Solar Eclipse, Seen Only By U.S. Satellite

Sun blocked by the moon as seen by NASA's orbiting Solar Dynamics Observatory. NASA/Goddard Space Flight Center

There was a solar eclipse on Tuesday, and we earthlings missed it.

It wasn't our fault, really. As happens most months of the year, the moon, passing between Earth and sun, just missed casting its shadow on us. Its orbit is slightly canted so that if you could see the disc of the new moon, it would usually pass a little north or south of the sun.

So nothing happened - at least as far as we earthbound humans are concerned - but an orbiting NASA probe called Solar Dynamics Observatory, 22,000 miles overhead, just happened to be in the right place at the right time. The result you see:

The moon's odd motion - seeming to come in from the top of the picture and then skittering off to the right - is created by the relative motion of the moon and the satellite. The moon orbits about 240,000 miles from Earth, circling us once every 29 days, while SDO is in a geosynchronous orbit, circling us once every 24 hours. Put their motions together and it gets complicated.

If you want to be pedantic, what SDO saw was really a transit instead of an eclipse. While the moon appeared to pass over the face of the sun, the word eclipse is often reserved for when that's seen from Earth. The next eclipse visible from down here (mostly over the Pacific, but ending at dusk over the southwestern U.S.) will be May 20, and it will be an unusual annular eclipse - the moon, whose orbit is not quite circular, will be far enough away that even if you're in the right place, the sun will appear to form a bright ring around the moon's disc.

After a period of relative quiet, the sun is becoming more active this year, with more sunspots and charged gas, or plasma, being hurtled out into space. But in truth, it's never quiet. SDO, which has been keeping watch for two years, has returned pictures showing a 30-hour period on the sun's surface Feb. 7-8, and the video has gone viral online.