Today the Moon is passing across the field of view of AIA on board SDO. AIA sees this as a partial obscuration of the disk of the Sun. If you look at an AIA image near 15:00 UTC (March 04, 2011) you can clearly see that a big round object – our Moon – is blocking a portion of the solar disk, and some off disk-emission.

Moon nibbles Sun: a partial eclipse of the Sun by the Moon as seen by SDO

If you zoom in to the image, you can see that the edge is not perfectly circular. Those are mountains and valleys on the Moon seen in silhouette.

This partial eclipse of the Sun as seen by SDO was expected, since we know the orbits of SDO, AIA, the Sun and the Moon. You can find out more about the eclipses SDO will see (and has seen) by checking out the SDO operations calendar.

As well as being stunning displays of orbital mechanics, partial eclipses of the Sun by the Moon are also very useful in helping us understand the data we are taking with AIA. If you look closely at where the disk of the Moon meets the Sun, you can see a little bit of color is in the dark disk of the Moon. Since we know that the Moon does not emit radiation (and it is not transparent!), the light causing that little bit of color must have come from the Sun. The only way that light could appear to have come from the Moon is due to slight imperfections in the telescope. By measuring how much light leaks from the bits of the image where the Sun is, over to the bits of the image where the Moon is, we can characterize the imperfections in the telescope. And once we have done that, we can use that to improve the images by enhancing the image to take out the effects of the telescope imperfections.