Sunday, August 30, 2009
September is the perfect time to discover Jupiter's four secrets
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.
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Blog
In the early years of the 17th century, our understanding of the universe was quite different than it is today. Before 1609, there were no telescopes used to reveal secrets that lay just beyond the reach of human eyesight.
In January 1610, Galileo Galilei first aimed his primitive, low-power telescope at bright Jupiter and discovered something that helped change his view -- and, ultimately, humanity's view -- of our solar system. He saw that Jupiter appeared, not as a stellar pinpoint, but as a tiny disk that was accompanied by four little "stars" that were always lined in a row.
In his work, Sidereus Nuncius, Galileo wrote about his discovery of what he termed the "Medicean stars" huddling closely to Jupiter, "I, therefore, concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun; which at length was established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions" around Jupiter.
The next three months give 21st century skywatchers a great opportunity to view this giant world and its attendant "Medicean stars" as they all slowly drift together in front of the dim, true stars in the constellation Capricornus. Just after darkness settles, Jupiter can be found shining brightly in the southeast. All that is needed to discover its fascinating retinue are a pair of typical household binoculars and a means of holding them steady such as a camera tripod.
Look closely at the tiny disk of Jupiter with your binoculars. Directly on either side of it, sometimes appearing to hug the planet, lie up to four dim "stars." Stars they are not. Those are moons, which, in reality, are as large or larger than our Earth's moon. Galileo's inaccurate and awkward term "Medicean stars" was soon replaced by the names that are still in use today: Io, Europa, Ganymede and Callisto.
Look again on succeeding nights at their changing positions as they each orbit the planet. The number of moons visible may vary. This is due to the likelihood that at least one of the four satellites may be situated directly in front of bright Jupiter, located behind it, or eclipsed in the shadow of the giant planet.
Continue looking for a few weeks and you will find that Jupiter's outermost moon, Callisto, takes just over two weeks to revolve once around the planet. The innermost one, Io -- the moon that is often the most difficult to discern in the planet's glare -- needs less than two days to complete a single orbit. Compare those orbital periods with the leisurely 27 days that our Earth's moon requires to move around us.
Jupiter, with its moons in tow, will be the only planet visible until the early morning hours. After it sets about 4 a.m., divert your attention to Mars moving near the bright stars Castor and Pollux high in the eastern sky. Brilliant Venus will follow, rising in the east about 5 a.m. An intriguing binocular sight occurs on Sept. 1 and 2 when Venus drifts beneath the dim "Beehive" star cluster in Cancer.
The moon moves near ruddy Mars on the morning of Sept. 13, allowing for a positive identification of that planet. Two mornings later, the crescent moon lies just south of the aforementioned "Beehive" star cluster giving another pretty binocular sight. One morning more, an even thinner crescent moon, aglow with Earthshine, floats next to Venus forming an eye-catching combination low in the east about 5:30 a.m.
