T
timbartlett
Guest
Just 'cos I'm feeling 'nitpicky'. what on earth is when it's at home....?
Armchair bluddy admirals! Nearly made me drop my toast! H'rumph!
I owe you a large pink gin, m'lady!
All you Astro Navigators out there
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I owe you a large pink gin, m'lady!
Lady Campanula
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binch
Well-Known Member
Sorry, that is not the way to get the mer pass.It is not accurate because the mement the sun is at greatest altitude is not usually the moment it is due south. Use the almanac and work it out to the second. The meridian sight is the simplest to do and plot and is the quickest onto the plot sheet.
If you do not know the exact time of mer pass , then take equal alts near noon, that is take one obs and then time the moment the sun is at the same altitude after noon. This also is easy to work out. This fix is easier at low latitudes.
If you do not know the exact time of mer pass , then take equal alts near noon, that is take one obs and then time the moment the sun is at the same altitude after noon. This also is easy to work out. This fix is easier at low latitudes.
binch
Well-Known Member
plotting sheet
As a pro with over 5,000 sights in my workbook, i recommend using a chart of the same lat and a scale of 1/80,000 or thereabouts. I have done this for over fifty years. I have had it condemned by captains as sloppy, but it's simpler and removes a possible source of error. I did not use it when teaching at the R.N. school of nav.
Another thing, it is rare for a pro not to be able to get a sun except in very heavy cloud.
Work in advance a relationship between your DR and predictions of sun altitude and azi for say 10 min intervals. Easy to do with tables like HO214. Give your mate the deck watch, and sit down, set the appropriate alt on the sext, fit the star telescope (3.5 mag) and point in the expected direction. You will have to wait, adjusting the set altitude from time to time, and then it is almost always the case that through the telescope you will see a hazy sun.
Shoot both the upper limb and the lower, average the two and apply the sun's semi diameter from the almanac and you will have a pretty good shot of the sun's lower limb.
This I have had to do quite often during the monsoon for example and when the right landfall is essential after several days without a sight.
Try it.
As a pro with over 5,000 sights in my workbook, i recommend using a chart of the same lat and a scale of 1/80,000 or thereabouts. I have done this for over fifty years. I have had it condemned by captains as sloppy, but it's simpler and removes a possible source of error. I did not use it when teaching at the R.N. school of nav.
Another thing, it is rare for a pro not to be able to get a sun except in very heavy cloud.
Work in advance a relationship between your DR and predictions of sun altitude and azi for say 10 min intervals. Easy to do with tables like HO214. Give your mate the deck watch, and sit down, set the appropriate alt on the sext, fit the star telescope (3.5 mag) and point in the expected direction. You will have to wait, adjusting the set altitude from time to time, and then it is almost always the case that through the telescope you will see a hazy sun.
Shoot both the upper limb and the lower, average the two and apply the sun's semi diameter from the almanac and you will have a pretty good shot of the sun's lower limb.
This I have had to do quite often during the monsoon for example and when the right landfall is essential after several days without a sight.
Try it.
binch
Well-Known Member
PS
I accept our Antarctic Friend's caveat about latitude scale in high latitudes. In the White Sea for example, one has to use the pukka method, but plotting on a local mercator works well for latitudes up to 55. Only a masochist navigates yachts above that.
I accept our Antarctic Friend's caveat about latitude scale in high latitudes. In the White Sea for example, one has to use the pukka method, but plotting on a local mercator works well for latitudes up to 55. Only a masochist navigates yachts above that.
Talbot
Well-Known Member
Havent bothered reading the wiki, but do know that David Lewis actually navigated by the polynesian method from Polynesia to New Zealand , all described at some length in his book on the first cruising catamaran circumnavigation in the early 60s I dont remember the xact title, but Amazon will bring up the details - may have been "daughters of the wind", but could have been oe of the others.