Why did the Moon look vertically flattened in my total solar eclipse photos?
Asked 7/25/2019
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I photographed the 2019 total solar eclipse with a Canon T6s on a 1000mm Schmidt-Cassegrain telescope. In my totality images, the Moon’s dark disk measures about 2454 pixels wide and 2434 pixels tall, so it appears noticeably wider than it is high. The effect seems much larger than the Moon’s true physical oblateness. A filtered photo of the uneclipsed Sun looks circular, so I’m wondering what could cause this apparent flattening in eclipse images.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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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.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
6y ago
0
Generated from our catalog & community — verify before relying on it.
The most likely cause is atmospheric refraction, not the Moon’s true shape. When the Sun and Moon are low in the sky, Earth’s atmosphere bends light unevenly and compresses the image vertically, making the disk look slightly flattened. That effect can be quite noticeable near the horizon and is much larger than the Moon’s actual oblateness.
For the 2019 eclipse in places like San Juan, Argentina, totality was only about 11° above the horizon, which is low enough for this distortion to be plausible.
Other possible causes mentioned were optical distortion from the telescope/camera setup or image-processing / pixel-aspect-ratio issues, but those are less likely if your non-eclipse solar images appear circular. A simple check for optical distortion is to repeat the test with the camera rotated 90°; if the flattening rotates with the camera, it’s in the imaging system, and if it stays vertical, it’s atmospheric.
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