September: Disappearing Acts

September 2009

Disappearing Acts

By Calvin L. Chrisman

Astronomical disappearing acts abound in September. Two of the favorite targets of amateur astronomers with small telescopes are Saturn’s rings and Jupiter’s four Galilean moons. In two rare events, the rings of Saturn and these four moons of Jupiter will be invisible from Earth. As Saturn moves through its orbit, its position to Earth shifts causing the rings to tilt up in one direction and then back in the other. Saturn orbits the Sun once every 29 years and every 14 years, the rings cross the center of our view from Earth. This will happen on the evening of September 4th making the rings virtually invisible as they are viewed edge-on. Unfortunately, Saturn is quite close to the Sun at the moment, so it will be very hard to see this event in the early evening sky. As Saturn moves away from the Sun over the next month or so, the rings will still appear quite slim compared to what we are used to seeing.

On the early morning of September 3rd (the night following the publication of this paper), none of Jupiter’s four Galilean moons (the ones Galileo first saw 400 years ago) will be visible. This will occur between 12:43 A.M. and 2:29 A.M. EDT on the early morning of the 3rd and will be the last time this happens until 2019. If you have a telescope and begin watching shortly before midnight, you will notice that Callisto has already passed behind the planet. You will see Io pass behind Jupiter’s edge (called the Limb) at 11:43 P.M. on September 2nd and Europa will disappear by transiting the planet’s face at 11:58 P.M. Finally Ganymede will transit the face fifteen minutes later, leaving Jupiter apparently moonless until 2:29 A.M. when Io emerges from Jupiter’s shadow. If you miss this event because of weather or you read the paper after Wednesday, use this as a reminder to take a look another time. Jupiter’s moons are always interesting. If you don’t have a telescope, check out Jupiter anyway. It is very bright at the moment and rises in the east early in the evening.

For early risers, take a look east an hour before dawn on the morning of September 20th. Bright Venus will be only ½ degree away from Regulus, the heart of Leo the Lion. This is so close, you will be able to cover the planet and the star with the tip of your finger held at arm’s length! It will be a bright, beautiful sight.

If you have wondered what the view through an astronomical telescope is like, mark your calendar for the October 22-24. This is the International Year of Astronomy, marking Galileo’s first use of a telescope to explore the night sky. The evenings of October 22-24 have been proclaimed Galilean nights and astronomy clubs around the globe will be hosting events. The Asheville Astronomy Club will be holding a Sky Party on the campus of UNC Asheville. Details will be provided next month.

Clear Skies!

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