According to round earth curvature drop-off, a distance of 275 miles (443km) would drop over 50,000 feet (15km). Pic Finestrelles is at 2.8km elevation and Pic Gaspard is at 3.8 km. With a curvature drop-off of 15km, Pic Gaspard would disappear behind the curve if earth were a ball. Yet from the photography below, both mountain peaks are visible, supporting the fact that the earth is not a sphere.
Earth Curve Calculator: https://dizzib.github.io/earth/curve-calc/?d0=275&h0=0&unit=imperial
By Marc Bret (Beyond Horizons team)
Camera Panasonic Lumix FZ72 @ ISO100 – 1/250
Lens Integrated Zoom @ 1200 mm – f./2.8
Date July 16th 2016
Simulation by Ulrich Deuschle.
https://beyondhorizons.eu/2016/08/03/pic-de-finestrelles-pic-gaspard-ecrins-443-km/