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  • “My advice to you is to get a sextant”

    Posted by Jonathan on July 12th, 2020 (All posts by )

    This was said to me by someone who had watched the Shackleton documentary on Amazon video.

    I’m wondering if we could substitute an iphone?

     

    23 Responses to ““My advice to you is to get a sextant””

    1. Kenneth Mitchell Says:

      Substitute an iPhone? Alas, no. The sextant is nearly a standalone device. That, plus an accurate watch and a set of tables, and you can determine your position anywhere on the Earth, WITHOUT reference to anything else on the Earth.

      An iPhone can only tell you where you are with the assistance of a few dozen Global Positioning System satellites. When everything is working properly, it’s far more accurate than a sextant. But if a Carrington Event solar flare wipes out the GPS satellites, or if a Chinese EMP explosion does something similar – or when your iPhone battery dies – you’re up the creek without a paddle.

    2. Gavin Longmuir Says:

      To be pedantic, a sextant (plus a whole lot of individual skill and experience) can give our lost person his latitude — North/South position. To fix his East/West position, he would also need an accurate time-keeping device. See, for example, Sobel’s book “Longitude”.

      It is also worth remembering that most explorers are well-armed when they trek into the unknown.

    3. David Foster Says:

      To be even more pedantic, you can calculate longitude *without* an accurate timepiece if you employ the method known as Lunars. Complicated calculations plus you need to hold your sextant at weird angles to take the measurement, but it can be done, and apparently was even after the chronometer was invented, on ships that couldn’t afford the latest nav technology.

    4. David Foster Says:

      It should also be noted that GPS is easily jammable over a specific area. The FAA regularly releases Notices to Airmen about military testing operations that will make GPS unreliable over a given radius from the test site, which increases with altitude.

    5. Mike K Says:

      I gave my sextant to my son when sailing was over for me. When I was sailing to Hawaii, I took two sextants, one plastic in case something happened to the good one. The new Texas Instruments digital watch had just come out (1981). I used it as a chronometer. I had done a lot of sailing around California and along the Mexican coast down as far as Manzanillo. Loran C worked well near California but was useless going to Hawaii from the east. I don’t know why but it was well known. There was another system called Omega, and I had the system but it never worked very well.

      One problem with the sextant the year we went was that, once in the Trade Winds, the sky became overcast every day about sundown. Longitude was easy. I took a sight each day about mid afternoon. We already had a knot log so distance was secondary. The crucial fix was latitude and Hawaii is about 20 degrees north, just beneath the noon sun at that time of the year. Doing a latitude fix with the sun 90 degrees over head is not very accurate. In Mexican races, which are run in winter, the latitude was easy.

      We were doing pretty much a dead reckoning course but I needed a good latitude near the end as we came into Hawaii. Fortunately, a couple of nights before the landfall, I got a Polaris sight through a hole in the clouds. We hit right on the money at Makapuu Point.

      As it turned out, we missed winning overall by 9 minutes. Our time was 11 days 20 hours.

      Here is a clip from the movie I made.

    6. Mike K Says:

      I edited the movie later and added a sound track of popular songs that summer. It is color although that clip is faded. Those kids are now all in their 50s.

    7. Grurray Says:

      Shackelton also had a compass, and, due to the exceedingly rough seas, his navigator Frank Worsley had to be held steady by two shipmates when he took the readings. Worsley also drew upon his experience, intuition, raw instinct, and probably divine providence to come up with some impressive dead reckoning.

      The James Caird was one of the greatest open boat voyages because of the harrowing conditions, but for shear distance the voyage of William Bligh and his loyalists after the mutiny on the Bounty is tough to beat. With only a compass, a quadrant, a pocket watch, and many prayers they sailed 3500 miles through and around hostile natives and dangerous obstacles. Bligh’s image has been tarnished over the years because of the mainly fictionalized versions of the story. He was actually well respected by most of his crew and his superiors. We now know that the mutineers weren’t breaking any chains of oppression, but they were simply hedonists who caught an acute case of jungle fever.

    8. David Foster Says:

      One might think it is almost impossible for a ship to run around when it is equipped with GPS *and* radar *and* a depth sounder…but then one would be wrong.

