This image is being spread on social media. This idea has been passed around before. Does it work? As far as I can tell it would give an reasonable estimate. The problem that is not mentioned though is that looking at the sun directly can cause eye damage (temporary or permanent) if done with the unaided eye.
This is why with a sextant (a navigational device using a similar principle) a filter is used to overcome the glare such as "shades" covering both index mirror and the horizon mirror designed to prevent eye damage.
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