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SYFY WIRE Bad Astronomy

Sic Transit Gloria Mercury

By Phil Plait

On Monday, Mercury passed directly between the Earth and the Sun, the last transit it will make until November 2019. Judging from my Twitter and Facebook feeds, it was watched by approximately eleventy bazillion people.

I got to watch it through my own small solar ‘scope, which was lovely (sorry, no pix; I still haven’t figured out how to get good shots with my phone or my DSLR). Seeing it with your own eyes through a properly protected telescope is amazing, but not possible for everyone.

Happily, it’s never cloudy for NASA. They were able to get pretty good views through the Solar Dynamics Observatory, orbiting the Earth high above the atmosphere. And the happier news is they made a really spiffy animation from the observations!

SDO sees the Sun at many different wavelengths, to capture the different ways our star emits energy. This in turn helps scientists understand how the Sun’s interior affects the surface, and how both affect the space around it. The Sun has a powerful and chaotic magnetic field that strongly affects the gas (really, plasma) on the surface. You can see that in several parts of the animations as huge, towering loops of magnetic field lines arc over the Sun. Mercury passed right over one from our view, though it was actually more than 65 million kilometers from the Sun at the time.

The Sun has an atmosphere, called the corona, and it glows in the far ultraviolet and X-rays. Just before ingress and just after egress, you can see Mercury silhouetted against the corona:

And as much as I like the nicely produced video above made by my friends at NASA’s Goddard Space Flight Center, I think I actually prefer this one: It shows the Sun centered in the frame as Mercury slides past.

Why do I like that one better? The scale. Mercury is a mere 4,880 kilometers across, the Sun a mighty 1.4 million. Mercury is a little closer to us, so perspective makes it look a bit bigger, but the Sun still appears about 150 times wider in this video. It’s hard to get a grip on all this; is Mercury just small, or is the Sun vast?

Answer: yes.

And this is one of the many reasons I love events like these. It gives us a sense of the Universe, ties us to the sizes, the scales, the cycles of things. The Universe is lovely to watch, and you know what else? It’s fun, too.

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