[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

BSkylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 13, 2006. 
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at
     http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
     http://www.astro.uiuc.edu/~kaler/sow/sow.html.

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Coming soon: The Cambridge Encyclopedia of Stars.

We begin the week on Friday the 13th with the last quarter Moon,
and then will watch it wane in the crescent phase toward new, which
is not reached until Skylights' next week, on Saturday the 21st. 
Watch in the morning hours as the Moon gets ever closer to the
eastern dawn horizon, the Earthlight on the lunar nighttime side
getting ever stronger until the thin crescent just disappears into
twilight.  The morning of Monday the 16th, the crescent will appear
just above Saturn.  Our lunar companion then takes a ride through
southern Leo, appearing down and to the left of Leo's luminary,
Regulus, the morning of Tuesday the 17th.

Saturn is the only one of the bright planets we have left to
admire, and you have either to stay up late or get up early to see
it.  If you do, the planet, rising around 2 AM daylight time,
precedes the rising of Regulus.  By the onset of dawn, they make a
fine couple in the eastern sky, Saturn (above Regulus) the brighter
of the two.  Even though Mercury passes its greatest eastern
elongation with the Sun on Monday the 16th, the low angle of the
evening ecliptic against the horizon renders the little planet --
though bright -- very difficult to see.  Mars (still to the east of
the Sun) and Venus (still to the west of the Sun) are completely
(and respectively) lost to evening and morning twilights.  Jupiter,
while still visible, is a tough find in evening, as it sets before
twilight ends.  That leaves the evening sky with Uranus and
Neptune, which take accurate maps and positions to locate in their
respective current constellations, Aquarius and Capricornus (Uranus
actually visible to the naked eye in a dark sky).

The fading Moon allows a wonderful opportunity to see the Orionid
meteor shower, whose meteors appear to emanate from the celestial
Hunter.  Though the broad peak runs from the morning of Friday the
20th through Sunday the 22nd, the shower will be building during
much of the current week (and actually runs through most of October
and into November), so take a look.  Under a dark sky, at maximum
you may see 20-25 meteors a minute.  They are caused by the debris
of Halley's Comet hitting the Earth's atmosphere.  Halley's
flakings come at us twice, and will hit us again next spring as
May's Eta Aquarids.

No matter what the time of year, northerners can always admire
circumpolar Ursa Minor and its Little Dipper, whose handle ends in
Polaris, the second magnitude Pole Star, which sits close to the
sky's northern apparent rotation pole (and guides the way north,
its angle above the horizon even providing us with our latitude). 
The southern analogue is not so easy to see.  Surrounding the
southern celestial pole, invisible to anyone north of the equator,
lies much fainter (fifth magnitude) Sigma Octantis in the modern
(and very dim) constellation Octans, the Octant, the dichotomy an
accident of nature.

STAR OF THE WEEK: NU OCT (Nu Octantis).  As if fourth magnitude
(3.76) Nu Octantis, the luminary of Octans (the Octant, one of the
sky's several navigational instruments), is not already obscure
enough for being in such a dim constellation, it sinks further into
obscurity by being quite overwhelmed by the fame of the
constellation's best-known star, the fifth magnitude (5.47, nearly
sixth) southern pole star, Sigma Octantis.  Way down the Greek
letter list, Nu Oct oddly beats out the next star down, fifth
magnitude (4.15) Beta Oct, while the Alpha star, usually a
constellation's brightest, is a full magnitude (5.15) fainter yet. 
In spite of its seeming anonymity, which is enhanced by being yet
one more yellow-orange class K (K0) giant, Nu Oct harbors a
pleasant surprise by providing us with a wonderful chance to see
what is going to happen to the Sun.  For a giant, its character is
modest indeed.  As befits this humble star, there is no temperature
measure, so 4700 Kelvin must be adopted from its K-giant class.