A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2013 Apr 3, 12:50 -0700
Hi Bruce, you wrote:
"I'm looking for a convenient site(s) where I can measure dip angles with heights of eye between 150 to 550 ft."
I'm curious what your goal would be with these observations. The proportionality of dip to the square root of the altitude is not significantly in doubt for you, is it? The constant of proportionality, which would be 0.97' in average conditions (for height of eye in feet), is somewhat variable depending on atmospheric conditions. I would say that a range from 0.90 to 1.05 should be considered "typical" if not normal. But unless you're able to repeat these observations from many altitudes over and over again, you won't be able to get much sense of the degree of variability.
I, too, would like to find sites like the ones you've requested, even if just for a nice view. I didn't realize that Blue Hill Observatory was located on such a highland. I'm sure I've seen that set of hills on maps many times before. I just never realized how high up they were! The only other locations that I can think of in southern New England that are so high and have some degree of water view are the rocky hills right around New Haven, Connecticut. West Rock ridge has many locations over 400 feet high and some over 600. They would all be in view of well-defined landmarks in New Haven, so it should be possible to determine altitudes by standard surveying procedures. Sleeping Giant, a little north of New Haven, is higher still, but I understand that the sound is rarely visible. I realize that New Haven is probably too far for your interests, but if you're down that way for any other reason, it might work.
One other option: there's an observation tower on the aptly named "Tower Hill" near Narragansett, not far from here (41.4951 N,71.4558 W). That would get you a range from about 200 feet to 260 feet above sea level. There are wide horizon views to the southeast.
John Huth mentioned that you should observe angular locations of relatively distant objects closer than the horizon, and this is a really good idea. The variability in terrestrial refraction that makes dip variable also implies that objects miles away, but closer than the horizon, will shift up and down relative to each other when observed from the exact same location. It would be great to find a location with several fixed landmarks, like prominent buildings that are miles apart in distance but appear perfectly lined up under normal conditions from one location. Then as atmospheric conditions change, and the Earth effectively becomes larger or smaller, the effective curvature would change and the visual alignment would shift. It's something I've been hoping to photograph for years...
You also wrote:
"I'd like to do several "heights" on one day so the dip data would be recorded under more or less constant meteorological conditions."
That's a tough call. The main factor in this variability of dip is the temperature structure of the atmosphere, specifically the rate at which the air temperature rises or falls with altitude. This certainly can change during the course of just a few hours. If the dip is given by k(T2-T1)*sqrt(h) where I am writing k as a function of temperature difference (average value 0.97'), and if the temperature difference is changing during the course of a few hours, then we would have some time-dependent k(t)*sqrt(h) (maybe 0.75' at 10am, 0.90 at noon, and 0.97 at 2pm). And if you're making observations steadily coming downhill, then that would mean k(t) would appear to be a function of height: k(h)*sqrt(h), which could easily create the misleading impression that the dip is NOT proportional to sqrt(h) at all. So you might end up creating a very complicated problem that would not lead to any general conclusions.
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