An eclipse from space with a two-way Moon

This is pretty wild: a partial solar eclipse on March 4, 2011 as seen from NASA’s Solar Dynamics Observatory! Watch as the Moon slips in front of the face of the Sun in this video:

Cool! In this far-ultraviolet view, you can see magnetic activity on the Sun’s surface, like arcing gas eruptions and sunspots (which are dark in visible light, but very bright in the UV), and the roiling, bubbling motion as pockets of hot solar gas rise and fall. Then, suddenly, the Moon makes its appearance!

When I first saw this, I was surprised the Moon appeared to reverse direction. But after a moment’s thought I figured it out: parallax! Here’s how it works:



On the right is a diagram (not to scale, duh) I put together to illustrate the situation. SDO orbits the Earth, and I drew it so that it orbits counterclockwise. When SDO is at the bottom of the diagram, it’s moving to the right (position 1). A little while later (position 2) it’s moving to the left.

Now imagine the Sun is way off the top of the diagram. SDO stares at ...

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