January 2004 Sky from the Keeble Observatory
Last month we wrote about Voyager 1 and promised to say more about Voyager 2 this
month. The latter craft was actually launched first, on the 20th of August in 1977.
A virtual twin of the other craft, it arrived at Jupiter on July 9th in 1979, four
months after Voyager 1. Actually, it's more accurate to say that it flew past Jupiter,
cameras and instruments programmed to image the giant planet and its moons, studying
their surfaces and near-space environment. The gravitational boost from this flyby
gave the probe the correct velocity to direct it to Saturn, where it made its encounter
in August of 1981. (Voyager 1 made its Saturn encounter in November 1980.) Saturn's
gravity was used to fling Voyager farther into the outer solar system, towards distant
During the Saturn encounter the camera platform jammed, requiring a complete reprogramming
of the on-board computer so that the entire craft could be tilted and swiveled to
point the cameras. Also, keep in mind that the cameras were designed for the light
levels at Saturn, about 1% as bright as sunlight on Earth. Uranus is twice as far
from the Sun, and the light levels are only one fourth those at Saturn! Exposure
times had to be extended, and the pointing accuracy had to be sufficient to prevent
blurring the images. Nonetheless, Voyager 2 made a successful flyby of Uranus and
its moons in January of 1986, almost nine years after launch! This encounter added
enough speed and redirected the probe to its final planetary encounter with distant
Neptune in August 1988. Neptune, almost thirty times as far from the Sun as Earth,
provided the last gravitational assist to send the probe below the plane of the
solar system and outward towards interstellar space.
The science return from these two probes is easily as impressive as the longevity
and durability of their 1970s technology. Jupiter was discovered to possess a thin
dusty ring system, mimicking the brighter and more familiar rings of Saturn. Io
was found to be the most volcanically active object in the solar system. Distant
Triton, orbiting Neptune, was also found to possess a thin atmosphere and geysers
of liquid nitrogen. Lightning and aurora were detected in the atmospheres of the
outer planets, while Uranus' and Neptune's magnetic fields were found to be mystifyingly
off-center and tilted with respect to the planets' rotation axes. For a more complete
look at the historic and ongoing missions of Voyagers, along with a treasure trove
of spectacular images, go to the JPL website at www.jpl.nasa.gov.
Lunar phases for January: Full Moon on the 7th, Last Quarter on the 14th; New Moon
on the 21st, First Quarter on the 29th.
January is a good month for evening planet watchers. As the sun sets, you'll see
Venus emerge from twilight to the southwest, about 25 degrees off the horizon early
in the month, higher by month's end. Mar is visible high above the southern horizon.
Saturn rises to the northwest at sunset early in the month, about half way between
Orion and the bright Twins of Gemini. By the end of January, Saturn emerges from
twilight about 25 degrees above the horizon. Jupiter rises at 10:30 pm on the first,
about 8:30 pm by the first of February. The ecliptic (the path on the sky followed
by Sun, Moon, and planets) is low in the sky during the day, high in the sky at
night during this season. Accordingly, the planets will traverse the night sky well
above the horizon clutter of trees and building, and your view will be relatively
unimpeded by growing light pollution.
About three hours after sunset, the Milky Way divides the sky from northwest to
southeast. To the northwest, you'll see bright Deneb marking the top of the northern
cross of Cygnus. The foot of the cross is below the horizon. Tracing upward along
the plane of the Galaxy takes you to the irregular M shape of Cassiopeia. Above
and to the left of Cassiopeia, you'll be able on a clear moonless night to see the
Andromeda Galaxy, at more than two million light years, this is the most distant
object visible without a telescope. Almost at zenith, the bright star in Perseus
is called Mirfak (note that most proper names for stars are Arabic!), though you're
more likely to see Capella in Auriga just to the east, because it's so much brighter
than Mirfak. High to the southeast is bright Aldebaran, in the constellation Taurus.
You're also likely to notice the compact dipper shape of the Pleiades cluster. Below
Taurus is Orion's distinctive belt. Use binoculars or a small telescope to view
the Orion Nebula, a star-forming region in the sword of Orion. The brightest star
visible (other than the Sun) is Sirius, just below Orion above the southeast 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.