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Why Does a Small Change in Height Cause the Full Moon to be Occulted by a Nearby Mountain?

-- | April 26, 2016

Question: I cannot find an answer to this conundrum anywhere on the Web, and it’s not at all a function of the famous Moon Illusion. Late last month (March), the full moon rose over a 7500-foot elevation ridgeline that’s located about 10 miles from my house patio viewing spot at 3500-foot elevation. While standing up I watched the top half of the moon clear the ridgeline, but upon sitting down in a chair I noticed the view of the moon had been reduced to a mere sliver.
I understand that refraction considerations might play into this, but really? to that great a degree? Essentially, the effect of a 3-foot vertical difference in my viewing height resulted in half the rising moon being obscured by the ridgeline. However, a tiny 3-foot difference distributed over 10 miles of horizontal distance seems to me as if it’d be way too small to result in any appreciable change of moon-viewing angle/perspective. What else could I be missing here? — Rick

Answer: I believe what you observe is due to your proximity to the mountain that is approximately 4000 feet above your point of observation and not due to refraction. Now, to cause the Moon to go from half-blocked to only slightly blocked by a nearby mountain, I would think that you would need to be closer than 10 miles, though.

Jeff Mangum

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