More on Asteroidal Occultation Primer

New: 2002 September 20

The message below from Joseph Sedlak gives more good 
background on asteroidal occultations for beginners,
explaining more of the "what" and "why" as well as
more on "how".  David
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Steve and Dave,

If your primer is being provided to a bunch of Scouts or other
middle and high school kids, you may need a simpler primer.  
Here is a short intro for real newbies:

The Goal:
The idea is to find the shape of an asteroid by watching as it passes in
front of a star.  If many people watch from several locations, you can get
a good idea of its shape.  The star is so far away that it can be thought
of as just a point, with no size. What you measure will be just the size of
one line, or "chord", through the asteroid.  The figure (beware, it's 
large, 859 kilobytes, so it'll take awhile to get with a phone modem)
here illustrates the geometry.

What to measure:
How long does it take the asteroid to pass the star? (It might be a few
seconds up to a few minutes)  Someone else will use your time measurement
and put it together with the speed of the asteroid and the speed of the
Earth to get the length of the line through it as seen from your location.

Location:
It is best to coordinate with other observers in order to get better
coverage of the whole asteroid shadow path.

How to measure:
(this is just one method; there are other ways to do this with video
equipment)
1) Find the star that will be covered. You will need good charts to be sure
you are looking at the right star.
2) Start your tape recorder a few minutes before the expected event and
start your time signal (WWV if possible, or else WTOP) and watch the star
3) When the star winks out, shout "D" (for disappear, or whatever short
word you want)
4) When the star reappears, shout "R" (for reappear)
5) Afterwards, review your tape recording to find when the D and R occurred
to within 0.1 seconds or so. Try to correct for your reaction time.  If
using WTOP, you will need to check with the person who made the "master
tape" to find the exact time when some key word or sound was made, and
measure your D adn R times from that sound.

Other Projects:
Similar measurements can be made for occultations of stars by the Moon.
This is especially interesting if the Moon just grazes the star - then you
can see the star disappear and reappear several times as it passes behind
mountains along the edge of the Moon.


-Joe Sedlak