|
Author
|
Topic: Do tide condtions effect reliabillity of Celestial nav?
|
Mark Callahan
|
posted July 19, 2004 07:02 AM
I take my sights from shore looking out to the sea/sky horizon, I know Dip-Short is not an issue, just height-of-eye. If I want to shoot Polaris 3 nights in a row, would I have to do it at the same tide condition (hi-lo-slack)to get the same reading.
|
|
David Burch
|
posted July 19, 2004 03:59 PM
Tide height can indeed make a small difference if the tide range is large. It changes the height of eye. The dip correction is 0.97' x sq root(HE in feet), so you can figure the possible effect.
Note that in the newsletter somewhere we have used this factor to evaluate some sights published by Curising World. Maybe find a link to it in the article on plastic sextants in the sextant page of the accessories catalog.
------
in passing, may i put on the teacher's hat and do some nit picking on terminology! the tide has indeed a Hi and a Lo, but no "slack." Slack is a term related to currents, not tides. It is in general not a good policy to associate slack water with any particular stage of the tide height. It may be possible to make such an association in some cases, during some cycles, but there is rarely any general rule that can be carried from one place to another, and as one travels into areas with strong tidal currents, this type of reasoning or associations can lead to serious errors. The point is discussed and illustrated in some detail in our coastal nav course.
We see the consequences ofen in our training trips north to Alaska, where we confront tidal currents as large as 15 kts, and frequently 10 kts. Trying to guess the state of the currents based on the state of the tides in these conditions canlead one very seriously astray.
Very often in navigation we use the word tide, as in "what is the tide doing?" when we in fact want to know what the current is doing. What i am doing here is encouraging a tidy use of language so that we keep the terms separate and precise in all usages and thus are more prepared for those cases where it might make a big difference to safe, efficient navigation.
--david
From: Starpath, Seattle, WA
|
|
|
