January 2005 Sky from the Keeble Observatory
What's that "smudge?" No, not on the page ... in the sky! Look for the familiar shape of Orion after sunset (it will be to the southeast, drifting higher and to the south over the next few hours. Then, find the Pleiades above and to the right (finder chart produced with Sienna Software's "Starry Night Pro"). With sharp eyes, clear skies, and a fortuitously absent Moon, you'll see Comet Machholz pass to the west of the Pleiades, with its tail pointing almost directly at the small cluster also known as the "seven sisters." (It's also known as Subaru, in Japanese, which is why you see a stylized constellation on the grill of that brand!) This comet is brightening more than originally expected, so you have a good chance to see it without binoculars. Even though it will fade steadily, you should be able to follow it north through January and February evening skies.
You can also find a more detailed finder chart for this comet at http://skyandtelescope.com.
But, we've not answered the question ... what is that smudge? Our understanding of comets is that they are leftover material from the formation of the solar system. Forming in the cold outer reaches of the proto-solar nebula, far from the nascent Sun, they are composed of ices and dust, largely unprocessed in five billion years. Imagine a large "dirty snowball," with a mix of water ices, frozen ammonia and methane, and other ices along with dust and "gravel" and the occasional boulder. Sometimes, their distant orbits can be perturbed, sending them falling into the inner solar system. As they approach the Sun the volatile ices sublimate (turn from solid to gas) and erupt from the surface of the comet. Exposed to direct sunlight, the gas is ionized by ultraviolet light and glows with the spectrum characteristic of its chemical makeup - which is how we know what the comet itself is made of. The erupting gas also entrains dust and other solids. Pressure of sunlight and the constant "solar wind" streaming outward from the Sun pushes the ion tail and the dust tail outward, which is why a comet's tails always point away from the Sun.
NASA scientists expect to learn more about comets in general from a probe to be launched this month. On 2005 January 12, Deep Impact is scheduled for launch from Cape Canaveral, beginning a 6-month journey to the comet Tempel 1, traveling some 431 million kilometers (268 million miles) before its encounter. The mission profile calls for release of a copper-clad impact probe which will smash into the 6 kilometer wide comet nucleus at over 10 kilometers per second (23,000 miles per hour) on July 4th. At those speeds, the probe should vaporize and excavate a crater the size of a football stadium. The main part of the spacecraft will monitor the impact and relay images and data to Earth, where other telescopes (including Hubble, Chandra, and Spitzer ... the three major orbiting observatories) will also be tasked to observe the fireworks.
The comet will acquire a new crater. Its orbit will also change infinitesimally.
(As I write this, the Huygens probe has been successfully released from the Cassini spacecraft, and is on schedule for a January 14th landing on Saturn's moon Titan. We'll have more next month.)
Lunar phases for January: Last Quarter on the 3rd, at 12:46 pm; New Moon at 7:03 am, on the 10th; First Quarter on the 17th, at 1:57 am; Full Moon on the 25th, at 5:32 am.
Evening sky watchers will see Saturn rising at about sunset, actually at opposition (180 degrees around the sky from the Sun) on the 13th. It won't have company until after midnight, when Jupiter finally rises to the east.
Mercury is back in the predawn sky, about 13 degrees above the southeast at sunrise. It is joined by Venus, which is steadily shifting closer to the Sun in the sky as it pulls ahead of us in its orbit. It will spend most of the month within about a degree of Mercury, actually within a third of a degree on the 13th. Both planets rise about an hour before the Sun. Mars is a bit higher, about 24 degrees above the southeast horizon. You'll have some possible confusion, because the bright red star Antares (whose name means "against Mars") is also in that general direction ... Mars is the one on the left. Jupiter is high to the southwest at sunrise.
Two hours after sunset we can follow the Milky Way from southeast to northwest, picking out bright and familiar constellations and star fields. Bright and low on the horizon is Sirius, the brightest star other than the Sun visible from Earth. Next we find the familiar rectangle of Orion, with its pinched waist marked by three bright stars of the "belt." Below the belt we find the "sword" marked by the diffuse Orion Nebula ... a nearby star-forming region, spectacular in a small telescope. We've already noted the Pleiedes in Taurus, above bright Aldebaran at the apex of the V which marks the face of the Bull. About 30 degrees to the east-northeast of zenith we find bright Capella, in the constellation Auriga. 20 degrees west of zenith we find the Andromeda Galaxy, above the now-familiar M of Cassiopeia. Cygnus is now clearly the "northern cross" as it settles to the northwest horizon.
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.