We managed to have a beautiful clear sky for the first hour of our viewing last night. Dark intermittent clouds moved in around 9:30pm, but by then all of our 18 guests had gotten a good look at the night’s viewing object, Jupiter and three of its four Galilean moons (so called because Galileo was the first to observe them and their motion around Jupiter).

For those who attended the viewing and would like to know which moons they saw, you can use a program like Stellarium to find out. The image is a screenshot from that program, showing Jupiter as it was seen at 9pm on April 6. (The view through our telescope was the mirror image because of how the optics of the telescope work.)

The moons of Jupiter that were visible during the April 6th public viewing.

The three moons that were visible during the viewing, in order of increasing orbital distance from Jupiter, were Io, Europa, and Ganymede. The Galilean moon that orbits the farthest out from Jupiter, Callisto, wasn’t visible during the viewing. However, the orbital period of all these moons is quite short – between about 2 to 17 days – so which moons are visible changes rapidly. If one were to look at Jupiter at 9pm tonight (if the clouds weren’t in the way!), one would be able to see the moon Callisto, but the moon Io would not be visible as its orbit takes it behind Jupiter.

The moons of Jupiter that would be visible on the evening of April 7th.