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Old 01-08-2020, 17:45   #31
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Re: wood ships , iron men

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Originally Posted by StuM View Post
Doh! That's what you get when thinking from a southern hemisphere perspective.



Ha! That reminds me of the time I bought a celestial navigation program (AstroNav by NavSoft, great program, BTW) and noticed that the diagram showing the illuminated fraction of the Moon was backwards. I emailed the developer to inform him of his "error", and he politely pointed out that in the Southern Hemisphere that's how it looks.
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Old 01-08-2020, 18:50   #32
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Re: wood ships , iron men

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Picky, picky, picky. My bad and what you posted is true; but completely missed the point of my post.

With out an ephemeris nothing else matters.
Not really, if you had a good clock and observed the local time when a star was at maximum altitude you could derive your longitude from the time difference.

Latitude can be worked out from first principals if you know the solistices.
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Old 02-08-2020, 04:35   #33
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Re: wood ships , iron men

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... But returning to a specific location such as one's home port was - I dare say - not an uncommon skill amongst pre-chronometer mariners. Otherwise I don't think sailing would have really taken off.
We used to go fishing beyond sight of land. After moving around from spot to spot, and drifting a lot, we didn't really know our starting location. We usually had a compass, but it wasn't much more help than just glancing at the position of the sun.

With that, we got close enough to see a landmark. Our best one was three hills which ran roughly North-South. If the big one was on the left, and they were evenly spaced, we were due East. If they were starting to line up, we were farther North. With that, we could adjust course a little. As we got closer, more landmarks would appear. By the time we got to our home harbor, we'd been steaming a straight line in for some time.

Granted, we weren't crossing oceans. And explorers back in the day did sometimes get blown off their DR course by storms and ended up seeing unfamiliar landmarks when approaching shore. This is why early charts always had profiles of the land, as they would first appear to an approaching sailor.
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Old 02-08-2020, 11:50   #34
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Re: wood ships , iron men

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Originally Posted by RaymondR View Post
Not really, if you had a good clock and observed the local time when a star was at maximum altitude you could derive your longitude from the time difference.

Latitude can be worked out from first principals if you know the solistices.
Please explain first principals.
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Old 02-08-2020, 15:15   #35
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Re: wood ships , iron men

The first principals in the derivation of latitude instance would be that an observation of the elevation of the sun when both observer and sun enjoy the same longitude compensated for seasonal factors may be used to derive the observers latitude.
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Old 02-08-2020, 15:38   #36
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Re: wood ships , iron men

Oh yeah, I knew that......
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Old 07-08-2020, 06:45   #37
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Re: wood ships , iron men

Read the book ‘Longitude’ - fascinating!
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Old 07-08-2020, 06:46   #38
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Re: wood ships , iron men

I have a Casio watch that lost 1 second in almost 2 years. The battery is dead now and it is in a drawer. I am afraid to fire it up again because I don’t want to find it is no longer perfect, like me say, and I don’t actually care what day it is anymore. I did a sun sight once that put me less than a 1,000 feet from where my GPS said I was.
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Old 07-08-2020, 07:04   #39
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Re: wood ships , iron men

Maybe you can't buy an hour-glass (they were 1/2 hour) which were corrected for time at sunrise.

You have beautifully encapsulated the problem in your own smart aleck remark. It is the other way around. When the sun comes up, you don't know where you are. Which also means that one hour earlier you didn't know where you were. Which means that maybe you had better have the lookout look down ahead of your vessel as soon as the light affords it!

At the end of a passage in my logs I always scrawl NBS for "navigate by sight". I then handle the vessel by what I see. One time in the Caymans I went back the next day to finish the log, and according to the brand new fancy equipment, I was anchored on the island. Not in the water, but on the island.

The GPS system and the pretty good charts that we have are a godsend, and still there are problems. Without knowing longitude to any degree of accuracy, there had to be a lot of NBS going on, and bold and courageous men were required, my friend.
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Old 07-08-2020, 07:07   #40
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Re: wood ships , iron men

I sleep more better. May be false though. I still get up and peer around at night and on windy nights I do it a lot.
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Old 07-08-2020, 07:32   #41
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Re: wood ships , iron men

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The wisdom is that clocks were needed for longitude. A uni prof of polar history tells me that early sailing ships were unable to return to a specified position. But hang on, that was before 1760 and since 1500 there were European colonies in Americas and Asia.

