our starry night skies
Be a considerate neighbor: Reduce
Shield all your outside lights downward
(or turn them off completely)
and enjoy the beautiful, starry night sky.
December 2007 Night Sky
Something wonderful, spectacular, and completely unexpected happened
on October 23rd. A small, unremarkable comet called P17/Holmes exploded.
But this is not the first time P17/Holmes has had an outburst.
In November 1892, astronomer Edwin Holmes was observing the Andromeda
galaxy. On the 6th of the month, an object brightened over the course
of several evenings to about 4th magnitude. Then, during the next
several weeks, it faded away. Holmes was the first to observe the
comet that now bears his name. More observations in 1892 showed
that P17/Holmes is a short-period comet orbiting the Sun every 6.9
years. The comet was observed again in 1899 and 1906, but it was
then lost for more than fifty years. Using a computer-aided orbital
prediction, the U.S. Naval Observatory in Flagstaff was able to
locate it again in 1964.
On the evening of October 23rd of this year, P17/Holmes exploded,
going from a very dim magnitude seventeen to bright magnitude 2.5
in just a few hours (smaller magnitude numbers are brighter). It
became easy to find in the northern constellation Perseus as a bright
yellow “star.” By October 25th, 17P/Holmes appeared
as the third brightest “star” in that constellation.
As October closed, its comet-like nature started to appear as the
cloud of dust and gas expanded, giving the object a fuzzy quality.
The cloud surrounding the rocky nucleus of a comet is called the
In late October, the coma appeared to be about half the diameter
of a full moon. By November 9th, the coma’s diameter was 869,900
miles, based on measurements from the University of Hawaii Institute
for Astronomy. By comparison, the sun’s diameter is about
864,900 miles. P17/Holmes currently has the largest extended atmosphere
in the solar system.
How far away is the comet now? P17/Holmes is 149 million miles
away from Earth. As it circles the Sun, its orbit never goes inside
Mars’ orbit; nor does it ever go outside Jupiter’s orbit.
How big is the nucleus? The actual solid body of the comet is estimated
to be 2.2 miles in diameter.
How long will the comet be visible? Well, it is already fading
as the cloud expands and dissipates into space. The dust and gas
expelled from the nucleus is racing away at eleven hundred miles
per hour. So the show is coming to a close—unless there is
What caused the outburst? Scientists can only speculate. The blast
may have resulted from a collision with a meteoroid, or, more likely,
a build-up of gas inside the comet’s nucleus eventually broke
through the surface. The asymmetrical nature of the coma suggests
a large chunk broke off the main nucleus body and disintegrated
into tiny dust particles.
Where is the tail? Most comets form a tail as the gas and dust
boil off the nucleus and the coma is pushed away from the Sun by
the solar wind. Recent images of P17/Holmes are beginning to show
a tail forming. Don’t expect to be able to see a tail unless
you have a professional telescope.
When will the comet be back? Look for P17/Holmes to be in our neighborhood
in 2013. Who knows, perhaps it will put on another spectacular show.
One of the most predictable meteor showers of the year is scheduled
to peak on December 14th before dawn. Astronomers are expecting
one to two meteors every minute for North American observers with
dark, clear skies. The Geminids are named for the constellation
of Gemini, the Twins. On the nights of December 13th and 14th, the
meteors appear to emanate from a spot in the sky near the bright
star Castor in Gemini. This year’s event should not be hampered
much by a crescent moon.
THE PLANETS AND THE MOON
• Mercury will be very low in the eastern sky about thirty
minutes before sunrise. It will be difficult to spot.
• Venus is rising in the east about 4:45 this month below
Virgo. It is hard to miss bright Venus at -3.8 magnitude.
• Mars rises about two hours after sunset in the east, just
above the constellation Gemini. Look for Mars just to the lower
left of the full moon on the 23rd near the northeastern horizon.
Mars is at its closest approach to Earth on the 24th—a good
time to look at the red planet through a telescope.
• Jupiter will set in the west about 6:30 p.m. If you look
with binoculars in the southeast, you might just be able to spot
the giant planet near the horizon.
• Saturn is rising around midnight in the east. Look for
it in the constellation Leo. The Moon will be three degrees to the
lower left of Saturn on the 1st. The bright star to the right of
Saturn is Regulus. The event almost repeats again on the 28th.
• The Moon is new on the 9th at 10:40 a.m. MST. The Full
Moon occurs at 6:16 p.m. on the 23rd.
• Winter officially begins on December 21st at 11:09 p.m.
Interested in meeting me in person? Come to the Placitas Library
on December 13 at 7:00 p.m. I’ll be giving an hour-long talk
on two diverse subjects: the Geminid meteor shower and our nearest
star, the Sun.
If you have a question or comment for Charlie, you may email him