Last night, May 15, I could have seen the ISS five times in a row. The first pass occurred at 9:14 p.m. and the last at 3:41 a.m. this morning. I opted for the 10:50 p.m. and 12:27 a.m. flybys, both of which crossed the northern sky before my bedtime.
Normally, only one or two passes are visible at dusk or dawn. But they actually happen all night. We don't see these because later in the evening the Earth's shadow reaches far up into the sky and blocks sunlight from reaching the space station. It's in eclipse. Since the ISS must be illuminated by the sun to see, without sunlight it's invisible.
But twice a year around the summer solstice — May to July in the northern hemisphere and November to January in the southern hemisphere — the International Space Station remains in full sunlight as it orbits the planet. This happens because its orbit and the Earth's day-night terminator — the border between day and night — nearly align. For the astronauts on board the station, the sun shines all day long, much like the midnight sun does north of the Arctic Circle.
Down on the ground, observers at mid-northern latitudes (from about 40°-55° north) can see every pass all night long for up to two weeks on either side of the summer solstice.
Traveling at around 17,200 mph, it takes the space station about 93 minutes to circle the Earth. If you see it, say at 9:30 p.m. on the first go-round, just add 93 minutes to the time, and you'll know when to watch next. For my location in northern Minnesota, the maximum number of appearances during the current ISS "marathon" is five, but observers in some locales will see up to six.
Hard-core space station fans really do stay up all night to see every pass, completing the marathon barely able to stand up. Others, like myself, sign up for the half-marathon. Obviously, I need to get into up my game!
The current ISS marathon runs for the next dozen days or so until early June. When you see the space station circle so often you get a feel for how many sunrises and sunsets the astronauts must see from orbit, up to 16 of both. You'll also see lots of other things up there, some unexpected.
Last night, I caught a bizarre pass of the latest Starlink launch (May 15). The satellites were so close together (packed into just 1°) they looked like a dash moving across the northern sky. Later, a brilliant fireball meteor as bright as Venus but much more colorful seared its way south across Scorpius.
Like many satellites, the ISS travels from west to east, the same direction as the Earth rotates. It begins its climb in the western sky (northwest to southwest) and moves east, slowly at first. Then it seems to pick up speed when it's overhead and slow down again as it drops toward the eastern horizon. The speed-up and slow-down is an illusion caused by the station's changing distance.
When it's low in the sky, it's farther away than when overhead. At 10° altitude (one fist above the horizon), the ISS is more than 900 miles (1,500 km) distant compared to just 250 miles (400 km) when it glides overhead. Things that are farther away appear to move more slowly. They're also fainter. That's why the space station looks brighter when it's higher up compared to near the horizon.
During last night's second pass, I noticed a fainter satellite preceding the station along nearly the same track. This may have been the Japanese Experiment Module called Kibo that was ejected from the ISS back in February but still remains in orbit.
To find out where and when to look for the station, go to Heavens Above and select your city by clicking on the blue Change your observing location and other settings link. Then return to the home page and click on the blue ISS link to see a 10-day table of passes that includes time, direction, brightness and altitude. Ten degrees (10°) of altitude is equal to one fist held at arm's length against the sky. The higher the negative number in the brightness column, the brighter the pass. Click on any pass time and a map will appear showing the station's path across the sky.
The map is a 2D version of the 3D sky. The outer edge is the horizon, and the center of the map is the overhead point. All times shown are local times on the 24-hour clock for your location, so 18:30 = 6:30 p.m. and 19:15 = 7:15 p.m. You can also get a list of customized passes and alerts by downloading the free ISS Spotter app for iPhone and ISS Detector for Android devices.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.