Keeble Observatory
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.
Copyright 2006
George Spagna