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Celestial mechanics works July 25, 2007

Posted by Peter Hornby in astronomy.

The Planetary Society blog, mostly written and researched by the seemingly indefatigable Emily Lakdawalla, is an outstanding resource if you’re interested in what’s happening in our exploration of the Solar System.  Emily seems to have everyone in the field of solar system research on her speed-dial list, and serves up the latest authoritative information from all the current and future projects and missions.

I was browsing the blog the other day when I ran across a remarkable image.  The image was in a post with the arresting title of “Hey, Moon! Get out of the way of Cassini”, and the point was to demonstrate, in a very graphic way, that this was a situation where our own moon almost got in the way of Cassini’s communication with Earth. The image was generated by a wonderful Solar System simulator, written by David Seal at JPL, which I hadn’t previously been aware of.  You can reproduce the image by going to the simulator and asking for the view of Earth as seen from Cassini on 2007 July 16 at 22:00 UTC. 

Understandably, Emily’s perspective in her blog entry was that of a space scientist – the pesky moon is obstructing Cassini’s line of sight to Earth.  It struck me, though, that if the moon was transiting Earth as seen from Cassini at Saturn, the same celestial geometry should show up on Earth as an occultation of Saturn.  And indeed it did.  Here’s an image showing the track of the occultation.  It turns out that most of the track was over the Pacific in daylight, which means that it wouldn’t have helped the scientists at Goldstone to look out of the window.  The image shows the occultation track crossing the South American coast around sunset, shortly after which the Moon and Saturn set.

I can’t quite put my finger on why this gives me a sense of comfort, but it does.  Maybe it’s just good to feel reassured that celestial mechanics works.



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