Sky & Telescope
Alan MacRobert, Senior Editor
Press Release: March 27, 2006
Moon to Sweep Through the Pleiades
Note to Editors/Producers: This release is accompanied by
publication-quality graphics; see details below.
A special celestial event to watch is coming up on Saturday evening,
April 1, 2006, for anyone who lives in the eastern or central part of
North America. That evening, if the sky is clear, you can watch the
waxing Moon eclipse, or "occult," a number of stars in the Pleiades
cluster in the western sky during and after dusk. You'll have a decent
view with your unaided eyes if your vision is sharp, but binoculars will
do much better. And if you have a telescope, now is certainly the time
to get it out.
Just keep watch on the Moon from twilight on April 1st until the Moon
sinks too low in the west to follow. You'll notice right away that the
Moon is next to, or among, the stars of the Pleiades cluster. Optical
aid will also show the Moon's dark, night side dimly visible by
earthshine -- the light of Earth's daylit face lighting up the Moon's
As time goes on, you'll see that the Moon's dim earthlit edge is
creeping toward the stars it's facing. Eventually, with a little luck,
you'll see the edge approach a star until the star seems to hang right
on the edge, like a tiny white fire on the Moon. Then suddenly --
instantly -- the star will snap out of view. You've just witnessed a
lunar occultation (from the Latin word occultare, to hide).
The Moon moves by about the width of its own diameter per hour against
the background stars. So the occulted star reappears out from behind the
Moon's other, sunlit edge up to an hour or so after disappearing. But
the reappearances are much harder to see, since they happen in the
bright glare of the Moon's daylit side. For these you really need a good
More on this beautiful event appears in the April 2006 issue of Sky &
Telescope and the April/May 2006 issue of Night Sky magazine. Included
in each is how to get time predictions of when individual stars will be
covered up by the Moon's dark edge at your particular location.
The Value of Occultations
Astronomers have tracked occultations for centuries. Aristotle told of
the Moon covering Mars on April 4, 357 BC -- proof that Mars was farther
away than the Moon. The suddenness of star occultations offered the
first proof that the Moon has no air and therefore cannot support life.
If the Moon had an atmosphere, stars would gradually dim as the Moon's
edge approached them, the same way the setting Sun dims before it
reaches Earth's horizon. Scrutinizing an occultation in 1843, Friedrich
Wilhelm Bessel found that a star's light rays did not bend at the Moon's
edge by any amount he could measure, a sign that the Moon could have no
more than 1/2,000th as much air as Earth.
For many years, precise timings of occultations gave the most accurate
fixes available on the Moon's orbital motion around the Earth. Also,
many close double stars were first discovered by their "stepwise"
occultations. In such an event, the star drops out of sight on the
Moon's edge in two distinct steps, as first one star of the double is
covered, then the other -- even though the stars are so close together
that they may look single even in the most powerful telescopes.
Most of these scientific uses for occultations have been superseded by
other, more modern techniques. But amateur astronomers still go on
expeditions to time grazing occultations -- when the Moon's edge barely
skims a star sideways. During such an event, the star may flash in and
out of view several times as lunar hills and valleys slide silently
across it. Timings of grazing occultations are still valuable for
mapping the Moon's profile very accurately.
Sky & Telescope is pleased to make the following publication-quality
graphics available to our colleagues in the news media
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broadcast media, as long as appropriate credits (as noted in the
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