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Moon motion

Watching our satellite as it glides in front of the Sun is a moving experience.
The great eclipse is getting close now. I hope you will not stay put and be content with a mere partial event, but will travel to witness the unbelievable majesty of totality.

One often-neglected aspect of the spectacle involves motion. This is more singular and special than popularly realized. That’s because a frozen visual scene is the norm in our profession or hobby. When we look through telescopes, it’s a slide show. The images are static. Nothing moves. We’re used to it.

The entire human race will come and go before the great globular cluster in Hercules looks any different. Even much closer to home at 410 light-years, the beautiful colored double star Albireo has not changed since the first telescopes were aimed its way in the 17th century. We know the blue-and-gold component stars whirl around each other at several miles a second. But that’s not enough to let us detect the slightest change in the stars’ positions, not even after four centuries.

By contrast, the tilt of Saturn’s rings changes noticeably from one year to the next, and they’re now wide open. We can remember just a few short years ago when the rings were nearly edge-on, making Saturn a very different animal. Jupiter is more animated, even if just barely. A few hours of observation reveal its spin and the whirling of its moon Io, which whizzes completely around that planet in less than two days.

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