A Really Weird Solar Eclipse

March 7, 2019: Earlier today, NASA’s Solar Dynamics Observatory (SDO) observed an eclipse of the sun–a strange kind of eclipse that you can only see while orbiting Earth. The black disk of the New Moon passed in front of the sun, reversed course, and did it again:

During the eclipse, which lasted just over 4 hours, as much as 82% of the sun was covered. Technically, that makes it an annular solar eclipse, not total. At maximum, an annulus or “ring of fire” completely surrounded the Moon.

The strange “double-dip” motion of the Moon across the sun is a result of orbital mechanics. Both SDO and the Moon are orbiting Earth, but at different speeds. SDO’s velocity of ~3 km/s is faster than the Moon’s velocity of 1 km/s. SDO thus overtakes the Moon first in one direction, then the other, during the long eclipse.

High-resolution images of the eclipse reveal that the Moon is not perfectly smooth. The little bumps and irregularities you see are lunar mountains backlit by solar plasma:

Images like these have practical value to the SDO science team. The sharp edge of the lunar disk helps researchers measure the in-orbit characteristics of the telescope–e.g., how light diffracts around the telescope’s optics and filter support grids. Once these are calibrated, it is possible to correct SDO data for instrumental effects and sharpen images of the sun even more than before.

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