LRO Images the May 2012 Solar Eclipse

On May 20, 2012, Lunar Reconnaissance Orbiter turned away from the Moon so that its camera (LROC) could point at the Earth. LRO periodically uses the Earth as a target for calibrating the cameras, but in this case, it was imaging the shadow of the Moon during an annular solar eclipse. The LROC narrow-angle camera (NAC) captured four images of the shadow on two successive lunar orbits.

This isn't as easy as pressing the shutter button. Each of the twin NACs comprises a single scanline of 5064 pixels. Ordinarily, the camera is pointed straight down at the Moon, and the orbital motion of the spacecraft sweeps the scanline over the lunar surface to build up an image. (This sweeping business is the reason such single-scanline cameras are often called pushbroom sensors.) To take a photo of the Earth, LRO must be continuously rotated, or slewed, to sweep the scanline across the disk of our home planet. After the first photo in each orbit, LRO's slew was reversed, sweeping the scanline in the opposite direction.

Because the Moon's orbit is elliptical, its distance from Earth and its apparent size in the sky can vary by about 14%. An annular eclipse is a solar eclipse that occurs when the Moon is at the far end of this range and isn't quite big enough to cover the Sun, leaving a ring, or annulus, of sunlight around the edge of the Moon. This is why there's no central black umbra in the LROC images.

See http://lroc.sese.asu.edu/posts/513