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The Cassini division

Dave North


“Cassini looks almost comical to me for some reason - the proportions are so odd ...”


Bill Arnett snapped this rare view of Mare Orientale on the morning of November 16. The libration in the direction of Orientale was unusually large during third quarter in November.


Cassini is that goofy-looking crater just a bit north of the AAA - Aristillus, Autolycus and Archimedes - on the east edge of Imbrium.

Goofy? You bet.

It's old and beat up, flooded with lava, and has since taken two solid hits, leaving it with a pair of chunky child craters on its floor.

So of course it's one of my favorites. In the right light, it ends up looking whimsical: the comic relief in an otherwise pretty spectacular area, crowded with rilles, mountains and spectacular views and lighting.

I've commented on this before. Here's my first note from the Hitchhiker's Guide To The Moon:

"Cassini looks almost comical to me for some reason - the proportions are so odd. But a few moment's contemplation will yield easy views of the two nearly obliterated central peaks that add a poignance, and the odd, fat, short rilles to the east of the encroaching central crater."

Jay Freeman exercised his considerable gift for understatement in describing it as "odd-looking."

The crater is about 57km across. That makes it an easy target, and intrinsically interesting without any gloss from me.

Just a pinch above it you'll find the Alpine Valley. A little southwest: Timocharis.

Good area.

It's named after Giovanni-Domenico (Jack) Cassini who did a bunch of things, including the discovery of four satellites of Saturn and is best known for noting the Cassini Division in Saturn's ring system.

He discovered the polar caps of Mars, and the odd dark area on Iapetus. He worked on the first realistic attempt to derive the distance of the sun, and was off by only about 10 percent. A final highlight of his career was the explanation of the zodiacal light.

He was a clever experimentalist. He made a rough determination of the eccentricities of the earth's orbit by measuring a projection of the sun's image (on a cathedral floor) throughout the year. By comparing the sizes he could calculate proportional distance, and therefore the shape of our orbit.

He was a lousy artist, which should inspire anyone today who thinks about sketching what they see in the sky. You cannot do worse than Cassini.

But that didn't stop him.

Like Messier, his primary interest was comets. Also like Messier, most people don't care about that much at all.

Enough of history.

The logical choice for a probe to Saturn would, of course, be Cassini. And that's how it works out.

Cassini is chugging to Saturn right now after wandering around the solar system to pick up some free gravity-assist momentum.

Experiments are already running, including a test of Einstein's work (no surprise, passed again) and recording the "sound" of a solar storm.

It's expected to arrive next July and four years of doodling around are planned. It will also drop the Huygens probe on Saturn's moon Titan, known to have an atmosphere.

What is learned about Titan and the rings alone will probably pose more questions, and hopefully four years will be enough to get some answers.

Though three space agencies and seventeen nations have been involved in making the Cassini probe real, when push comes to shove its the Jet Propulsion Laboratory that's sitting in the pilot seat.

JPL, as most of you know, is in the Pasadena area of Los Angeles and tied closely to Caltech, nestled under Mount Wilson where so many things began.

So we have an enigmatic crater named after an important astronomer who inspired the name for Nasa's latest slambang mission - to Saturn.

Why is that in this month's Mooning?

Because on a recent flyby, the Cassini mission scooped up Mojo and Jane Houston Jones, our editors and among the most active contributors to Bay Area amateur astronomy. Arguably the most energetic and inspiring pair of people we've got.

Jane is joining the Cassini Division. And that will put a division of distance between us.

AANC President Jane will be part of the outreach program, and it's hard to imagine someone better suited for the role.

By the time you read this, they will already be down there moving into a new place and getting up to speed.

In spite of the loss to our local groups, this is a good thing for the larger amateur - and professional - community. No, it's terrific!

Besides, we have reasonably strong ties to the Los Angeles Astronomical Society (whose longtime former president has spoken at SJAA and works at JPL) so they're not disappearing into some unknown realm.

Other editors have moved on and the Ephemeris has grown stronger each time. I'm sure that trend will continue, though the bar has been raised with each new editing staff. It's a bit tougher now than when some previous folks took it on (though I doubt anyone will ever equal John Gleason's run...). But I'm sure Paul and Mary Kohlmiller will show us some new tricks.

Mojo and Jane also run out of their now-for-sale house. Not only that, they run websites for the SF Sidewalk Astronomers, the AANC, our own Ephemeris, the San Jose Bridge Club, the Fremont Peak Observatory Association, Litebox Telescopes, East Bay Astro, Jotabout Labs, and the Mount Diablo Astronomical Society. At least. There are also redirects for Bill Arnett's incomparable, and a sketching website that has played a monster role in promoting amateur artwork of the universe.

All this hardly takes into account the tireless, enthusiastic, friendly and outright wonderful realtime contributions they make at star parties, helping John Dobson with his classes, storing equipment, speaking (and listening with the patience of Job), attending meetings ... their unflagging support for oldtimers and newbies alike.

Aside from being two of the best friends anyone could ever ask for, I will forever be grateful for them taking on the Ephemeris at a time when I did not want (or think it right) to both do that and handle the SJAA presidency at the same time.

Certainly, after seeing the example they've set, I can hardly complain that in my case it was too much work!

So what does this have to do with the Mooning column?

Without them, it wouldn't be here. And this is the last one they'll produce.

Such a passing should not go unsung.

They've done one hell of a job, and deserve a huge thanks from SJAA, all the Bay Area, JPL, NASA and Cassini.

Thank you, MoJane.


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