A last hurrah at Uranus: the 7th planet yielded one final discovery for visual observers just before its revealing Voyager close-up.
Although Uranus is roughly four times the diameter of Earth, it is so remote that its apparent diameter never exceeds 3.7 arcseconds--13 times smaller than Jupiter and five times smaller than Saturn at opposition. The planet's low apparent surface brightness, almost 14 times dimmer than Jupiter's, poses an even greater handicap for the observer. The lack of contrast between Uranus's pallid disk and the dark sky background also accentuates the planet's limb darkening, making it difficult to confirm whether its image is even in focus.
Few observers have ever managed to make out any markings on Uranus. Dusky belts and bright zones of extremely muted contrast have been reported for well over a century, but they were usually only glimpsed as uncertain, fleeting impressions. Belief in the presence of markings on Uranus waned in 1970 after the Stratoscope II balloon lofted a 36-inch telescope to an altitude of 80,000 feet and obtained images of Uranus free from the effects of atmospheric turbulence. The planet proved to be utterly featureless in visible light, although the Stratoscope telescope detected a dappling of prominent dark spots on the far smaller disk of Jupiter's satellite Io.
Unable to follow the motion of any well-defined markings, astronomers attempting to determine the velocity of Uranus's axial rotation employed spectrographs to measure the magnitude of the equal but opposite Doppler shifts in sunlight reflected from the planet's approaching and receding limbs. The accuracy of this technique is extremely sensitive to guiding errors and atmospheric turbulence, and with Uranus it's further complicated by the fact that the planet's rotational axis is inclined by a whopping 98[degrees] to the plane of its orbit. Uranus rolls along in its orbit on its side almost pole-first while making its 84-year-long circuit of the Sun. The planet's poles are alternately pointed almost directly at the Earth at 42-year intervals. In 1944 the planet's north pole appeared near the center of its disk, while in 1986 its south pole was similarly presented. At the intervening equinoxes the planet's equator faces Earth, as was the case in 1966 and 2007.
Given these daunting challenges, it's hardly surprising that rotation periods determined spectrographically were wildly discordant. Early in the 20th century a period of 10.8 hours was widely accepted, but 1977 brought a flurry of new values. Astronomers from Caltech and the University of Wisconsin suggested a rotation period of 12.3 hours, almost twice the spin rate derived by a team of University of Texas investigators, who proposed a period of just under 24 hours. Robert Brown and Richard Goody of Harvard University's Center for Earth and Planetary Physics derived a period of 15.6 hours from spectrograms made with the Kitt Peak National Observatory's 4-meter reflector, which they soon revised to 16.3 hours.
1977 was also the year that NASA launched the Voyager 1 and Voyager 2 spacecraft on a "Grand Tour" mission to the outer planets that featured a Voyager 2 close encounter with Uranus in January 1986. When the outbound Voyagers flew past Saturn they returned images that verified the existence of the ghostly "spokes" in Saturn's B ring that O'Meara had reported three years earlier (S&T: July 2011, page 50). Bradford Smith, the leader of the Voyager imaging team, marveled at O'Meara's prowess as an observer and challenged him to try to determine the rotation period of Uranus.
For months O'Meara saw only the limb darkening of a disappointingly bland globe through the Harvard College Observatory's venerable 9-inch Clark refractor, until two brilliant spots suddenly appeared on the night of July 22, 1981. One of these features seemed to be stationary, marking the position of the planet's south pole, while the other was located closer to the limb. Taken aback, O'Meara rushed out of the dome, sprinted across a catwalk, and ran down a spiral staircase to the office of Michael Rudenko, a frequent observing companion.
"Another member of the staff, Peter Collins, was also there," O'Meara recalls. "I asked them to come up to the dome and take a look at Uranus. I didn't tell them anything about what I saw and merely asked what they could see. They both told me that they could see a pair of "stars" on the planet. ... In fact, Peter Collins said that Uranus looked like the Ring Nebula with two central stars rather than one. That's how bright and obvious these clouds were!"
