Three Rings and an "Appulsar"

Last month was the annual NASA Lunar conference – a great chance to hobnob with lunar scientists and hear the latest on lunar research.

Orientale was big, with several talks on how its vast triple-ring structure formed. Orientale is my favorite lunar formation – a relatively young basin from a huge impact that raised three concentric rings of mountains. The outer ring, Montes Cordillera, is 930 km (580 miles) in diameter).

Orientale is also interesting to observers because it’s technically on the far side of the moon. We can only see it in profile, and only when the moon’s elliptical, inclined orbit gives it a tilt (called “libration”) that lets us see a little bit over the edge to the far side. (It’s probably best that it’s just over the edge. If we could see Orientale face on, like spacecraft do, it might look like a gigantic eye on the moon staring down at us, and human mythology and religion might have ended up quite different.)

It turns out there are several competing theories about exactly how the Orientale impact created its three rings, and about which mountain ring marked the edge of the original crater. Did material slumping back into a nested melt cavity cause fault blocks to rotate, creating mountains like a circular version of Nevada’s Basin and Range fault blocks? How big was the initial crater, and which mountain ranges were part of it? The GRAIL mission is measuring gravitational anomalies in the middle of the basin to try to pin down the details.

Happily for us, the structure is easily big enough to see with a small telescope. Since Orientale is on the far western edge of the moon, it’s best lit just before full moon and just before new moon. This month, the time around full moon isn’t a great time to look at Orientale – it’s turned away from us then – but you can get a glimpse it in sunset light, just before the new moon, on the nights of Sep 13 and 14.

Interestingly, these multiple-ring basins we see on the moon (and even on Earth – like Upheaval Dome in Utah’s Canyonlands National Park) don’t exist on Mercury. There are basins with two rings, but any possible third rings are subtle and difficult to detect. Why? Nobody’s sure. Have they been covered by volcanic activity? Does the hotter rock make it react differently to impacts? Fascinating stuff.

Speaking of Mercury, you won’t see it this month – it’s too close to the sun. The other twilight planet, Venus, is in the morning sky. Meanwhile, evening twilight sports Saturn and Mars, both low and getting lower – Saturn will disappear in the glare before the month is out.

Jupiter rises around midnight and is a late night and early morning object, as are the outer planets. Pluto is well placed for viewing, and if you get frustrated looking for it you can always divert to M25 right next door, or nearby M18, M24, M22, M16 NGC6595 and a wealth of other deep sky objects. Neptune is just past opposition and should be easy to locate off the eastern tip of Capricornus (though it’s actually over the border in Aquarius).

Uranus reaches opposition right around midnight on the evening of Friday, Sep 28-29. But more interesting is what happens the previous Sunday, September 23rd, when Uranus passes within one arcminute of the star 44 Piscium.

Mark Gingrich, describing the upcoming event on the Shallow Sky list, coined the term “appulsar” for this fairly rare event. (An “appulse” is when two astronomical objects make a close approach.)

Both Uranus and the star are magnitude 5.75. Normally, each one is just barely visible with the naked eye from a fairly dark sky – a tough hunt for most of us from light polluted San Jose. With the two objects so close together, how much brighter will they look? Surprisingly, not that much brighter. Together, they’ll look like a single star around 5th magnitude. But that’s bright enough to see even in town – so it might be a novel chance to locate Uranus with the naked eye without leaving the Bay area.

Of course, it’s not just a naked eye event – it will be of interest to binocular and telescope users as well. One arc minute is a little bigger than Jupiter’s disk. So in binoculars, the pair should appear as a close double star – you may need to use a tripod or rest the binocular on a fence to separate the two. Can you tell, in binoculars, which one of the pair is the planet and which is the star? Uranus’s steady green glow, with no twinkling, should make it easy to tell which is which, at least if the air is at all steady. That’s my prediction, anyway. Let’s see if it turns out to be right.

Of course, in a telescope you’ll have no trouble telling the pair apart. But it should still be an interesting view. We know they’re almost exactly the same magnitude. So will a point source *look* the same brightness as a colored disk right next to it? Or will it look brighter?

Anyway, this should be a fun and unusual event, just a day after the Sep 22 autumnal equinox – a nice way to ring in the fall.