Dave Siemens wrote:
I haven't tried to check the math, but I have stood on the sea shore on a clear day and seen just the tops of the masts of a vessel, or seen a vessel sail over the horizon. To what extent does the refraction of the atmosphere affect this?
Dave (ASA)
A lot! Since we moved to the North Shore Boston three years ago, I've been fascinated by the visual appearance of ships, lighthouses, the South Shore of Boston, the Boston Harbor Islands, etc. as observed looking across Massachusetts Bay. One of my reference points has become a feature known on the nautical maps as "Halfway Rock." Depending on meteorological conditions, it sometimes appears to rest precisely on the horizon, sometimes floating well above the horizon, other times the water and the horizon are easily visible behind and above the island, still other times the edges are floating and seemingly curled up, and every position in between. Similar effects are seen for all the other objects.
I suppose it's been only about a century or so that this effect has been understand to be refraction of electromagnetic waves through the temperature differential of the air near the surface, just like the desert mirages. With more experimental observation, I suspect I could learn to calibrate the temperature differential between the air and the water based on this effect.
The net is that this effect is so strong and variable, it really dominates. With no other knowledge or background, I'm not sure that an argument for the sphericity of the earth would be very obvious or compelling, considering solely the height of a distant ship.
Randy
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Received on Thu Aug 21 14:04:43 2008
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