Last Friday, NASA's Solar Dynamics Observatory captured this video of a partial solar eclipse, which shows the Moon blotting out the edges of the Sun in most unusual fashion, seemingly reversing direction as it moves. What's going on here?
As Phil Plait over at Bad Astronomy explains, the Moon's back-and-forth movement is all about parallax. The Solar Dynamics Observatory orbits the Earth in what, for our purposes, can be called a counterclockwise rotation. When the SDO is in the "bottom" half of its orbit (in clock terms, when it's moving from 9:00 to 6:00 to 3:00), it is moving rightwards, while in the "top" half of the orbit (3:00 to 12:00 to 9:00) it is moving left.
The SDO's job is to keep its cameras locked on the Sun at all times, so there appears to be no motion relative to our star. Not so for the Moon, which actually has its orbital motion exaggerated and somewhat distorted by the SDO - when the satellite is moving "right", the Moon appears to move left, while when the satellite moves "left", the Moon appears to be moving to the right. And that, as Phil Plait explains, is why the Moon appears to quite suddenly reverse direction:
So at different points in its orbit, SDO sees the Moon moving in opposite directions. The point where the Moon's direction just switches direction is about when the Earth, SDO, and the Moon make a right angle, at about the 3:00 position in its orbit (and again at 9:00). When SDO saw the partial eclipse, it must have just been at the point in its orbit where the Moon switched directions. That's why in the video you see it moving one way, slow, stop, then reverse.
Via Bad Astronomy.