In this one-minute time exposure made on Sept. 16 from Duluth, Minn., the International Space Station looks like a streak of light under Jupiter and Saturn before it disappears in Earth's shadow (left). The Milky Way, diluted by light pollution, is visible at right. Bob King
In this one-minute time exposure made on Sept. 16 from Duluth, Minn., the International Space Station looks like a streak of light under Jupiter and Saturn before it disappears in Earth's shadow (left). The Milky Way, diluted by light pollution, is visible at right. Bob King

Just a quick heads up. The International Space Station (ISS) will be making evening passes now through early October for northern hemisphere skywatchers. With earlier sunsets we'll see the station during convenient evening viewing hours. If you have children and COVID restrictions force you to remain at home, herd them outside for a look. What better introduction to outer space and space travel than watching the space station and its current crew of three speed across the night sky?

Washington DC (left) and Baltimore look like spiral galaxies in this photo from the ISS during a night pass. You can see that Washington primarily uses older technology, orange-colored sodium vapor lights while Baltimore has largely converted to the whiter LEDs. NASA
Washington DC (left) and Baltimore look like spiral galaxies in this photo from the ISS during a night pass. You can see that Washington primarily uses older technology, orange-colored sodium vapor lights while Baltimore has largely converted to the whiter LEDs. NASA

You might even get lucky and see the ISS pass near the bright Saturn-Jupiter duo like I did last night. The station is always bright and unmistakable, but that brightness varies depending on its horizontal distance from an observer. Currently, the station orbits the Earth at an altitude of 261 miles (420 km). When it passes overhead, it's 261 miles away.

You can find the distance to a relatively nearby object in the sky like an airplane or the space station if you know its altitude and horizontal distance. Just plunk those numbers into the Pythagorean Theorem(a2 + b2 = c2)and solve. Even though the Earth's surface is curved, a flat triangle is a close approximation.CC-BY-NC-SA-3.0
You can find the distance to a relatively nearby object in the sky like an airplane or the space station if you know its altitude and horizontal distance. Just plunk those numbers into the Pythagorean Theorem(a2 + b2 = c2)and solve. Even though the Earth's surface is curved, a flat triangle is a close approximation.CC-BY-NC-SA-3.0

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Overhead passes are the brightest because the ISS is closest. It easily exceeds Jupiter and nearly equals Venus, the most brilliant nighttime object besides the moon. The lower in the sky we view the station the farther away it is because you have to factor in its horizontal distance plus altitude to arrive at its true line-of-sight distance. The ISS may only be 261 miles up, but it can also be hundreds of miles away.

For instance, during last night's pass of Jupiter and Saturn, the station's line-of-sight distance was around 800 miles (1,300 km). That's why the ISS only reached magnitude -2.1, about as bright as Jupiter. No matter where you live, when the space station first "rises" in the western sky (it always travels west to east) its line-of-sight distance is more than 1,400 miles (2,300 km). Isn't it amazing you can see it from so far away? Its large size and high orbit are the reasons why.

The ISS Spotter app provides a live view of exactly where the space station is passing over the Earth, called its ground track.
The ISS Spotter app provides a live view of exactly where the space station is passing over the Earth, called its ground track.

You can easily find the distance to the space station during any pass for your location by going to Heavens Above. Under Configuration on the left side of the page, click the observing location and settings link and add your city. Return to the home page and click the blue ISS link for a list of passes. Click on the next pass and a map and timeline of the station's path will pop up. The times shown are local.

Scroll down to the bottom and you'll see distances in kilometers at three different points of its track. To convert to miles clickhere.

Phone apps are another good way of keeping track of the space station. They also include distances for selected spots along its path. Here are two:

I wrote a short tutorial on how to use these apps here.

September and October are beautiful months to be outside. Cooler nights put an end to nuisance mosquitos, and earlier sunsets mean you don't have to stay up so late to enjoy the stars. Wishing you clear skies!