At 12:18 3-8-98 +0100, you wrote: -----Original Message/Oorspronkelijk bericht-------------- >Fellow Shadow Watchers, > Information boards are being prepared for a nearly-completed large dial on a hill near the Northumberland (UK) coast which is 94 metres above mean sea level at Latitude 55° 1' 38" North Longitude 1° 30' 16" West. > > The latest question is "How far away is the sea horizon?" School geography taught me that the earth is an 'oblate speroid' so I suppose the true distance varies slightly depending on the direction in which the observer is looking but so little as to be unimportant perhaps? The sea is only visible in a generally easterly direction. > >Can any list member supply the mean sea level radius of the earth at this location on which to base the necessary trig calculation plus any subtleties I may have overlooked as I don't have ready access to specialist reference material of this sort. > >With thanks in anticipation of any helpful response. > >Tony Moss > Tony, I saw already some nice formulas for the calculations. However your starting point is constant changing: your heigth is 94 meters above mean sea level you say. So that is only 4 times a day. All other moments your height above the actual water level will be more or less thanm the 94 meters! Each meter more or less changes the distance to the horizon by ca. 0.1 nautical miles. Is that heigth of 94 meters at ground level or is it the heigth of the eye of the observer? How much is the difference between high tide and low tide at that spot? In Britain the sea-levels are related to the Chart datum, which usually approximates to Mean Low Water Springs (which occurs roughly once a month). Normally a sailor doesn't have to take this into account because he floats on the water, but you have a fixed observationpoint on the land. I am afraid you have to make a few corrections to the 94 meters! The refraction is dependant on the airpressure and the temperature.
----------------------------------------------------------------- Thibaud Taudin-Chabot, home email: [EMAIL PROTECTED] (attachments max 1.2MB, in case of larger attachments contact me)