The spots persisted for several weeks. Their relative motion between July 23rd and August 28th suggested a rotation period of 16.0 hours, while a second series of observations between September 8th and 15th yielded a value of 16.4 hours. A third remarkably bright but very-short-lived cloud observed on August 27th and 28th indicated a 16.0-hour period.
When Richard Goody learned that O'Meara's visual observations confirmed the rotation period that he had determined spectrographically several years earlier, he implored O'Meara to publish his results. But the cautious Brian Marsden, Director of the Central Bureau for Astronomical Telegrams, balked at issuing an announcement. To convince Marsden that O'Meara's observations were credible and worthy of publication, Rudenko and another member of the observatory staff, Daniel Green, devised an experiment. Seven artists' depictions of Uranus that closely approximated the planet's tiny apparent size were affixed to the railing of a tall building located one kilometer from the refractor's dome. O'Meara, Rudenko, and Green took turns at the eyepiece and made sketches. O'Meara's were consistently the most detailed, accurately recording features on the order of only 0.5 arcsecond wide.
His skepticism mollified by these results, Marsden announced O'Meara's visual discovery in International Astronomical Union Circular 3912, issued in early 1984. Two years later the motions of several bright clouds were recorded in contrast-enhanced Voyager 2 images. They revealed that features in the temperate latitudes (where two of O'Meara's three spots appeared) circle the planet within minutes of the periods he derived.
The Voyager 2 images were as perplexing as they were gratifying. The level of contrast for the handful of features that could be distinguished amidst the hydrocarbon hazes of Uranus's cold, deep atmosphere in visible wavelengths was so low that it seemed utterly implausible that any visual observer could have seen so much as a hint of them. In the decades following his 1981 observations, O'Meara has never seen another well-defined feature on Uranus, even though he has studied the planet through telescopes as large as the 60-inch Mount Wilson reflector.
In retrospect, Uranus seems to have been unusually quiescent during the 1986 Voyager 2 flyby. In 1994 the Hubble Space Telescope recorded a pair of bright convective clouds that were strikingly similar to the features observed by O'Meara in 1981. Since 2004 several large ground-based telescopes equipped with adaptive optics have recorded belts, zones, and periodic outbreaks of bright spots in near-infrared wavelengths, where these features display much higher contrast than in visible light. Uranus is proving to be far more dynamic than once thought. Long-term studies may reveal that the wildly exaggerated seasons that result from the planet's 98[degrees] axial tilt greatly influence its level of atmospheric activity.
Although Uranus is not a memorable spectacle, bear in mind the experience of the first observer to report seeing "fairly definite features," British amateur T. H. Buffham. Using a 9-inch Newtonian reflector at magnifications of 212x and 320x on two nights in January 1870, Buffham noticed "two round bright spots" on Uranus. Over the course of an hour on the first night he was able to discern that they were moving. Based on their nearly identical positions two nights later, he guessed that Uranus had rotated on its axis four times during the intervening 48 hours and proposed a period of about 12 hours in a report published in The Astronomical Register three years later.
Had Buffham made the equally plausible guess of three rotations, corresponding to a period of about 16 hours, he would have beat O'Meara to the punch by more than a century. Buffham and O'Meara employed telescopes of the same modest aperture at similar magnifications and saw transient bright spots moving at the same rate. Is this merely a remarkable coincidence, or proof that on rare occasions Uranus can be a rewarding target for telescopes of modest aperture?
Uranus comes to opposition this month (see page 50). See if you can spot any features in the planet's usually bland atmosphere.
Contributing editor Thomas Dobbins glimpsed a diffuse spot on Uranus when he observed the planet with Stephen O'Meara through the 36-inch Lick refractor in 1995. He's never seen another feature on Uranus before or since.
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|Title Annotation:||Exploring the Solar System|
|Publication:||Sky & Telescope|
|Date:||Sep 1, 2012|
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