ISS Visibility Tracking

What Is the International Space Station?

The International Space Station (ISS) is the largest human-made object in orbit. Spanning roughly the size of a football field, this orbital laboratory circles the Earth every 90 minutes at an altitude of approximately 250 miles (400 km). Since its first component launched in 1998, the ISS has been continuously occupied by rotating crews of astronauts conducting science experiments in microgravity.

What makes the ISS special for skywatchers is its size and reflectivity. Those massive solar panels reflect sunlight brilliantly, making the station one of the brightest objects visible in the night sky — often brighter than any star or planet. On a good pass, it's unmistakable: a steady, bright light gliding smoothly from horizon to horizon in just a few minutes.

When Can You See the ISS?

ISS visibility depends on a precise alignment of conditions. The station must be passing over your area, your sky must be dark (after sunset or before sunrise), and the ISS must be in sunlight so it reflects light down to you. This creates specific visibility windows — typically in the 1-3 hours after sunset or before sunrise.

During the middle of the night, the ISS passes through Earth's shadow and becomes invisible, even when it's directly overhead. And during the day, it's washed out by sunlight. That narrow window around twilight is when the geometry works in your favor.

Starglow calculates all of this automatically for your location, showing you upcoming visible passes with their exact timing, how long they'll last, and how bright they'll appear.

How to Spot the ISS

Spotting the ISS is one of the easiest and most rewarding skywatching activities. Here's how to make the most of it:

• Check Starglow for the next pass. The app shows the start time, direction, and predicted brightness. A pass rated as "bright" or magnitude -3 or better will be very easy to spot.
• Go outside 2-3 minutes early. Give your eyes a moment to adjust and find the correct part of the sky. The ISS moves fast, so being ready ahead of time matters.
• Look for a steady, moving light. The ISS doesn't blink or flash — it moves smoothly and steadily. If a light is blinking, it's an airplane. The ISS typically takes 3-5 minutes to cross the sky.
• Find an open area. Buildings and trees can block your view, especially when the ISS is low on the horizon. A spot with a clear view in the direction of the pass gives you the longest viewing window.
• Share the experience. ISS passes are a great way to introduce friends and family to skywatching. The wow factor of seeing a crewed space station fly overhead is universal.

Understanding ISS Pass Details

Starglow provides several pieces of information for each ISS pass:

• Brightness (magnitude) — Lower numbers mean brighter. A pass at magnitude -3.5 will be stunning; magnitude -1 is still easily visible.
• Duration — How long the ISS will be visible, from a few seconds for a brief appearance to over 5 minutes for a full overhead pass.
• Direction — Where the ISS will appear in the sky, where it reaches maximum elevation, and where it disappears.
• Maximum elevation — How high the ISS gets above the horizon. Passes above 40° are considered good; directly overhead (90°) passes are the best.