes a curved path on Earth.
A second type of occultation is a lunar occultation, where Earth’s Moon eclipses distant stars. A very special type of lunar occultation is a grazing occultation, where the star just touches the edge of the moon on a tangent and winks on & off as it disappears behind lunar mountains.

Why are occultations important?

Stars are very distant, so they appear as points of light. The light beams from the star are essentially parallel, so we can define dimensions and speed:

Occultations by Asteroids, Planets, and moons (other than the Earth’s):

* See the outline of the asteroid, which can help determine its composition.

* Detect companion asteroids (moonlets).

* Detect a companion star that is too close to see directly.

* Define the asteroid’s orbit, which is very important for near-Earth objects.

* Detect dust around the asteroid or moon.

* Detect atmosphere around planets or moons

Lunar Occultations:

* Examine a dim companion star for position and orbit

* Refine mapping data for lunar mountains and valleys on the limb of the Moon, which has not been well-mapped.

* Roughly determine a very close and large star’s diameter

How do we observe them?

At the most basic, the equipment required is pretty simple – An optical instrument capable of seeing the target star, a voice recorder, and a short wave radio tuned to the WWV time signals. Of course, you would need a terrestrial map showing the appropriate observing location and a celestial map showing the location of the target star. You watch through the telescope with the radio and recorder running, calling out when you see the star wink off & on. More advanced observers use video recording equipment.

The kind organizers of the International Occultation Timing Association (IOTA) create predictions of the shadow paths, dates, and other parameters, then post them on thier predictions web site. Only a few of the millions of asteroids’ orbital paths are know to high accuracy, so their special software also ranks the event to show how well defined the path is.

With this information, volunteers along the path organize themselves to pick a point along the path and a specified distance away from the predicted centerline. Some volunteers travel hundreds of miles for these events!

Many times we will gather in small groups along the path. As long as we each pick a different distance from the centerline at our observing location, it really doesn’t matter how distant we are from each other.

Think about an egg slicer. In this case, the wire strands are observers, each with a different distance from the centerline of the path. Just like the egg slicer, we want a few observers to be just outside the path to verify that we have the maximum size defined.

It doesn’t matter if I am in Texas and other observers are in Mexico, New York, Florida, or Canada. If each of us has a different distance from the local centerline, the observations can be combined to slice the egg. The observations will
have some error, but it will be easy to see the
shape of the asteroid with enough observations.

Why volunteer?

* 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 that they can transport to remote locations for occultation observations.

* 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 asteroids!

Additional Information on Occultations:

* Asteroidal Occultations: http://lunar-occultations.com/iota/asteroids/astrndx.htm

* An explanation of the prediction data: http://asteroidoccultation.com/asteroid_help.htm

* Occult – the prediction software: http://lunar-occultations.com/iota/occult3.htm