Sunday, August 29, 2010
Bright Jupiter points way to another world
John Goss
John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.
Recent columns
- When were the stars born?
- Moon, Venus, Jupiter in a predictable dance
- Look far beyond the stars of the Milky Way
- The planets lead nomadic existences
Blog
There exists a world where night can extend for 40 years, where a year lasts 30,000 of our days, and where only one-quarter of 1 percent of the sunlight that falls on our fair planet strikes its blue-green cloud tops. That darkly colored, cold world is the seventh planet from the sun, Uranus.
Spotting this planetary pinpoint flickering near the edge of visibility is normally a challenge for persistent skywatchers. It is not an exercise for the novice stargazer. However, the odds of seeing Uranus over the next few weeks increase dramatically to where a person armed only with a pair of binoculars should be able to pick it out drifting among the numerous background stars. The trick is finding Jupiter. That's the easy part.
Bright Jupiter rises in the east as evening twilight fades. When it climbs above our planet's atmospheric muck, center it in binoculars. Faint, unremarkable Uranus lies somewhere in the same field of view as dazzling Jupiter. Deciding which point of light is the elusive planet and which is an ordinary star is the hard part.
Over the next 10 nights at 10 p.m., Uranus lies to Jupiter's upper right, halfway to the edge of the binocular field. The small disk of Jupiter will be plainly evident, but Uranus will remain starlike. Their differences in appearance are due to Jupiter's diameter being nearly three times wider than Uranus' and Jupiter being almost five times closer. Hence, Jupiter shines large and bright, while Uranus appears tiny and faint.
Both planets slide westward nightly with respect to the background stars. Because it takes Jupiter 12 years to complete one orbit around the sun, and it takes Uranus 83, Jupiter moves more rapidly in our night sky, allowing it to pass underneath Uranus. On Sept. 20, the two planets appear closest to each other. Unfortunately, between Sept. 17 and 27, bright moonlight washes out the sky, impairing the view of dim stars -- and of dim Uranus.
Sept. 22 features several special events
First, the glaring full moon is situated directly above the easily seen Jupiter and the likely invisible Uranus. The sky position halfway between it and those two planets marks the vernal equinox, which is where the sun sits on the first day of spring, March 20.
Next, although it is of no particular astronomical importance, a very rare celestial alignment occurs when a straight line can be drawn beginning at the 2 billion-mile distant Uranus, passing through the 400 million-mile distant Jupiter, then glancing off the 240,000-mile distant moon, before striking us in Southwest Virginia. However, the linear shot doesn't end here. It continues traveling through the 8,000-mile diameter Earth, nearly intersecting the 42 million-mile distant Venus, almost hitting the 93 million-mile distant sun, just missing the 200 million-mile distant Mars, and finally hitting the 1 billion-mile distant Saturn. Sorry, but neither Mercury nor Neptune enter into this arrangement.
Finally, Sept. 22 at 11:13 p.m. marks the autumnal equinox, when summer officially ends and autumn begins. The full moon -- the so-called "harvest moon" -- occurs six hours later. This most well-known of full moons can happen up to two weeks before or after the equinox, but this year it nearly coincides with it. Their timing won't be this close again until 2029.
Farewell to 'Star Gazer'
Skywatchers lost a special friend on Aug. 20 with the passing of Jack Horkheimer. He will be remembered for his sometimes wacky, always engaging five-minute "Star Gazer" episodes seen on many PBS stations. Programs that were produced before his death can be viewed at www.jackstargazer.com. In Jack's own words, "Keep looking up!"
John Goss writes a monthly astronomy column for The Roanoke Times.