Maybe you can't buy an hour-glass (they were 1/2 hour) which were corrected for time at sunrise. But how real was the problem? One minute-time is about 17miles spin at equator, say 8 miles either direction. So a 20ft mast gives sight to a 20ft hill at about 10miles. The destination can only be ahead , not astern. Latitude is fairly definite. 5 minutes error is say 85miles spin. Running to a N-S coast at night is a worry but probably not usual , slacken off and wait for tomorrow.

I know nothing about the subject and would like any input about what I don't know .

To calculate longitude with a clock you need an accurate clock set to grenich mean time and then you take a reading at noon each day to determine the difference, each hour = 15 degrees. And that noon is not the noon on your ipad, it is the sun directly overhead. Difficult to measure accurately, esp on a moving vessel, but I think they used the masts as sights to determine when the sun was directly overhead.
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Old 07-08-2020, 08:10   #42
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Re: wood ships , iron men

Dead reckoning comes from deduced reckoning.

Back in 1970 I was heading south to the Med in a wooden ketch built in 1934. Teak on oak and copper clenched with pitch pine decks and spruce spars. Celestial navigation was the order of the day with my trusty Plath sextant, and a Walker log to tally the miles sailed.

As we exited the Channel the wind started to build from the NE and we were soon running under reduced canvas across the Bay of Biscay on a course that should take us well clear of Cape Finisterre. The wind built to gale force and on the second night we were pooped by a big following sea that left me at the wheel with only two masts sticking out of the water briefly. Low scudding cloud stopped any sun sights for four days. Eventually the wind dropped and the sun came out. I did a running fix and discovered that I was only about eight miles out on my DR position!
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Old 07-08-2020, 08:17   #43
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Re: wood ships , iron men

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Dead reckoning comes from deduced reckoning.

Back in 1970 I was heading south to the Med in a wooden ketch built in 1934. Teak on oak and copper clenched with pitch pine decks and spruce spars. Celestial navigation was the order of the day with my trusty Plath sextant, and a Walker log to tally the miles sailed.

As we exited the Channel the wind started to build from the NE and we were soon running under reduced canvas across the Bay of Biscay on a course that should take us well clear of Cape Finisterre. The wind built to gale force and on the second night we were pooped by a big following sea that left me at the wheel with only two masts sticking out of the water briefly. Low scudding cloud stopped any sun sights for four days. Eventually the wind dropped and the sun came out. I did a running fix and discovered that I was only about eight miles out on my DR position!

THAT is a great story! I recently made the decision to revert back to the old ways as my primary navigational system (with digital stuff and wind reports as my back up) thinking that the day may not be far off when you can't buy diesel for pleasure boats and the digital stuff doesn't work anymore. My boat is almost completely autonomous but relies on satellites for navigation......that could be a critical mistake, in fact I think we are all collectively making that critical mistake, relying on vulnerable systems for our life support.



But feel free to tell me 3-4 more stories and see if you can change my mind! :-)
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Old 07-08-2020, 08:35   #44
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Re: wood ships , iron men

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Dead reckoning comes from deduced reckoning.

....

I did a running fix and discovered that I was only about eight miles out on my DR position!
If you get lucky, dead reckoning will put you spot on but it's kind of like playing the lottery. If you win, it's great but can you reliably pick the right numbers every time? Can you even get 75% of the numbers right every time?
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Old 07-08-2020, 08:42   #45
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Re: wood ships , iron men

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It is my understanding that the pioneer in establishing ocean currents was Matthew Fontaine Maury of the US Navy. He analyzed the logbooks from thousands of voyages that were sent to the US government but before Maury basically ignored.
It's a relative thing...even before longitude, they knew about currents.The North Atlantic Gyre was known in simple terms long before but it was often a far more imperfect understanding. They might wander left or right looking for the warm waters of the Gulf Stream to speed them along but a good captain with both lat & long would be able aim for a known area where the stream existed on prior trips.
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