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Moon to Sweep Through the Pleiades (Forwarded)



 
 
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Old March 31st 06, 03:44 PM posted to sci.space.news
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Default Moon to Sweep Through the Pleiades (Forwarded)

Sky & Telescope
Cambridge, Massachusetts

Contact:
Alan MacRobert, Senior Editor
617-864-7360 x151

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 night landscape.

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
telescope.

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
[http://skyandtelescope.com/aboutsky/...le_1700_1.asp].
Permission is granted for one-time, nonexclusive use in print and
broadcast media, as long as appropriate credits (as noted in the captions)
are included. Web publication must include a link to SkyandTelescope.com .

Sky Publishing was founded in 1941 by Charles A. Federer Jr. and Helen
Spence Federer, the original editors of Sky & Telescope magazine. The
company's headquarters are in Cambridge, Massachusetts, near the
Harvard-Smithsonian Center for Astrophysics. In addition to Sky &
Telescope and SkyandTelescope.com, the company publishes Night Sky
magazine (a bimonthly for beginners with a Web site at NightSkyMag.com),
two annuals (Beautiful Universe and SkyWatch), as well as books, star
atlases, posters, prints, globes, and other fine astronomy products.


 




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