Question about the moons of Jupiter.?

Question

Question about the moons of Jupiter. I'm confused about which one's I'm seeing. It's midnight PDT between the 17th and 18th of July and I'm in Oregon. I can see two moons to the left (east) of Jupiter and one moon to the right (west) of Jupiter. Page 41 of the July issue of Astronomy magazine makes it look like there should be just one moon to the left and a couple right next to each other to the right. Huh?

Answer 1

Remember that, depending on the optics, your telescopic image may be reversed. Magazines and software programs tend to show the image you would see if you could use no optics at all. The arrangements of lenses and mirrors in telescopes often leads to inverted images.

To see if this is indeed the case, look at the Moon through your scope and compare it to your naked eye view. That will give you a good indication of the effects of optics on the image you are seeing.

Answer 2

Jupiter's moons move quickly, so you have to make sure the chart you're looking at is correct for not just the date, but the time of your observation as well. Judging from the chart in the July Sky & Telescope, we're seeing Io and Callisto to the east, and Europa to the west. Ganymede should be coming out of eclipse soon.

Answer 3

From the left, you would see Callisto, Io, Jupiter and Europa, all equally spaced. Ganymede was behind Jupiter at the time. Either the diagram in Astronomy magazine shows the view in an astronomical telescope, with the image inverted, or it shows the positions of the moons for a different hour of the day, for example 0 hours Universal Time, which is 7 hours ahead of PDT.