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Now, the atmosphere at mean sea level is 40 times denser than water
The density of water is 1 gram per cc, or 1 kg per litre, or 1 tonne per cubic meter. The weight of 1 cubic meter of dry air at sea level, at 15 deg C, is approximately 1.226 kg, or less than 1/800th the density of water.
The atmospheric pressure at sea level is approximately 101 kPa, but pressure is not the same as density. The weight of a column of "atmosphere" with a base area of 1 square centimeter is approximately 1 kg, but I'm not sure what you mean by atmosphere is 40 times denser than water?
I can recall my physics master at school, sometime in the late 1950s, explaining that the moon looked bigger at moonrise because of the refraction of the denser air at just a few degrees of elevation, whereas when the moon is overhead, the light is striking the atmosphere at 90 degrees, and therefore there is no (or minimal) diffraction. However, I have heard that if you photograph the moon at moonrsie, or at its zenith, the size of the image is identical. That's another project to be undertaken sometime...........