Intro to Occultations
Introduction
An occultation occurs when a solar-system body passes in
front of a more distant object (e.g. a star or another solar
system body), partially or totally hiding the more distant
object and momentarily blocking its light. Each occultation
can be seen only at the right time and from a limited part
of the Earth. There are two main areas of
occultation astronomy: lunar occultations and asteroid
occultations.
Lunar Occultations
Observing lunar occultations tends to be fun and easy to
learn. These events do provide novice observers with a
training tool from which they can make timings using visual
means or more advanced methods such as with video. Lunar
occultations are more easily observed by those with small
telescopes or less experience. The Moon makes a very
convenient guide to identifying the area in the sky in which
to find the star which is to be occulted. Lunar
occultations can be classified into two groups: total lunar
occultations and grazing lunar occultations. In a total
lunar occultation the observer will see only one occultation
event, either a disappearance of the star as the moon passes
in front of the star or a reappearance of the star as the
moon moves away from in front of the star. In a grazing
lunar occultation, the very edge of the moon passes in front
of the star and the star may disappear and reappear many
times as the mountains on the edge of the moon individually
occult the star.
Asteroid Occultations
For asteroid occultations the star is usually the brightest
component of the occultation. The asteroid is usually
several magnitudes fainter than the star and often too faint
to be detected in a small telescope. In an asteroid
occultation, the observer must find the star to be occulted
and monitor the star to watch for any drop in brightness
that would signal an occultation. Asteroid occultation
events typically last several seconds but observers may
record much shorter or much longer events in rare cases.
An observer will only see an event (drop in
the brightness of the star) if they are located inside the
path of the asteroid's shadow. Since asteroids are
generally much smaller than the moon, choosing a location
for observing an asteroid occultation is more important than
location in lunar occultations. In addition, asteroids
subtend a much smaller angular size on the sky and this
leads to more uncertainty in the actual location of the
asteroid's shadow. Asteroid occultation predictions
provide information on the expected location of the
shadow path, expected time of the occultation, the level of
drop in the star's light and the expected duration of the
occultation event. An observer can expect to see a single
disappearance (or drop in starlight) and a single
reappearance though it is possible to see step events.
Some asteroids actually have one or more satellites (also
known as moons) orbiting them. As the asteroid moves in its
orbit, it projects a shadow across Earth. Rarely, an unknown
asteroid moon moving with it creates opportunity for discovery. Such
secondary occultations should be expected to occur within
+/- one minute of the actual asteroid occultation and be of
very short duration,
perhaps a fraction of a second and may be quite distant from
the predicted asteroid shadow line. For this reason, if you
cannot travel you should consider observing an asteroid
occultation even if
you are some distance from the predicted shadow line. You
may be the person who discovers a new moon of the asteroid!
