Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 26, 2001.
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.
As a result of error and travel, Skylights is late this week.
My apologies.
The Moon passes through its full and brightest phase this week the
night of Wednesday, October 31. This one, more than the last full
Moon of October 2, deserves to be called the "Hunter's Moon," as
the early evening around the time of the full Moon is still
dominated by moonlight. As a result of various misinterpretations,
the second full Moon in a month is sometimes called a "blue Moon."
This full Moon is curious in that the technical full phase occurs
on the morning of Thursday, November 1, at 5 hours 41 minutes
Greenwich Time (better known as Universal Time), and therefore does
not qualify, nor does it in Eastern Time, which is 5 hours behind
Greenwich. However, for the remainder of the Americas (Central
Standard Time, 6 hours behind Greenwich, and west), this one is
indeed a "blue Moon," as it takes place at 11:41 PM CST. Such
timing differences can also confound the much more important date
of Easter, which falls on the first Sunday following the first full
Moon after the Sun passes the vernal equinox in Pisces.
The big news of the week, however, involve a remarkable interplay
between Mercury and Venus. If you have never seen Mercury, now is
the time, as brilliant Venus, which now rises in the east just as
dawn starts to light the sky, shows the way. On Monday the 29th,
Mercury reaches its greatest western elongation from the Sun. From
Saturday, October 27th until Wednesday November 7, Venus and
Mercury will be within a degree of each other, an extraordinary
event that is rarely repeated. Just find Venus, which is hard to
miss as it will be the brightest body in eastern morning twilight,
and the next brightest thing close to it will be Mercury! What
makes the close pass even rarer is that the two are never in formal
conjunction, wherein one planet lies due north of the other. Of
much lesser interest, since it cannot be seen (not in Moonlight
anyway), is an event that involves Uranus, which begins its
retrograde motion within the confines of dim Capricornus on the
night of Monday, October 29.
On the morning of Sunday October 28, Daylight Savings Time ends in
the UK, Canada, and the US, that is, we drop back an hour to our
own Standard Time rather than using the one to the east of us. The
sky darkens an hour earlier (a strictly artificial event, as it is
the clock changing, not the sky), but we gain daylight back in the
morning. Look to the south now about 8 PM STANDARD Time, and you
will see the bright star Fomalhaut crossing the meridian. If you
are far enough south, from just above 40 degrees north on down, you
can see -- once the Moon is out of the way -- the modern
constellation Grus, the Crane, which, unlike most of the
constellations in the sky, rather looks like what it is supposed to
be, a great bird walking along the southern horizon -- providing
your horizon is clear of trees, corn, or even soybeans.
STAR OF THE WEEK. ANWAR AL FARKADAIN (Eta Ursae Minoris). The Big
Dipper (the Plough in England), the major figure of Ursa Major (the
Great Bear), is so well known that few except the dedicated pay
much attention to its counterpart, the Little Dipper, which in
parallel is the most (in fact the only) recognizable portion of
Ursa Minor, the Small Bear. And no wonder, since the Little Dipper
is so faint that it cannot be seen in any town with bright lights.
The figure is recognizable mostly by the North Star (Polaris, Alpha
Ursae Minoris) and by the two front bowl stars, Kochab and Pherkad
(Beta and Gamma). Kochab was at one time called "Nair (or Anwar)
al Farkadain," meaning the "bright one" (or "the lights") of the
"two calves," and Pherkad was called "Alifa al Farkadain," meaning
the "dim one of the two calves" (hence the name "Pherkad"). As so
often happenes, the names have been transferred to other stars, in
this case to Zeta and Eta Ursae Minoris. And here, at the bottom
of the Little Dipper, we find dim fifth magnitude (4.95) "Anwar,"
the faintest star of the Dipper's seven. Physically, the star is
nearly (but not quite) sunlike, a class F (in the middle of the
range, F5) dwarf with an estimated temperature of 6400 Kelvin,
right at the point at which we do not have to correct for infrared
or ultraviolet radiation. The star's luminosity of only 7.4 times
that of the Sun leads to a radius twice solar and a mass 1.4 solar.
Rather well along in its hydrogen-fusing lifetime, Anwar is a bit
brighter than normal for its temperature, and seems close
to becoming a "subdwarf," a star that has shut down hydrogen
fusion, if it has not already done so. Anwar is listed as a double
star with a rather distant companion, but the cool nature of the
star and its dim 12th magnitude status place it at only about half
Anwar's distance, showing the pairing to be merely a line-of-sight
coincidence. Lower-mass solar type stars rotate slowly (the sun
taking 25 days at an equatorial speed of 2 kilometers per second),
while high-mass stars rotate quickly. The division in rotation is
rather sharp, the "rotation break" falling in the middle of class
F. Anwar, rotating at least 76 kilometers per second (with a
period under 1.4 days), falls just above the limit. Like the Sun,
however, the rotation (and convection in its outer layers) give
Anwar an X-ray-emitting hot corona.
****************************************************************
Jim Kaler
Professor of Astronomy Phone: (217) 333-9382
University of Illinois Fax: (217) 244-7638
Department of Astronomy email: [EMAIL PROTECTED]
103 Astronomy Bldg. web: http://www.astro.uiuc.edu/~kaler/
1002 West Green St.
Urbana, IL 61801
USA
Visit: http://www.astro.uiuc.edu/~kaler/ for links to:
Skylights (Weekly Sky News updated each Friday)
Stars (Portraits of Stars and the Constellations)
Astronomy! A Brief Edition (links and updates)
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