Flat-Earthers compared the apparent size of the ISS and an airplane that appeared in photographs and then incorrectly concluded that the ISS could not be 410 km above us. Their mistake was equating altitude with distance and failing to account for both the ISS’ and the airplane’s downrange distance.

If an airplane is at an altitude of 12 km, it does not mean its distance from an observer is also 12 km. Its altitude and distance are equal only if the airplane is directly above the observer. If the plane is not directly above, then its distance is greater than its altitude.

In a meme circulated by flat-Earthers, they showed two pictures of the ISS & an airplane in front of the moon. Because it appears the airplane is only twice as large as the ISS, they incorrectly concluded that the ISS could not be 410 km above us.

From the picture & the known dimension of both objects, we can calculate their distance ratio.

- size = distance × angular size
- ISS angular size = 109 m / ISS distance
- airplane angular size = 71 m / airplane distance
- airplane angular size / ISS angular size = 44 / 22 = 2
- (71 m / airplane distance) / (109 m / ISS distance) = 2
- 71 m / airplane distance = 218 m / ISS distance
- airplane distance / ISS distance = 71 m / 218 m = 1:3

Therefore, if the ISS is 500 km away, then the airplane’s distance is about 167 km, which is possible if the airplane is close to the horizon, not straight above like what these flat-Earthers imagined. Using any curvature calculator, we can also determine that an airplane at a 12 km altitude is still visible above the horizon at that distance.