This e-mail is from Gary A. Becker, Director of the ASD Planetarium, and Michael E. Stump, ASD Planetarium Assistant, B.S. Astrophysics, Villanova '01, on behalf of their William Allen High School Astronomy class. You had requested some information regarding the planets Venus, Uranus, and Pluto, and their clockwise rotations. In the realm of astronomy, this motion is called "retrograde," i.e., the opposite of the norm, counterclockwise. Retrograde rotation is due to the rotational axis of a planet being inclined, or tilted, to the perpendicular of that planet's orbital plane, in excess of an angle of 90°. For reference, the Earth's axial tilt is approximately 23.5° to the perpendicular of its orbital plane. Venus most definitely rotates in a retrograde fashion. Its axial tilt is well in excess of 90°, measuring 177° (some sources say 178°), nearly putting Venus on its head. Venus's axial tilt was determined in 1962 by Earth-based radar observations. The planet Uranus is a bit trickier. Uranus is nearly on its side. The axial tilt of Uranus is approximately 98°, so that the planet's rotational axis is nearly in its orbital plane. There has been some debate about the planet's exact axial tilt, but historically and by current consensus, Uranus's axial inclination is noted as 98°, and thus its rotation is retrograde or clockwise. The spacecraft Voyager 2 made the definitive discovery of the axial tilt of the planet in 1986, though there were other pieces of information, such as the clockwise orbit of Uranus's satellites, available prior to the Voyager 2 flybys that astronomers may have used to deduce the retrograde rotation of the planet. Pluto falls into a similar category as Uranus, though there is no real debate about its rotational direction. Its axial tilt is approximately 120°, tipping it well past the required 90° that it needs to have its rotation considered retrograde. Pluto's axial tilt was determined within a reasonable range in 1988 by observing eclipses of Pluto by its moon Charon and eclipses of Charon by Pluto. It is most commonly believed in today's astronomical community that the retrograde rotations were caused by the planets being impacted by relatively large asteroid-sized objects in the chaotic history of the early solar system. These planets were literally knocked over. Impacting by large objects is the general explanation for why all planets have axial tilts.
Michael E. Stump, Gary A. Becker, Tonya M. Barrett, Jarryd Homick, Michael D. Roberts, Shannon A. Ruhe, Jennifer R. Torres, Monica L. Ward, and Kristin B. Weaver--William Allen High School, First Period Astronomy, Room 205 Main
|The May 7 Sunrise Transit of Mercury is a rare celestial event that must be observed with a telescope equipped with either a solar projection system or a front-end solar filter. Never observe the sun with a filter that is threaded into the eyepiece. A telescope concentrates the sun's light, as well as the sun's heat at the eyepiece position. Solar filters housed in the eyepiece have been known to crack while in use. If this happens, your observing eye will become toast. A variety of safe, front-end solar filters are available at Dan's Camera City, 1439 West Fairmont Street, Allentown, PA 18102--610-434-2313. Ask for John Evrard or Larry Moyer.|