Video: When Earth Eclipses the Sun

Solar eclipses (when the moon blocks out the sun) and lunar eclipses (when Earth’s shadow engulfs the moon) can be awe-inspiring sights for us surface dwellers.

But Earth-orbiting satellites sometimes get a bonus view: terran eclipses, when Earth blocks out the sun.

The above video was captured by the Solar X-ray Imager (SXI) on the 15th Geostationary Operational Environmental Satellite, also known as GOES-15.

As the name implies, the GOES satellites are in so-called geostationary orbits more than 22,000 miles (35,400 kilometers) above Earth. This type of orbit allows each GOES craft to remain at essentially the same points above Earth 24/7.

The National Oceanic and Atmospheric Administration has five working GOES satellites in orbit, which are mostly used for monitoring severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes.

GOES-14 is orbiting but remains inactive, a condition known as on-orbit storage.

GOES-12 is dedicated to keeping its eye on South America, GOES-11 watches the western half of the U.S. and the eastern Pacific, and GOES-13 is trained on the eastern half of the U.S. and the western Atlantic.

GOES-15, meanwhile, is focused solely on the sun. It uses its SXI to monitor the sun’s x-rays for early detection of solar flares, coronal mass ejections, and other space weather phenomena.

[UPDATE: My contact at NOAA emails me the following—GOES-15 has the same Earth-observing mission as the other GOES satellites, but it’s somewhat interesting as to why it is the one currently taking SXI images.

The SXI on GOES-12 bit the dust a while back, so SXI duties were supposed to be taken up by GOES-13. However, the big X-class solar flare of December 2006 swamped the SXI on 13 and damaged the sensor. GOES-14 was launched in 2009 and was taking SXI pictures until GOES-15 was launched in March 2010. When GOES-15 was undergoing post-launch testing late last summer, the SXI would not turn on. Just when everyone was ready to throw in the towel, the SXI finally came to life. Because of this, folks are afraid that if the GOES-15 SXI is powered off to put the satellite into storage, it may never come back on. Thus, the decision was made to put GOES-14 into storage and keep the SXI on GOES-15 active.]

Via the GOES program, NOAA is keeping a sharp eye on space weather to help forecast extreme solar storms, which can interfere with communications satellites and—if they’re strong enough—even disrupt power grids on Earth.

(See “What If the Biggest Solar Storm on Record Happened Today?”)

In that sense, terran eclipses—while lovely—can be a huge pain for the satellite.

The above video of an inky interruption on February 26 marked the start of the 2011 spring eclipse season for GOES-15.

Due to the orbital geometry, eclipse seasons for GOES satellites occur twice each year around the spring and fall equinoxes, and each season lasts about 48 days.

This particular animation captured a partial eclipse, since it was still early in the spring season. But at the height of a season, the GOES satellites can be in total darkness for more than an hour.

The main problem with that is all the GOES satellites are solar-powered. No sun, no power.

GOES 13, 14, and 15 have more advanced batteries than their older siblings, so they have enough juice to keep taking pictures even during an eclipse.

Still, that’s small consolation for the sun-watching GOES-15, which can’t take pictures of much when—as a NOAA scientist tells me—”the pesky Earth is in the way.”

Luckily for NOAA—and all of us, really—GOES has a lot of help right now watching the sun. NASA has STEREO, SOHO, and SDO in orbital action, and several ground-based observatories are using advanced optics to cut through Earth’s atmospheric distortion and add to the solar database.

BONUS PHOTO: In January NOAA scientists got to see GOES-15 get whacked by a cosmic ray—it’s the comet-looking streak on the left. My contact at NOAA says the ray (really a charged subatomic particle) was traveling from right to left and got “smeared across the sensor like a bug on a windshield.” How cool is that?

Changing Planet