R.H. van Gent wrote: (and others as well have stated substantially the same thing regarding dip) > > The small angle between both horizons is known as the 'dip', and can be > approximated by the following relation found in almost any astronomical > or navigational handbook: > > dip (minutes of arc) = 0.97 sqrt(h[ft]) > > with 'h' denoting the height of the observers eye above sea level in > feet.
I think, that for beginners on the list, to avoid confusion, it would be well to state clearly that the above dip formula refers to the sea level as being one's local horizon of reference. and relates to one's vertical elevation of eyepoint above sea level. It must be pointed out that this formula will not be correct if one is on a horizontal plane at some considerable distance above sea level where the local horizon is also well above sea level. Put another way, someone living in Denver or other high plateau regions would have to calculate dip based upon height above the local horizontal plane, not referred to sea level. I am of course, referring to that portion of dip which is related to atmospheric refraction. Naturally, the eye level position above the horizontal will be the same. Even at "sea level" there would be differences in dip, as it has been determined that there are areas of the ocean's surface which are below mean sea level because of mass concentrations within the earth's crust or mantle. Tom McHugh [EMAIL PROTECTED]