      When Humans and Robots Communicate

    9. Mike K Says:

      Bligh’s image has been tarnished over the years because of the mainly fictionalized versions of the story. He was actually well respected by most of his crew and his superiors. We now know that the mutineers weren’t breaking any chains of oppression, but they were simply hedonists who caught an acute case of jungle fever.

      To some degree but Bligh was also typical of some British sea captains who were great floggers and tyrants. As I recall, Blight got into more trouble later as a tyrant.

      Seventeen years after the Bounty mutiny, on 13 August 1806, he was appointed Governor of New South Wales in Australia, with orders to clean up the corrupt rum trade of the New South Wales Corps. His actions directed against the trade resulted in the so-called Rum Rebellion, during which Bligh was placed under arrest on 26 January 1808 by the New South Wales Corps and deposed from his command, an act which the British Foreign Office later declared to be illegal.

      The guards at the penal colony in Australia were notoriously corrupt but Bligh was also no diplomat. The unofficial motto of the Royal Navy was “Rum, Buggery and the Lash.”

      I agree his epic small boat voyage is historic.

    10. Mike K Says:

      David, I had forgotten that post.

      On one occasion sailing at night in the fog, we were trying to round the east end of Catalina Island. We could not see 100 yards in front but started to hear machinery noise. I turned on the depth sounder and we were in 60 feet of water. Catalina is in very deep water, thousands of feet, and the bottom rises pretty close to the island so we tacked. Soon we were in deeper than 100 feet, the limit of the depth sounder, so tacked back. To make a long story short, we short tacked along the north side of the island without seeing it. There is a gravel pit on the north side near the east end that was the noise. Good thing it was there.

      My partner used to go far offshore fishing. He was going out of San Diego one time and his son and another guy were on watch.Tom and his wife were asleep in the bow bunk. They were going about 30 knots when they saw a small freighter ahead. They had it on radar but had the radar set on the wrong range. The freighter was much smaller and closer than they realized. They hit it at 30 knots. They were 50 miles offshore. Fortunately the boat did not sink but the bow was smashed back to the deck hatch. To get home they had to back the boat 50 miles to San Diego.

    11. Gavin Longmuir Says:

      David F: “… you can calculate longitude *without* an accurate timepiece if you employ the method known as Lunars.”

      Humans have tremendous ingenuity! I have never used that method, but I believe it requires tables of information on the predicted movements of the Moon in addition to the sextant readings. A sextant alone would not be enough for longitude.

      Interestingly, I read somewhere that standard practice in the early days of sailing across the Atlantic was for navigators deliberately to aim for a landfall some way north or south of the intended destination, with the offset being based on the expected uncertainty in position when reaching the other side. This enabled the navigator to answer the challenging question — ‘Land Ho! Do we turn the ship to port or starboard?’

    12. Grurray Says:

      To some degree but Bligh was also typical of some British sea captains who were great floggers and tyrants.”

      Things change. There’s no longer the daily tot, but some traditions are slower to catch up. The Navy only recently ended the punishment of 3 days in the brig with bread and water.

    13. John henry Says:

      Joshua Slocum sailed around the world 120 years ago with compass sextant and an a tin clock that had only an hour hand and didn’t work when he bought it.

      He had to boil it to get it to tell the hours.

      Sailing alone around the world

      Another great Slocum book is voyage of the liberdade.

      Shiprecked with his family in Brazil, he and his son (and former first mate of the wrecked Aquidneck) built a sailing canoe and, with wife and other children sailed to NY (Boston?)

      Fiddlesticks on Navigational aids, he was heard to mutter

      John Henry

    14. john Henry Says:

      A few years after Slocum’s famous solo circumnavigation, he set out again and was lost at sea.

      Speculation is the the hour hand fell off his tin clock.

      John Henry

    15. MCS Says:

      The standard “clock” until fairly modern times was the exact moment that Jupiter’s different moons would pass behind it (occultation). This required a fairly good telescope and, especially, a more stable platform than could be provided by a ship. Because of the distance, parallax wasn’t a problem and because there are 12 moons the observation period was manageable.

      Lunars substituted the Moon for Jupiter and various bright stars for the moons. Then, because it was seldom convenient to wait for the moon to actually pass in front of a star, the angular separation was measured. Actually, a fix required several stars. Then parallax and the changing apparent size of the Moon made the whole exercise very complicated.

