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PostPosted: Mon May 07, 2018 1:58 pm 
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Joined: Thu Oct 13, 2011 12:53 pm
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Location: Maple Shade, NJ
Tonight, Monday, 07-May-2018, there will be concurrent transits of Jupiter’s Great Red Spot (GRS) and its Galilean satellite, Io, plus Io’s shadow. Since opposition is on Tuesday evening, 08-May-2018, at 8:39 pm EDT, Io will be nearly superimposed on its shadow. Opposition also results in Jupiter being a plump 44.8 arc seconds in apparent diameter.

These transits all begin at the eastern limb about 11 pm Monday. Io and its shadow will cross Jupiter’s central meridian (CM) shortly after midnight, then egress from the western limb just after 1 am Tuesday. The GRS transits the CM at 1:42 am Tuesday, and due to foreshortening near the limbs, is best observed about +/- 1 hr from the CM transit.

Again because of its proximity to opposition, Jupiter transits our southern celestial meridian about 1 am EDT at a nominal 34 deg altitude (for 40N latitude).

If the weather cooperates, it should be worth taking a look with a scope.

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PostPosted: Tue May 08, 2018 1:02 pm 
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Joined: Thu Oct 13, 2011 12:53 pm
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Location: Maple Shade, NJ
There were heavy-looking broken clouds here (Maple Shade, NJ) around sunset on Monday, May 7, 2018, but by the end of astronomical twilight, they cleared out leaving generally good transparency. Ultimately, I was picking up stars in the magnitude 3.5 to perhaps 4 range with unaided eyes, which isn’t too bad for the suburbs.

Using my Stellarvue 130 mm apo refractor set up in my back yard, I observed Jupiter on-and-off from about 10:45 pm EDT on May 7 until about 2:15 am on May 8. The seeing was woozy at the start, but settled down as the night wore on. For the most part, I was using 4.7 and 3.7 mm Stellarvue Optimus 110 deg eyepieces, which yielded 194x and 253x respectively (with 34 and 26 arc minute true fields respectively).

I saw Io creep onto the eastern limb of Jupiter’s disc around 11 pm, and after it crossed onto the Jovian disc, Io itself was an obvious little disc. Io’s shadow was not evident near the limb, but as Io moved farther in, and for much of the transit, Io’s disc disappeared and it’s shadow became readily visible as it looked like a pitch-black bullet hole, despite being partially covered by Io. As it neared Jupiter’s western limb and egress just after 1 am, Io’s little disc reappeared and the shadow disappeared, the reverse of what happened after ingress.

I’ve never seen such a well-defined Galilean satellite disc before. Even the other three satellites looked relatively disc-like against the dark sky, but against the limb-darkened edge of the Jovian disc, Io’s little disc really stood out. WinJUPOS indicated it was 1.14 arc seconds in diameter. I suppose Io’s shadow was not seen near the limb due to foreshortening.

At 11:35 pm, the Great Red Spot was vaguely visible inside Jupiter’s eastern limb, but I did not get a solid view until I was back out at 12:30 am on May 8. It was well defined inside the “eye socket” on the South Equatorial Belt and it had a distinct, truly reddish color, much more vivid than it was a few years ago when it could have been called the Great Peach Spot. After Jupiter’s nominal 1 am transit of the celestial meridian, when the seeing had settled down, considerable detail was seen on Jupiter’s belts, although it took patient watching to see it. I could have spent a lot of time sketching!

I made a couple of side trips too. The double star Rasalgethei (Alpha Herculis) was an easy split at 4.7 arc seconds separation, but somewhat unequal magnitudes of 3.1 and 5.4. It was pretty nevertheless. While I was in that neighborhood of the sky, I took a moment to find magnitude 9.5 Barnard’s Star, which is near 66 Oph, and not far from the coarse open cluster IC 4665, which was also seen in the scope (and can be seen with unaided eyes from a dark site). Note to SkySafari users — it shows Barnard’s Star in the approximate year 2000 position. Since then, it has moved about 3 arc minutes, which would make positive identification difficult in a magnified scope view. I used a labeled photo I took last year to confirm the position.

I’m still not using a finder scope or reflex finder with the SV-130T, but the 20 mm 100 deg Optimus eyepiece turns the scope into a big finder (it yields 46x with a 2.2 deg true field).

Finally, I looked at Antares just after 2 am, but it was still low and way too woozy to make a definitive identification of the secondary. I thought I saw a faint dot popping out periodically at the 9:30 o’clock position (which a later check with SkyTools confirmed as the right place in the vertically correct, horizontally reversed refractor + diagonal view), but there were just too many diffraction ring segments fluttering in and out to be certain.

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Joe Stieber
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