Skylights featured three times on Earth Science
Picture of the Day: 1
, 2
, 3
, 4
.
Photo of the Week.. Coming in from the gigantic Oort
Cloud of comets that extends far beyond the planetary system,
Comet
Hyakutake of 1996 sports a delightful long gas tail caused by
sunlight heating the icy comet's tiny nucleus and the
solar wind blowing it backwards. The bright star at upper left
is Arcturus in Bootes.
Astronomy news for the week starting Friday, February 18, 2005.
We begin, as always, with the wanderings of the Moon, which of
course does not wander at all, but is under the strict
gravitational control of the Earth and Sun. Meander might be a
better word, since the
orbit is not close to a simple ellipse, but is quite amazingly
complex. Most of the week sees it waxing in its gibbous
phase until it passes full the
night of Wednesday, the 23rd, about midnight (as it crosses the
meridian) in central Leo, to the
east of the "Sickle," which makes the Lion's head, and the star Regulus.
Earlier in the week, the nights of Friday the 18th, Saturday the
19th, and Sunday the 20th, watch the Moon pass through central Gemini and above the planet Saturn, these
two and Gemini's Castor and Pollux making a wonderful quartet.
The next planet out,
Uranus, passes conjunction with the Sun the night of
Thursday, the 24th.
For all of ancient times, Saturn was the last of the planets,
fainter than all the rest (though still quite bright) and with the
longest period relative to the stars, 29.5 years. Then in 1781,
with his telescope,
William Herschel came across much dimmer Uranus, which is just
barely visible to the unaided eye. Though seen before, Herschel
was the first to make out its tiny disk. From the motion of Uranus
(which, like all the other planets is gravitationally perturbed by
its neighbors),
Neptune was found in 1846. Finally, the "last planet," Pluto
was discovered by Clyde Tombaugh at
Lowell Observatory on February
18, 1930 (from photographs made on January 23 and 29), making this
date the 75th anniversary of its discovery.
With its highly tilted orbit, Pluto (faint, requiring a decent
telescope to see) is well off the ecliptic and is now in the
morning sky in the constellation Serpens (the eastern part, Serpens Cauda). Averaging
40 Astronomical Units away, for a brief 20 years of its 248-year
orbit, it actually comes closer to us than Neptune (30 AU away, the
AU the average distance between Earth and Sun). Pluto is also
gravitationally controlled by Neptune: averaged over a long period
of time, Pluto goes around the Sun twice for every time Neptune
goes around thrice. Planet or not? Pluto is really the largest
body of the Kuiper
Belt of small bodies that lies outside the orbit of Neptune
(which supplies us with short-period comets). The stuff was too
thinly spaced to assemble into a larger planet like Neptune or
Uranus. There is no reason, however, why Pluto cannot be both a
Kuiper Belt object and a planet at the same time.
Not of course to forget the others, very bright
Jupiter (which now rises around 9:30 PM) and Mars,
which rises in the southeast around 4 AM, an hour after Jupiter
crosses the meridian.
Admire Orion, now crossing the
meridian to the south around 8 PM. It is the top of a stack of
constellations to the south of it that begins with Lepus (the Hare) and continues with
the triangle that makes Columba
(the Dove). Farther south and visible only from southern climes is
Pictor (the Easel). Plunging
farther down we find Dorado (the
Swordfish), Mensa (the Table),
finally the sky's south pole, which is
surrounded by Octans (the Octant).
