By
David Dunham - Astroalert News
Spectacular
occultations of the 3rd-mag. red giant stars eta and mu Geminorum by a thin crescent
Moon will be visible from most of North America Friday evening, April 27. They
can be observed with most binoculars and recorded with most camcorders without
the need for a telescope, and those with good eyesight might not even need binoculars.
This
is a fitting evening complement to the current naked-eye (with suitable protection)
sunspot group, and a good prelude to Astronomy Day activities on Sat., April 28.
Please copy this message to your friends and colleagues.
These stars are
in the feet of Castor, with eta also sometimes known as Propus and mu as Tejat.
The angular diameters of the stars, around 0.01", can be measured with high-speed
photoelectric recordings. Times of the occultations for large North American cities
are given on p. 118 of the January issue of Sky and Telescope; that page also
includes a graze map that shows the southern limit of the eta Gem event across
northern Mexico.
Much information about these occultations, including naked-eye/camcorder
views showing the paths of the star behind the Moon for several North American
cities, are now on IOTA's Web site at http://www.lunar-occultations.com/iota
Unless
clouds (or, in some areas, bright twilight or daylight) reduce contrast, you don't
need these diagrams to know where to look for the star - it will be the only bright
star just above, or above and to the left, of the Moon's dark side (direct, not
inverted, view). It would be of some interest to see if you can time the disappearance
without any optical aid, as observers did in the centuries before the telescope
was invented.
Camcorder timings of the occultations are valuable for studies
of the lunar profile, information needed to reduce solar eclipse Bailey's bead
timings used for solar radius variation studies. Occultations of eta and mu Gem
are especially important for this since they are near the ecliptic, providing
a lunar profile, or orientation ("librations" is the astronomical term), similar
to that for eclipses.
The camcorder record must include an accurate time
base, such as audio recordings of the short-wave time signals from stations WWV
or CHU, or a local broadcast, if it has been calibrated (see next paragraph).
Some (but not all) GPS displays also give accurate time.
Let me know if
you can record either a local TV broadcast (must be over-the-air, not cable) or
a strong AM radio station along with WWV or CHU time signals for one of these
events so that we can post and distribute this information to other potential
observers in your region who don't have short-wave receivers. That is, these other
observers can then use the local TV broadcast or AM radio station as the accurate
time base since it will have been calibrated with the master tape that you might
make.
The disappearance of eta Gem behind the dark edge of the 22% sunlit
Moon will be visible from most of North America from the Rocky Mountains eastward,
while the disappearance of mu Gem will be visible mainly west of the Rocky Mountains.
Predictions
for the larger North American cities are given below; predictions for hundreds
more North American cities are given on IOTA's Web site above. Predictions for
phenomena in daylight have been removed below, but are in the predictions on the
IOTA Web site. Also not included here, but given on the Web site, are predictions
for the much more difficult reappearance (hard even with good telescopes), since
it will occur under poorer conditions with the Moon lower in the sky; glare from
the sunlit side of the Moon will overwhelm the star for most observers. Use of
a red filter for these M giant stars will increase contrast to aid reappearance
observations. |
