Author

Topic: HBS....approach to Pailolo Channel

Jon

posted December 07, 2020 01:42 PM
First post. I've been using Covid to complete the CN home study book and am now working through HBS (which is great fun). I'm attempting to remain pure, e.g., do all sight reductions by hand, not reading ahead or looking at the answers. One thing I'd like to try is to estimating when the peaks of both Molokai and Maui become visible from sea. If I were serving as a navigator (no experience), I might try to give the crew a sense of time range and bearing of those peaks. (I have NOAA chart 19004 of the HI Islands and was simply going to draw arcs out to sea based upon the calculation I mention below.) As I haven't read ahead in HBS, was this something that was done aboard SV Passages? Also, can anyone direct me to a formula that incorporates both the dip at sea as well as a single peak's height in determining the distance at which that peak becomes visible from sea? (I know there are formulas now on the internet, but I'd like to attempt by hand as if it were 1982!) Thank you.
From: Parallel 47



David Burch

posted December 07, 2020 09:44 PM
These formulas are of course in our own textbooks, but they were also readily available in 1982, and indeed 100 years before that.
You can check a 1980 volume of Bowditch, for example. The modern computed versions we have at www.starpath.com/calc are essentially what has been known for decades. The factor of 1.17 has varied a bit over time as one learns more about refraction and indeed the formula does depend on air temp and pressure when done best.
To see older presentations check this link to Lecky 1918 version (19th ed)
https://play.google.com/books/reader?id=dmbOAAAAMAAJ&hl=en&pg=GBS.PA694
but this was essentially the same in the first ed from 1881.The actual methods date from Euclid's invention of geometry in ~250 BC. So this is old stuff.
Our best presentation of all sextant piloting is this book: How to use Plastic Sextants — with Applications to Metal Sextants and a Review of Sextant Piloting. This gives you the formulas to work by hand.
We put this in this book because the cheapest plastic sextant is plenty good for most sextant piloting. The only exception is when using very small angles (<2º or so) for long distance off.
And to answer part 1 of your question. The formulas were all in hand on the boat at that time, but not often used because we went where we intended to go. And on those approach charts the circles of fixed distances off were already plotted on the chart before we left. Indeed of the many times I have done that voyage the charts all show these plots. The primary ones being those that marked the visible range to the lights whose heights are all well known. An obvious target in the days before GPS would be toward a region where the visibility of two major lights overlapped.
From: Starpath, Seattle, WA


