Last night (July 13-14) the International Space Station passed over my house five times between 10:05 p.m. and 4:35 a.m. I wish I could have stayed up to see each occasion but I had to clock out after the third at 1:18 a.m.. Otherwise I'd be sleeping right now instead of writing.

These two earlier ISS passes from July 13-14 are separated by about 90 minutes. Times are shown at lower left. The path the station takes varies at each pass because of Earth's rotation. (Bob King)
These two earlier ISS passes from July 13-14 are separated by about 90 minutes. Times are shown at lower left. The path the station takes varies at each pass because of Earth's rotation. (Bob King)

The space station has entered what NASA calls the high beta-angle season, when it remains in sunlight around its entire orbit. For observers in the northern hemisphere this means we get to see successive passes throughout the night starting at dusk and ending at dawn. That's once about every 90 minutes, the time it takes the space station to orbit the Earth.

The number of flybys varies from 3-5 per night depending on location. So does the path the ISS takes across the sky due to Earth's rotation. During those 90 minutes, the Earth continues to spin, so when it's time for the next pass we see the ISS at a different point along its orbit. On the first pass yesterday night, the space station tracked low in the southern sky, the second appeared nearly overhead, and the third arced across the northern sky.

The space station's steeply inclined orbit makes it visible from many locations on Earth. The Hubble Space Telescope's lesser inclination limits its visibility to the southern half of the U.S. south to mid-southern hemisphere latitudes. (Gary Meader)
The space station's steeply inclined orbit makes it visible from many locations on Earth. The Hubble Space Telescope's lesser inclination limits its visibility to the southern half of the U.S. south to mid-southern hemisphere latitudes. (Gary Meader)

Newsletter signup for email alerts

Because the space station's orbit is tilted 51° to Earth's equator — a steep angle! — it's visible from latitude 60° north to 60° south, where 99 percent of us live. That angle combined with the sunward tilt of Earth's axis during summer means a couple weeks of 24-hour sunshine for the ISS astronauts. Sounds like a day at the beach, but it's not. The extra heating caused by so much sun exposure can be a hazard, with temperatures pinned at a steady 250° F (121° C) on station's exterior.

To stay cool the ISS uses heat exchangers to dump excess heat. Astronauts also feather the solar arrays to reduce heat build-up. Without its cooling system they would either fry or freeze — when the station passes into Earth's shadow the exterior temperature plummets to -250° F (-157° C).

The ISS looks like a bright star moving west to east across the sky. When near the horizon it appears to travel more slowly because it's much farther away. When overhead, there's no horizontal distance between you and the station, just 250 vertical miles (400 km), so it appears to move faster.

This is a frame grab of the live ISS stream made on July 14, 2021 when the station crossed over the Atlantic Ocean near the southwest coast of Africa. (NASA)
This is a frame grab of the live ISS stream made on July 14, 2021 when the station crossed over the Atlantic Ocean near the southwest coast of Africa. (NASA)

The crew is currently studying nanoparticles and microscopic animals including those cute, virtually indestructible creatures called tardigrades. On July 29 a new Russian science module station named Nauka will dock with the station. Check out the space station's live streaming video of the Earth to get a feel for what's out the window. It includes a map showing a live view of its orbital track so you'll know exactly where you're looking.

Here are several ways to find out when and where the International Space Station is visible:

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. 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.

All times shown are local times on the 24-hour clock for your location, so 22:30 = 10:30 p.m. and 2:15 = 2:15 a.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. Or you can sign up for alerts at NASA's Spotthestation site. The current window of visibility runs through about August 1st.

Clear skies!

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.