September 2006 Sky from the Keeble Observatory
By now you’ve heard that the International Astronomical Union has demoted Pluto
from its prior status as a planet. Instead, it becomes a “dwarf planet” and perhaps
prototype for the many so-called Kuiper Belt Objects (KBOs). This action followed
the failure of another controversial proposal to redefine planet in a way which
would have extended that status to Pluto’s satellite Charon, to the large asteroid
Ceres, and to another recently discovered KBO informally known as Xena.
Just what was the controversy? What does it ultimately mean? This month’s column
will provide some background, and then next month I’ll go into more detail about
the new definition and its impact.
First, let’s consider the origins of the term “planet,” which comes from a Greek
word meaning “wanderer.” To the ancient Greeks, it was clear that there were two
broad categories of celestial object: the stars, which maintained their relative
positions year after year, night after night, and those objects which appeared to
move with respect to the fixed stars. Over a year’s time, the Sun moves through
a band of twelve constellations known as the Zodiac. This is the basis for the ancient
Babylonian practice of astrology. The Moon similarly follows the zodiac, traversing
its constellations in 28 days – the origin of our calendar months. There were five
other “planets” known to the ancients, each of which traversed the zodiac in progressively
longer times: Mercury, Venus, Mars, Jupiter, and Saturn. Their motions were not
always straightforward. Though they generally share the Sun and Moon’s drift to
the east, these planets would sometimes back up and head west, undergoing what is
known as “retrograde motion.” The positions of Sun, Moon, and planets were said
to influence events on Earth – your astrological “sign” is supposedly the constellation
of the zodiac where the Sun was found at the time of your birth. (A quick check
of the real sky versus the traditional birth signs will show that the Sun is almost
never in the right constellation! It was there 2000-5000 years ago!)
The work of Nicolaus Copernicus in the 16th Century moved the center of our perceived
universe to the location of the Sun – which meant that Earth became the wanderer
and the Sun a fixed center. The number of known planets orbiting the Sun was then
raised to six. We know now that the Sun also moves in its own orbit in the disk
of the Galaxy, which also moves as part of the great celestial ballet. There is
no center of the Universe (except Ashland!). William Herschel accidentally discovered
a seventh planet, Uranus, in 1781. For 65 years, this marked the outer edge of the
known solar system.
However, there was a problem – Uranus was not following its predicted path in the
sky. Either Newton’s laws of gravity and motion were wrong, or there must be another
planet even further from the Sun, tugging on Uranus and giving rise to its “unacceptable”
behavior. John Couch Adams in England (not the American patriot and president!)
and Urbain-Jean-Joseph Le Verrier in France independently calculated the location
and size of the necessary 8th planet to perturb Uranus. In 1846, a German astronomer
named Johann Galle found Neptune, using Le Verrier’s calculations. (Neither Adams
nor Le Verrier was able to convince astronomers in their own countries to look for
the planet they had predicted.)
For many decades, it appeared that even Neptune was insufficient to fully explain
the discrepancies in Uranus’ position, so various researchers undertook to calculate
and search for the purported 10th planet. In 1930, Pluto was discovered by American
astronomer Clyde Tombaugh, almost exactly where it was predicted to be. Curiously,
it wasn’t the big gas giant planet assumed in the calculations. Rather, it has turned
out to be a smallish ice ball – smaller even than our own Moon. It could not possibly
be the source of the orbital discrepancies that led to its discovery! And, to make
obvious that its discovery was purely serendipitous, the orbital discrepancies for
Uranus disappeared on reanalysis. Pluto turns out to be the wrong planet, in the
wrong place, discovered for the wrong reasons!
Subsequent discoveries of KBOs, including Xena which is larger than Pluto and Sedna
which is slightly smaller have fueled the discussion of demoting Pluto. Next month
we’ll see how that demotion took place.
Lunar phases for September: Full Moon at 2:42 pm, on the 7th; Last Quarter on the
14th, at 7:15 am; New Moon on the 22nd, at 7:45 am, and First Quarter on the 30th,
at 7:04 am. There will be a partial lunar eclipse at Full Moon – but the Moon will
be below the horizon until 7:42 pm, so it will not be visible from central Virginia.
Similarly, there will be an annular solar eclipse at New Moon, but visible only
from Guyana, Suriname, and French Guiana.
Early in the month, predawn sky watchers will see Venus rising about an hour before
the Sun, and about a half hour after Saturn rises. Venus moves closer to the Sun
on the sky, and Saturn further away as the month progresses. By month’s end, Venus
will be almost lost in the Sun’s glare, while Saturn will be rising a full four
hours before sunrise.
Evening planet watchers will have little to cheer for, since Jupiter is low (about
27 degrees from the horizon) to the southwest at sunset, and Mars is even lower
to the west and probably lost in horizon clutter and haze. Mercury returns to the
evening twilight by month’s end, but is also too low for good viewing.
Our overhead look at mid-month, about two hours after sunset, finds the bright star
Vega near zenith, and the Milky Way dividing the sky roughly NNE to SSW, with a
slight bow to the east. About 20 degrees toward the northeast from Vega lies Deneb
in the constellation Cygnus. 40 degrees to the south is the bright Altair in Aquila.
If we start at the northeast end of the Milky Way, we find in order the constellations
Cassiopeia (that crooked W shape), then Cygnus running from Deneb at the tail to
Albireo at the head of the Swan, Aquila (noted only for the presence of Altair!),
and finally Sagittarius (looks like a teapot). From Vega to the west we encounter
the small square of Hercules, then Bootes, marked by the bright star Arcturus, about
25 degrees above the horizon. To the east from Deneb we find the Great Square of
Pegasus – though there are really few bright stars in this direction, since we’re
looking out of the plane of the Galaxy. Andromeda lies to the northeast. On a clear,
moonless night you’ll be able to pick out the faint glow of the Andromeda galaxy
– over two million light years distant, this is the furthest object which you can
see without a telescope. Like our own Galaxy, this is a spiral galaxy of several
hundred billion stars.
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.