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Distances: The distances
can be mind boggling:
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The moon is light-seconds away.
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Asteroids and planets can be light- minutes
or hours away.
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The stars are dozens to many thousands of light
years away.
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An occultation involves objects many light years apart
aligning briefly:
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A relatively bright point of light many light years
away
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A very small (almost a point) dark moving disk many
light-minutes away.
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A point (the observer) on the Earth, which is moving
and spinning.
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A curved line of shadow on the surface of the Earth.
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Some asteroids will resolve into a disk and be visible in
the eyepiece. The dimmed portion of the event has the
brightness of the asteroid (or planet or moon).
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In theory, a bright asteroid occulting a dim star
could actually cause the star to brighten!
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You may be able to see the asteroid moving near the
star.
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The star is a point source of light, providing a very fine
detector:
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Asteroids & airless moons - it will suddenly
blink off and back on
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Very close, very large stars can sometimes produce a
stepped event where the star gradually disappears
rather than winking on or off.
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Planets and moons with an atmosphere will also dim
gradually
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Double stars or double asteroids provide steps in
brightness as one of the pair enters the event
first. This provides several quick science
results:
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During the time a double star is split,
scientists take spectrums to verify the star
types.
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The event can split star or asteroid pairs
that we can't split otherwise.
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Like an eclipse, the "shadow" of the star follows
a curved path on the surface of the Earth.
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Occultation paths can start or end with the event
start or end.
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Occultation paths can start or end with the horizon
(passing off the edge, as viewed from the star or
asteroid.
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The path's curve is affected by the blocking
object's and Earth's rotation and orbit.
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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!
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Most events move very swiftly across the Earth - The
shadow can travel hundreds or thousands of miles
across the Earth in only a few seconds.
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