Navigators use the term "dip" to refer to the effect of height of eye above sea
level has on the altitude of a celestial body above the apparent horizon.
Similarly , the distance to horizon is greater at higher eye elevations. A
sailor on ship deck has an advantage in spotting land over one at sea level;
but the sailor in the crow's nest
sees land first.
Any nautical celestial navigation book will have a table. The Nautical Almanac
also.
The simple formula is D = 1.169 SQR(h) where D is distance to horizon in
nautical miles and h = height of observers eye in feet. [ If using meters the
constant is 2.07]
Use of this simplified formula is usually all that is needed. It requires of
course a clear day to the horizon, perhaps questionable in Great Britain :)
For greater accuracy try:
D= SQR( [2*r*h] / [6076.1 *B] )
That 'B' will be a beta in the books. Again: D is distance to the horizon
in nautical miles; r is the mean radius of the earth ( 3440.1 nautical miles) ;
h is the height of eye IN FEET; and B is a contant relating to terrestrial
refraction, 0.8279
Seems to me that B could vary depending on atmospheric conditions but I never
studied it to that detail. However, Bowditch, "The American Practical
Navigator" (a classic tome) states the " error in refraction is generally less
than that introduced by nonstandard atmospheric conditions"
Good Luck
DAVE
Tony Moss wrote:
> 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
