Oh, now he sends it!

lunar eclipse progression
One (or perhaps eight) more from Jim, showing the progression of the eclipse, with two curious traits.

These were taken with a fixed camera, shooting with a wider field of view than the images from the earlier post. An intervalometer was used to snap a frame every 150 seconds, and the resulting eight frames were stacked together into this one image. The camera didn’t move, the frames were not shifted – the moon actually moved this far between each image. As I have said before, the moon and sun move their own width across the sky in 150 seconds, just two and a half minutes. Actually, it’s the rotation of the Earth that’s (mostly) responsible, but you get the gist.

Then they should all be contacting one another, like beads on a string, right? Certainly – the only reason they do not appear so is because of the shadow hiding one of the contact edges. If we were to take one of the images and rotate so its non-shadowed side faced its neighbor, we’d see them touching.

Don’t bother trying, because I already did, and it doesn’t work – there really is a gap between them. Turns out, the whole “150 second” thing is not entirely accurate. The moon’s orbit is elliptical, which means at times it’s farther away from the Earth than at others, and of course this makes it appear smaller in size. Apogee, the time when the moon was farthest, occurred April 8th, while perigee (the closest) will be April 23rd. Thus it was roughly one-third of the way up from its smallest size. Note also that the moon is not perfectly fixed in the sky, only showing apparent movement because of the rotation of the Earth. It’s moving too, otherwise the phases wouldn’t change, but this movement is tiny compared to the rotation of the Earth.

I played around with angular size and time and all that, always a risky thing for someone who’s pretty bad at math, then got smart and booted Stellarium again, which will show the sky’s motion at any speed you like. A sticky note attached to the monitor confirmed that 150 seconds takes the moon more than its own width, producing a pretty good match for this image. In fact, using Stellarium to plot the time needed for an exact ‘beaded line’ is probably a pretty easy way to plan a cool photo sequence.

That was all trait one. Trait two is, the shadows are going the opposite way than what you’d expect. The moon is moving right, but the shadow is overtaking it from the left.

Most of what you are seeing is the moon’s own orbital motion as it revolves around the Earth in a little over 27 days. But a very small part of it is Earth’s orbit around the sun, which shifts the shadow it throws. This doesn’t account for much, since the whole orbit takes a year to accomplish, but it affects the speed and duration of the eclipse nonetheless.

The video found here illustrates this to a certain extent, but the scale for all bodies and distances are way off; the sun is loads bigger than that but much, much further off, while the moon is also significantly more distant. Thus the shadows thrown by the Earth and the moon are much smaller, and coupled with the inclination of the moon’s orbit, this means it only catches the shadow sporadically, thus the rarity of both lunar and solar eclipses (rather than occurring every new and full moon.) If you want to see the actual shadow cast by the moon during a solar eclipse, well, thank the Mir 27 crewmembers.

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