      As Gavin pointed out, the marine chronometer was the high tech innovation of its day. Very accurate pendulum clocks had been available for some time but wouldn’t operate on ships. The development of pendulum clocks continued into the middle of the last century. The advent of electronics allowed them to measure the variation in the pendulum clock at the Naval Observatory from tidal distortion of the Earth. There is a lot of information at NIST.gov if you’re interested.

    16. OBloodyHell Says:

      Which is it? I assume the former:

      https://www.amazon.com/Shackletons-Captain-Craig-Parker/dp/B00ABNFTO8

      https://www.amazon.com/Chasing-Shackleton/dp/B00H8A5AK0

      :-D

    17. Christopher Says:

      The Apollo astronauts carried one on board. The viewing window had a mount for it.

    18. Alaska Paul Says:

      The sextant is a great tool about learning about stars and planets. You find that the stars and planets are your friends and guides. Without a timepiece, the sextant will determine latitude. Before the invention of the chronometer, early navigators would determine the latitude and beat down that latitude until they crossed the ocean and then run down the coast to their destination.

      If you are interested in a sextant, Davis Instrument makes an inexpensive instrument. You can also get an artificial horizon for land navigation, when you are not on a large body of water where you see the horizon as the surface of the ocean. A great place to get a beginner’s sextant, like the Davis is from Celestaire https://www.celestaire.com/

      The neat thing about celestial navigation is that it gets you tuned to your surroundings. I had a GPS in my plane for years, but it did wink out once, but I had my navigation and pilotage skills, so no big deal.

      I branched out into other things when flying. I wanted to fly my plane in the Canadian arctic, up by Resolute, where the Franklin expedition graves were at Latitude 75 deg N, 900 miles from the pole. The north magnetic pole is close so the magnetic compass is worthless, so you use the sun and other things for navigation. That got me into the astrocompass. I bought a 1943 Canadian Air Force surplus unit from Celestaire. There are still a few units available.

      After 27 years as a private pilot, I got into the whys and wherefores of polar navigation. I dug up information on exactly how Amundsen and Scott did their celestial navigation on the way to the south pole. Methods for that work are different and simplified when you get near the poles.

      All in all, it started with the sextant. I look up at the stars at night (when it is dark in Alaska) and see my twinkling friends, and think about my place in the world.

    19. David Foster Says:

      Alaska Paul…have you tried an aircraft-type bubble sextant? I see that Celestaire has one without, the averaging mechanism, which should be fine for training and messing around with, only it’s not lighted so not usable at night. They’ve also got reconditioned units *with* the averaging and lighting, but those are a lot more expensive.

    20. Mike K Says:

      Before the invention of the chronometer, early navigators would determine the latitude and beat down that latitude until they crossed the ocean and then run down the coast to their destination.

      This was called “Latitude Sailing”and it is what Columbus was doing. He had the Latitude right but his Longitude was off by about 180 degrees.

    21. Alaska Paul Says:

      David Foster—
      A bubble sextant worked well in large transports, knew an air force navigator who did it. Put the sextant lens tube through a port in the plane’s hull and there you go. Accelerations bring out circles of error of 30 nautical miles. In a small plane you do not have a chance unless you have an autopilot, and still it is sketchy.

      I had a friend who ferried twins from Oakland CA to Australia and NZ. I asked how he navigated to Hawaii. He used navaids out of Oakland till he was too far, then dead reckoning until it got light, then followed jet contrails on the SFO Honolulu route.

    22. MCS Says:

      Alaska Paul,
      My father flew aerial survey missions over the Amazon after WWII. Some was in Beach 18’s a lot was B17’s. He said he would listen for each cylinder on each engine fire on each revolution. I expect your friend would understand.

      How did he find Australia? It’s a bigger target but there’s an awful lot of empty, even after you hit land. Not as many flights either.

    23. Alaska Paul Says:

      MCS–

      I hear you on firing cylinders. I did 100 miles overwater in Canada. I heard every cylinder fire. Set you power at 65%, lean the mixture to 50 deg rich of peak EGT and do not touch anything. ;<}

      My friend did island hopping to non direction beacons (NDB). One time he was headed for Tarawa NDB and was waiting for it to come on but it didn't. He then called on the radio and the guy answered and was eating dinner. He scrambled to the generator shed, fired it up and turned on the NDB just before my friend was thinking of ditching.