SJAA Ephemeris September 2010 | SJAA Home | Contents | Previous | Next

The Shallow Sky

In and Out of the Ecliptic

Akkana Peck


September skies are full of planets as we approach the autumnal equinox, on the night of the 22nd.

“Jupiter season” begins this month. Jupiter hits opposition on the 21st, so it’s well placed for viewing most of the night. Alas, no multiple transits of the Galilean moons this month; but there are plenty of single moon transits, plus the big mystery: how visible will the South Equatorial Band be? You may recall that it vanished mysteriously a few months ago during Jupiter’s pass behind the sun – just one of those wild Jupiter atmospheric changes that makes the giant planet so much fun to watch. Will the Great Red Spot be standing alone in a sea of off-white? Or will the band darken by September to hide the GRS? We report; you observe AND decide.

Uranus hangs a scant degree to the north of Jupiter. It should be easy to find them in the same low-power telescope field. Uranus’s magnitude is around 5.7 – just a little brighter than Ganymede. But it won’t be hard to tell Uranus apart from Jupiter’s moons: Uranus will show a disc of 2.3” (that’s arcseconds, not inches) compared to Ganymede’s 1.7. More important, Uranus’s striking green color proclaims it as an entirely different beast.

Even aside from its odd color, Uranus will be well out of the plane of Jupiter’s moons. But wait: why is that? Aren’t they all orbiting along the ecliptic? Well, yes – more or less – but planets never stick exactly to the ecliptic (except Earth, since our orbit defines the ecliptic). Jupiter and Uranus are both pretty orderly as planets go. Jupiter’s inclination is 1.31 degrees, while Uranus’s is only 0.77. That sounds like a pretty small deviation. But a degree or so is a healthy distance when you’re looking through the eyepiece at 100x and the moons are all staying within a few arcminutes of Jupiter.

While you’re waiting for Jupiter and Uranus to get high enough for good viewing, there’s plenty else to command your attention in the early evening sky. Venus continues its long and beautiful evening twilight show, reaching its maximum brightness of -4.8 on September 23rd. Mars, much dimmer at magnitude 1.5, hovers nearby. And you can still barely catch Saturn at sunset as September begins, but it’ll disappear into the twilight as the month progresses.

Neptune is about three and half degrees up and left of the left horn of Capricornus. Don’t confuse it with the 5th magnitude star to its right: Neptune is magnitude 7.8, and with enough magnification you should see it as a small blue disk, though it’s much harder than Uranus to resolve or identify.

Pluto is visible in the early evening, but not as high or well placed as it was a few months back. Still, if you missed it and want your chance, there’s still time – go for it!

Early riser? Not all the action is in the morning sky. Mercury moves into the morning sky starting in mid-September, though it’s gibbous, so there’s not much detail to see, but it’s a pretty naked-eye or binocular sight if you have to be up early for work.


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