StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

APRIL 2003

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345 APRIL 6, 2003: Moon Bright: It seems that we are into the "April showers" part of the rhyme that goes, "April showers brings May flowers." Not to worry, because there is something big, bright, and beautiful in the sky this week that may be able to add some sparkle to the gloomy weather that we have been experiencing. It is the moon. Situated at an average distance of 239,000 miles from earth, our moon is the only natural satellite in the solar system's growing family of 118 moons that is officially called "the Moon." Yes, that's its English name! Luna, Latin for the moon, starts off the week as a rather thick, banana-shaped waxing (growing) crescent in the southwestern sky and continues to brighten until it is a rather bulbous gibbous moon by the week's end. The moon is full on Wednesday, April 16 at 3:36 p.m. EDT. In between, this Wednesday at 7:41 p.m., the moon passes through its first quarter phase. The moon will have completed one-quarter of its phase cycle, but it will appear to be half lit with its other hemisphere in its own shadow. The terminator, which represents the sunrise or sunset locations on the moon, always appears as a straight line when the moon is at first quarter (rising sun) or third quarter (setting sun). Watching the moon proceed through a complete cycle of phases is precisely the same as observing the rhythm of day and night on the earth. The difference is in the period of time. For the moon, sunrise-to-sunrise occurs about every 29-1/2 earth days or about one "moonth." As the moon circles the earth, the sun always brightens half of its surface; however, the lit portion that we see depends upon the moon's orbital location. Presently, as the moon waxes, more of its sunlit hemisphere will be pointed in our direction.

346 APRIL 13, 2003: Spin Doctor: There's a local radio station that only has nine listeners. The show hosts do a bang up job of making my weekday rides to school enjoyable. I just can't imagine how they've managed to keep their listening audience so low. Anyway, the buzz is that they blew an astronomical question last Wednesday, and I felt the necessity of setting the record straight for the other eight listeners who might also "bee" Morning Call subscribers. The query was, "What planet in our solar system rotates in a clockwise (backward) direction?" The correct answer would win a T-shirt and entrance into a drawing for 104 gallons of gasoline. Twenty minutes later, there still was no response as I was walking into my Allen homeroom. Some of my students had heard the question, but it was impossible to get through on the phone until Monica L. Ward, a senior in my first period astronomy class, made it her mission to do just that. About 30 minutes later she established "first contact" and politely informed the duo that the question should have been read as "What three planets in our solar system rotate in a clockwise direction?" A winner had been declared; the correct answer having been deemed Venus. We were told to provide proof and my student observer, Mike Stump, with a BS in astrophysics from Villanova University, drafted the response that was e-mailed to the station several hours later and can be read at Web StarWatch. The solar system was a rough and tumble place five billion years ago, and three of the accreting planets literally were knocked over in the bombardment of crashing objects. They spin backwards. Oh, the correct answers are: Venus, Pluto, and yes, that anatomically pronounced planet, Uranus. "Hey, Ken and Sydney, how about Astronomers in the Morning?"
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

347 APRIL 20, 2003: Mercury To Transit Sun: May is a busy month for astronomy aficionados. There is a transit of Mercury across the sun on Wednesday, May 7, National Astronomy Day on the 10, a total lunar eclipse on the 15-16, and the swan song of Jupiter and Saturn in the west after sundown occurring all month. We'll start with the sunrise (5:53 a.m.) May 7 Mercury transit. Transits occur when a much smaller body passes in front of a larger object. In this case Mercury will be passing in front of the sun. Europe and western Asia are especially well positioned for this rare celestial event that happens between 13 and 14 times a century, and only in early May and November. Mercury's orbital plane is tilted seven degrees to the plane of Earth's path around the sun. This means that on most occasions when Mercury passes between the Earth and the sun, Mercury is either above or below the sun and no transit will occur. But on May 8 and November 10 of each year, Earth is looking down the gun site, so that if Mercury just happens to be there, it must also pass in front of the sun. Many Mercury transits are missed because we might be experiencing night, or the weather is inclement. I have only seen one of these rare events and that was at sunrise on November 10, 1973. Dieruff students Paul Hallowell, Paul Nyce, and Richard Walters accompanied me according to my notes. To see the Mercury transit, you'll need an open eastern horizon, a telescope equipped with a solar filter or sun projection system, and a little bit of weather luck. Go to Web StarWatch from the URL listed below to discover where you can purchase a solar filter locally and to link with my tips about safely viewing the sun. Unfortunately, transits of Mercury are not visible to the unaided eye.

348 APRIL 27, 2003: Mercury Transit Invitation: Astronomical events wait for no one. The clockwork of the universe unfolds with immense precision, and if you're not at the correct place at the correct time and do not possess the proper equipment, the event becomes a mere exercise in verbiage. For most people that's how the transit of Mercury will be at sunrise on Wednesday, May 7. So this StarWatch amounts to an invitation from my astronomy students at Dieruff and William Allen high schools and me to come to view the transit of Mercury with us at sunrise, 5:50 a.m. in the main parking lot of Dieruff High School, 815 N. Irving St., Allentown. We will have the proper equipment: telescopes, solar filters, and projection screens to view the small, black disk of Mercury against the face of the sun, an event that happens 13-14 times a century, but few astronomy enthusiasts get to see more than once or twice during their lifetimes. My last Mercury transit was on November 10, 1973. But keep in mind that although the clockwork universe will insure the event occurs with exacting precision, there are still lots of factors that could muck up the event, even with the best of planning. Foremost among these obstacles is the weather. Wednesday morning, May 7 will have to present a clear and transparent eastern horizon, so that when the sun rises, its brightness will be intense enough to be seen through solar filters or to be projected onto a screen. You should be on campus by 5:45 a.m. I will post a go/no go message in big letters on the front page of my web site, www.astronomy.org by 4:30 a.m. Wednesday, May 7. I will also post an update to this article on Thursday if there is new information. For directions to Dieruff H.S. and a map, go to the URL above and click on "map."

April Star Map

April Moon Phase Calendar