NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sadler
From: Nicol�s de Hilster
Date: 2008 Dec 23, 15:53 +0100
From: Nicol�s de Hilster
Date: 2008 Dec 23, 15:53 +0100
frankreed@HistoricalAtlas.net wrote: > Jeremy, you wrote: > "the effect of tides on dip seems very academic to me. " > > Oh HELL yeah! It's completely academic. But it's at least worth noting that folks do calculate these sorts of issues so that we can sort out trivial things from important things. And this tidal business is definitely trivial... Two centuries ago, people even tried to calculate the difference in dip in a north-south direction relative to an east-west direction due to the oblateness of the Earth. Can you imagine?? They had no sense of scale. > > On board of a vessel the whole issue is indeed academic, but let us not forget that Sadler's original idea was "...to observe the dip, accurately by theodolite, from fixed locations on the seashore ... using the height of the tide to give varying heights above sea-level.". Tidal ranges of more than 10 metres are not uncommon and can be very useful in such an experiment, as this will produce a difference in dip of more than 3 arc minutes (assuming the observer stands at 3m above high water and thus 13m above low water). Sadler only pointed out that it was difficult to determine the relation between the tide measured near the shore with the one measured at the horizon, an unknown perhaps not measurable with a sextant, but surely with a theodolite. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---