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I took a few pictures during the total solar eclipse of 2019. I noticed in my images that the "width" of the Moon is greater than the "height". I know that the Moon is oblate by about 0.0012 (wikipedia). However, I am noticing a greater effect (0.040).

The width of the dark disk of the Moon during totality is approximately 2454 pixels (though there is some blurring so the exact dimension is hard to tell).

The height of the dark disk is approximately 2434 pixels.

This results in a flatness of 0.04 (which is about 38 times greater than the value posted in wikipedia).

Any ideas why?

NOTE: The image of the Sun (with solar filter, no eclipse) appears to be circular. So, I don't think it is lens distortion.

Canon T6s, 1/50 shutter, 100iso, 1000mm Schmidt-Casegrain telescope with T-mount, no solar filter.

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If celestial objects are low in sky, you are viewing them through about 20 more miles of atmosphere. Light waves from celestial objects must pass through about 100 miles of atmosphere to reach our eye / camera. If objects are low in sky (near the horizon), light waves travel through an additional 20 miles of atmosphere. This extra air induces distortion as it acts to modify the actual position of objects. As an example, when the sun or the moon are seen low in the sky, we still see them when in fact they are actually below the horizon. The shape of these objects often appear distorted, you see the atmosphere acts like a lens and modifies the path of light rays.

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I would guess at one of three causes:

  • the lens has some distortion which is stretching things horizontally;
  • there was distortion due to the atmosphere which had the same effect;
  • one or more of pixels not being square in the camera or some image-processing trouble.

Of these I guess the third is unlikely. The first would be easy to test for by taking pairs of pictures under controlled conditions with the camera rotated by 90 degrees between pictures in each pair & then measuring them (of course you can also just take pictures of accurately-circular objects, but the rotating-the-camera trick removes any error due to the objects not being circular.

The second one I don't know how you can test for, or how likely it is, but perhaps fairly likely.

  • 1
    Eclipse occurred about 11 degrees above the horizon. It could definitely be "number 2" on your list. Thanks! – AeroMac Jul 25 at 22:21
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Celestial objects that are low in the sky (i.e., near the horizon) experience atmospheric refraction causing an apparent vertical compression. The eclipse occurred around 11 degrees above the horizon (near San Juan, Argentina, 2019).

Thanks to @agtoever for suggesting this answer in the comments. I'm just moving his response from the comments to the answer sections. If you post your answer here, then I'll delete mine so you can get the credit.

Flattened Sun link: https://www.atoptics.co.uk/atoptics/sunflat.htm

Atmospheric refraction: https://en.m.wikipedia.org/wiki/Atmospheric_refraction

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