Observing Earth’s Curvature on a Flight

Earth’s curvature is hard to observe from the surface. Even from the altitude of a commercial passenger flight, about 30000-40000 ft, Earth’s curvature is still too small to notice. However, under careful observation, it is not impossible to confirm the curve from the cruising altitude of a jet airliner.

Observing Earth’s curvature on a flight is easier by taking a picture of the horizon, then do a careful examination of the resulting photo. Use a DSLR/mirrorless camera with a rectilinear ultra-wide angle lens, like 16 mm on a full frame camera, or 10 mm on an APS-C camera. The wider the field of view, the more visible the curvature will appear. Our eyes have the visual attention of about 55°-65°. On the other hand, ultra-wide lenses can cover more than 100°.

The biggest challenge is that on such altitude, the horizon will appear blurred most of the time and it is hard to distinguish the surface from the sky. To alleviate the problem, we can do one of these:

  1. Increase the contrast of the image of a blurred horizon. This can be easy or difficult depending on the resulting image.
  2. Use an infrared or night vision camera. It can penetrate the haze and reveal the clearly defined horizon.
  3. Find the right time where the horizon appears sharp, like after sunset, or before sunrise.

Any lens has some distortions even with rectilinear ones. To minimize distortions, place the horizon on the center of the frame. There are also cameras that can correct distortions digitally. Others have the same function in post-processing, especially when the photo was taken using the RAW format.

Some flat-Earthers invented an “explanation” that airplane windows were deliberately designed to make horizon appear curved. While it is implausible aircraft designers would intentionally do such a thing, it is possible that the windows can introduce distortions. We can verify this while the airplane is still on the ground by observing known straight objects outside the plane.

During the observation, we can also observe the dip of the horizon, which is much easier than discerning the curvature. We can use an app like Dioptra (Android) or Theodolite (iOS) to do it.

To make it easier to discern the curvature, we can magnify the resulting photos vertically during the post-processing.