Interesting. At around #sunset, the #contrails of passenger #aeroplanes glows very brightly and is visible from very far out. I spotted an #airliner near the #horizon which was heading this way and very roughly measured the time it took to pass by. That was around 15 minutes. Assuming that it was cruising at about 1000 km/h, it means that I must have spotted it about 250 km in the distance! Even if my timing was grossly overestimated at it was a mere 10 minutes, that still means I saw it around 170 km away when I first spotted it.