Jupiter's moons (July 3 @ 22:30 EST)

Factual Questions
1

I was taking a peak at Jupiter tonight and saw something that I don’t ever recall seeing before. I could swear that there were five moons visible.

Now, of course, I’m not really suggesting that I saw an extra moon, especially since I was only using my binoculars (16 X 70). Still, there is an object, about four Jovian diameters north-east of the planet that, for all intents and purposes, appeared identical to one of its moons. Indeed, its brightness and ‘size’ were indistinguishable from a typical Galilean moon.

I assume it’s a star. Still, makes for a very, very nice image. If you’ve got a pair of binoculars handy, take a look. I’m curious if anyone can identify it more specifically.

Thanks!

2

There’s some high wispy clouds here that prevent just about any astronomical observation.

I don’t observe much because of keratoconus, but my memory is that the Galilean moons look more like planets than stars in that they don’t twinkle much. So if this object resembles the moons in that way, then it’s probably an asteroid. It’s not one of the other planets; I checked the July issue of Sky&Telescope and none are anywhere near Jup.

3

My copy of Celestia implies that it was a sixth-magnitude star called HD 126766. (I highly recommend this software, BTW — it’s remarkably full-featured for a piece of open-source software.)

4

Thanks! I’m hoping it’s clear tonight so I can “confirm” that what I saw was HD 126766 (as you know, Jupiter and its moons will have moved relative to that ‘fixed’ star).

I most definitely will also check out Celestia. Thanks for the tip.

5

Wow! Celestia looks excellent.

I can’t access your e-mail address, MikeS, so maybe you can tell me here (or e-mail me) how to get the background stars to update (or revert to a given date) and not just the planet and its moon.

Thanks.

6

I hope it’s not poor form to resurrect this fairly old thread that I started, but I think there’s an interesting denouement.

Turns out that my sighting of a “5[sup]th[/sup] Galilean moon” :wink: was due to a combination of three coincidences*.

  1. a star (HD 70714) was at an apparent distance from Jupiter such that it resembled a Galilean moon

  2. the star had approximately the brightness of a ‘typical’ Galilean moon

  3. in the first week of July, Jupiter was almost stationary against the background stars as it neared the end of its March-to-July retrograde motion (in fact, on July 6 it was stationary, or darn close to it)

The third point above, in particular, did a lot to disguise the fact that what I was seeing was merely a background star. When the “moon” seemed to stay with Jupiter day after day, and didn’t move with the background stars, I was totally confused.

I thought this was neat. YMMV.

*actually, there was a fourth coincidence, i.e. I discovered Stellarium, a free and wonderful planetarium for my computer. I can’t recommend it enough.