Two events in the August sky, a planetary opposition and a planetary occultation, involve Uranus, the first planet discovered with the telescope. Uranus was discovered by the English astronomer William Herschel in 1781. He was making a survey in the region of the sky containing Uranus and immediately recognized the pale green disk as a planet.
The best observations of Uranus are made when the planet is as close as possible. This occurs when Earth is "passing" Uranus on the "inside lane," between Sun and Uranus. To quantify this, astronomers measure ecliptic longitude along Sun's apparent annual path around the sky. When the ecliptic longitudes of Uranus and Sun differ by 180 degrees Uranus is literally opposite Sun in our sky. We call this configuration "opposition." The slight departure from perfect circles of Uranus and Earth's orbits and the fact that the orbital planes of the two planets are not quite coincident result in the minimum separation occurring at a slightly different time from when the ecliptic longitudes differ by 180 degrees. This year, Uranus is closest, 18.85 times the average Earth-Sun distance, at 6 a.m.on Aug. 2. Uranus is at opposition at 3 a.m. on Aug. 3.
Uranus's brightness varies from about 5.7 when closest to about 5.9 when most distant. A magnitude slightly brighter than 6 means that under ideal conditions we can see the planet with our naked eye, but it blends in with the myriad of faint stars. Consequently ancient astronomers did not notice its "wanderings" with respect to the fixed stars and did not classify it as a planet. At the time of Herschel's discovery it had, in fact, been plotted as a star on sky charts at least 20 times since 1690.
This year it is located in the triangle shape associated with Capricornus the Sea Goat. At opposition it rises approximately at sunset and is up all night, highest above the south point of the horizon about midnight. It's quite easy to see as a small disk in ordinary binoculars. The August issues of Sky & Telescope and Astronomy have charts showing the location of Uranus; see one of them for help in finding the planet.
The second event involving Uranus takes place on the evening of Aug. 26. An occultation of the 9.5-magnitude star PPM 237981 by Uranus will be visible in our part of the world. Sky &Telescope reports that the star will disappear just after 8:55 p.m. EDT along the east coast of the U.S. The star will remain behind the planet for almost 42 minutes, reappearing about 9:38 p.m. The magazine states that it "will be a tough observation that may require a large telescope" since the sky will not be totally dark as the occultation begins. If you try to observe this occultation, begin watching as soon as you can see the planet in your telescope.
A feature associated with the occultation that Sky & Telescope says may be easier to see is a slight dimming of the star for one or two seconds as it passes behind Uranus's Epsilon ring about 5 minutes before the occultation by the planet begins. About three minutes later, "smaller dimmings" may be seen as the other rings pass in front of the star. The succession of dimmings will be repeated after the star reappears from behind the disk of Uranus. The discovery of Uranus's rings was from dimmings of this sort as astronomers on the Kuiper Airborne Observatory prepared to observe an occultation of a star by the planet in 1977.
Additional information may become available from Sky & Telescope athttp://www.skypub.com/occults/occults.html or from the International Occultation Timing Association (IOTA) as the date of the occultation nears. There is no information at IOTA's Web siteat the time of this writing on July 23. IOTA's URL is http://www.anomalies.com/iota/splash.htm and they may have a voice recording at 301-474-4945 a few days or hours before the occultation.
