ISS Transit

The International Space Station flies off the limb of the Sun on March 23rd, as seen in hydrogen-alpha light from Cambridge, MA.

S&T: Sean Walker

It was an exciting morning here last Friday. I would have told you about it then, but I had already prepared something.

In the February Sky & Telescope I wrote about the impressive Celestial Observer ( website, which not only serves as an online astronomical almanac, but will also send you e-mail alerts for all manner of sky events. I signed up for alerts for Iridium flares and flyovers of the International Space Station. They pop into my inbox around 8:30 each morning. Friday was a little different.

At 8:37, a CalSKY alert for the ISS came in, and I almost didn't open it, because the forecast predicted a cloudy evening. I opened it anyway, and saw that it was different. It wasn't for an evening passage. It was an alert that the station was going to transit the Sun from my location — in 90 minutes!

Because this was the first transit alert I had received, I had to make sure that it actually said what I thought it said. I recalled from writing about these events before that sometimes you have to travel a ways to be in the right spot to see it.

The bulletin included some geographical coordinates for a centerline transit, which turned out to be about a mile away from the S&T offices. The report also said that the path was nearly 5 miles wide. So we should see it here at 10:05:26 a.m.! The transit would last all of a second.

I told everyone as they came into the office. Since this wasn't something easily seen with a filtered naked-eye view of the Sun, we needed some magnification. Kelly Beatty had a small reflector that he jerry-rigged with a projection screen. Sean Walker snagged a Coronado PST out of David Tytell's office, dug up a webcam out of his own office, mounted them on a tripod, and connected it all to his laptop. Our set up area was out the back door in the parking lot.

I went around and gave people a 10-minute warning, but I still had some doubts about the validity of the prediction. I remember years ago when Iridium flares were fairly new. I used to calculate predictions and told everyone about a flare just before 5 p.m. on a winter evening. Nearly everyone went outside to look up — which slowed traffic on Bay State Rd. Nothing happened. I was never sure why. A few days later, there was another opportunity, and we all saw it — vindication!

Anyway, Kelly's solar projector was working great. Sean was unhappy about the seeing, but there was nothing he could do about that. Alan MacRobert was going to use eclipse glasses.

At 10:05 we quieted and waited. Just as three people came out of the building to join us, I saw a speck race across the Sun, like a gnat flitting by. A cheer went up from me, Kelly, Sean, Tony Flanders, and Kristin Beaudoin. I looked up to see Rick Fienberg, Lauren Darby, and Pat Coppola coming through the doorway and told them. "Sorry, you just missed it." Alan didn't see it either.

Sean was able to capture a bit of it with the PST (as seen here). And for about an hour afterward, there were smiles of satisfaction, as if we witnessed something rare.

Hopefully next time (if there ever is one), we'll be better prepared.


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