On June 2, the International Space Station (ISS) passed directly over the sun from my location. I had seen photos others had taken of ISS solar transits but had never observed one myself. A couple nights before the date, a friend tipped me off that it would practically fly over my house. Time to get off the couch and check this out.
So I headed over to transit-finder.com and confirmed that my small plot of earth stood almost directly on the center line of a visibility zone measuring just 2.8 miles (4.5 kilometers) wide and extending about 150 miles (240 kilometers) from north-central Minnesota into Lake Superior.
Similar to a total solar eclipse, anyone within that zone would see the space station zip directly across the sun that Friday afternoon. The closer to the center line the more centrally it would traverse the solar disk. And it would do so faster than a bat out of hell.
Traveling at more than 17,000 miles (27,000 kilometers) an hour the ISS would complete the passage in just 0.7 second. When the sun is low in the sky, the trip takes longer because in addition to the vertical distance to the space station (250 miles) you also have to add in the horizontal or line-of-site distance from the observer. The farther away the ISS is, the smaller its apparent size and the slower its apparent motion. This transit was exceptionally brief because the sun was up high with the space station just 300 miles away.
I needed a clock accurate to a second to make sure I didn't miss it. I downloaded the free app Atomic Clock: NTP Time ( Android version ) with a nifty digital display accurate to 1/100 second. Not than you need that kind of accuracy to anticipate a transit, but it's sure fun watching the hundredths dial spin while you wait.
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I also wanted to see it with my own eyes. That meant setting up two telescopes: one for the camera and the other for observing. Because the space station would moving quickly I set the camera's shutter speed to 1/1,250 of a second to prevent motion blur. That fast speed forced me to dial up the ISO to 1000, a little higher than I'd have liked, but not objectionable. Higher ISOs make images look slightly grainy.
The only way to guarantee snagging at least one image of the space station was to photograph the transit like I would a sporting event — at a high frame rate. Frame rate is number of times the shutter clicks in a second. The higher the fps (frames per second) the better chance a photographer has at capturing peak action. I set my Canon 5D Mark IV camera to 7 fps and used a remote shutter release (we used to call them cable releases) to automatically trip the shutter. All I had to do was press the remote button and the camera would start clicking away.
I removed the camera's lens and attached an adapter to connect it to the telescope, a small 94mm refractor capped with a safe solar filter. The scope rested on a mount with a motor that tracked the sun so it wouldn't drift out of camera view as the magical time approached.
After carefully focusing the sun I set up a second scope right next to the first and then parked my butt in a lawn chair. Comfort is essential to staying on your toes. I had written down the transit time 2:43:31 p.m. on a torn piece of paper and taped it to the scope's tripod so I wouldn't forget, then knocked off a few test exposures to make sure all was in order. As the atoms ticked off the time I held the remote in one hand and used the other to follow the sun in the second scope, all the while keeping an eye on the clock.
About 10 seconds before the predicted pass I pressed the remote shutter and let it rip. The intense anticipation made it the longest 10 seconds I've ever experienced at the eyepiece. At the predicted time the ISS seemed to fly out of nowhere. Its black silhouette tore across the sun as if shot from a cannon. And that shape! It instantly recalled an imperial fighter ship from the "Star Wars" movies, lacking only the hoarse screech of the bird-of-prey's engines.
After my heart rate dropped, I reviewed my take and was delighted to see the ISS show up in four separate exposures. The amount of detail visible was surprising. If you look closely you can discern eight individual solar panel arrays plus other structures. I also like how similar in size the space station is to the largest sunspot. Since that spot was visible with the naked eye (protected with a #14 welders glass) I suspected that the transit also would have been visible without a telescope.
Now I'm on fire and can't wait for the next one. Transit-finder.com displays transits across both the sun and moon for the ISS, the Chinese Tiangong space station and Hubble Space Telescope. Space station transits are the easiest to see because it's physically so much bigger than the others.
Once at the site select your location manually or simply click "Auto-detect." Then set the time span on the calendar and choose how far you're willing to travel to see a transit. Remember that the path of visibility is quite narrow so the chances of your house sitting right on the center line are infrequent. Click the "Calculate" button and a list of detailed transit predictions will appear. The small maps show the satellite's predicted path for your location. You'll immediately notice that most are misses. To find out where to drive to see a "hit," click the "Show on map" button and scroll your mouse to zoom in.
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Once you've identified a choice transit, keep in mind that the time and path shown are approximate 10-14 days out. On the big day be sure to check again some hours beforehand when the time will be accurate to a fraction of a second and center line good to within 328 feet (100 meters). For more information click the site's help link.
If it's a lunar transit you don't need a special filter. Just set your binoculars or telescope up and wait for the flyby. Solar transits require a safe, undamaged telescope or binocular solar filter. These are available at many online telescope shops .
Even if you don't take pictures I hope sometime in the next few months you'll get to see one of these awesome flybys and experience the same thrill I did.
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