Guide
Seeing the ISS and other satellites
You can see a satellite with your own eyes when it is lit by the Sun while you are in the dark. That narrows it to the hours around dusk and dawn.
Why only at dusk and dawn
NASA's Spot the Station page says all International Space Station sightings occur within a few hours before or after sunrise or sunset. A satellite shines by reflecting sunlight, so it has to be in sunlight while the ground below is dark enough to see it. The app marks a pass as visible when the satellite is sunlit and the Sun is more than 6 degrees below the horizon, the rule the Heavens-Above FAQ uses. Passes also need to rise at least 10 degrees above the horizon, which is our choice.
How positions are calculated
The app uses the SGP4 model, designed for NORAD element sets. The model's own report warns that feeding those sets into a different model degrades predictions. Positions are only as good as the age of the element set, so the app shows how old each one is. NASA says the ISS's trajectory data is updated about three times a week.
We do not have a verified figure for how quickly SGP4 predictions drift. Some sources give a few kilometres a day, but we could only find that on a secondary page, so we do not quote it.
Sources
- NASA Spot the Station. When the ISS can be seen
- Spacetrack Report No. 3. The SGP4 model used to follow satellites
- CelesTrak. Orbital element sets
- Heavens-Above FAQ. The visibility rule