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Old 28-10-2011, 19:39   #1
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Taking Multiple Sun Shots

Hi, how many sextant shots do you guys take each time? I've started learning celestial navigation and at first didn't get a very good triangle when I plotted my LOPs. Yesterday I took lots of sights and put the times and altitudes into an Excel file to both average my shots and so I could plot them and see if my errors were systematic or not. I took late morning, noon, and early afternoon sun sights in my driveway and ended up with a pretty good fix. I'm attaching my plotting sheet. So I thought I would put that question out there. How many sights do you take each time? I'm also attaching a picture of the file in Excel that shows each shot. I also measure sextant index error by touching lower sun limb of the reflected image to the upper limb of the sun and reverse it. I do this 4-5 times to get my index error. I take the reading of each sight for index purposes and put it into the Excel file as well. I then use this average to get my Ha by subtrating this error from the average altitude reading and then divide in half.
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Old 28-10-2011, 20:24   #2
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Re: Taking Multiple Sun Shots

Think you are obscessing on something that's just not that accurate on the deck of a dancing boat in the tradewinds. I took my first actual sun shot at noon the second day out of San Diego on the way to the Marquesas. Hiva Oa magically appeared 20 some days later. Only took one shot at a time but was careful to rock the sun onto the horizon with my wife taking the time. It got easier with more practice but didn't take all that much practice to get decent at it. Made all of our landfalls pretty much spot on.
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Old 28-10-2011, 22:48   #3
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Three.
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Old 28-10-2011, 23:31   #4
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Re: Taking Multiple Sun Shots

Generally one per noon sight per day, when the sun shines at any rate.
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Old 28-10-2011, 23:44   #5
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Re: Taking Multiple Sun Shots

One at local noon, but if I have a handle on local noon and my DR is decent, one at 10 and 2 pm local time.

Having that handle is in large part reliant on how I've been doing prior to that!
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Old 28-10-2011, 23:56   #6
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I think Bajardine wants to know how many shots you do when you are taking a sight. So for your morning sight, he wants to know how many times you write down the time and respective altitude. He does 5, I do 3.

For a noon sight, it is obviously just one. He wouldn't ask a dumb question like that. Anyone who has ever taken a noon sight and anyone with a little grasp of geometry will know that the sun only hits it's maximum point ONCE each day.
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Old 29-10-2011, 00:05   #7
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Re: Taking Multiple Sun Shots

If you are talking how many separate sights, I took an LOP around 2 hours before and 2 hours after the noon shot, and a noon sight. That way I always had the DR position with the crossing LOP to give a relatively accurate idea of where we were just in case clouds obscured the noon sun. The LOP's work to give you a reasonably accurate position if you have a calibrated knotmeter/log and/or taff rail log and a good handle on your heading so you have a pretty accurate DR position for the LOP to cross.

We missed the noon sight maybe 3 days out of more than 2 months at sea cruising from San Diego to French Polynesia and back to Hawaii. Of course, it seemed we missed the noon sight the day before a landfall, without fail. Sailing to the US mainland from Hawaii, you may go a week or more not seeing the sun at all. Makes keeping an accurate DR important. Sure do love my gps's, now. Back in the good old days, we were always lost, it was just a matter of how lost.

Still took only one shot for each sight.
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Old 29-10-2011, 07:14   #8
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Re: Taking Multiple Sun Shots

As an apprentice and junior officer on cargo ships tramping around the world we used to take three sights in the morning over a 30 - 60 second period then take the average intercept. That position line would be run up to Noon for the daily noon position.

Personally I used to prefer doing the star sights at sunrise & sunset, to see how small a cross could be achieved, it was far more satisfying and seemed less of a flog...

These days celestial navigation is no longer a requirement for many administrations, something I find totally amazing. If the satellites are turned off it will revert to the old Flag of Convenience method, hit the land, launch the lifeboat and send someone ashore to see where you are!!!
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Old 29-10-2011, 07:16   #9
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Re: Taking Multiple Sun Shots

Greetings and welcome aboard the CF, bajardine.
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Old 29-10-2011, 07:55   #10
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Re: Taking Multiple Sun Shots

While serving as a navigator in the navy I would do what was referred to as the "Navigator's Day" (morning stars, noon LAN, and evening stars) each and every day. This was back in the day when they actually had sextants and wound chronometers on war ships. Many years later as a delivery captain I tried doing the same from the deck of a boat but quickly learned the many splendid benefits of GPS.

To answer your question though, I typically would take at least 3 shots. If the weather was bad or the horizon unclear, I might shoot quite a few and take the best 3.
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Old 29-10-2011, 08:20   #11
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Re: Taking Multiple Sun Shots

Please...don't listen to any "and that's why I use GPS" type comments. Keep this thread going. I have a David White sextant and need some positive reinforcement to learn how to use it. William F. Buckley's DVD just isn't enough.
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Old 29-10-2011, 08:26   #12
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Re: Taking Multiple Sun Shots

Good for you guys who see celestial as valuable, just in case. If the satellites were shut down there would be a lot of lost souls on the oceans.
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Old 29-10-2011, 08:44   #13
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Re: Taking Multiple Sun Shots

I enjoyed learning celestial in the 80's. Most of the enjoyment came from the astronomy aspect which I found fascinating (and still do). We all know the "star to steer her by" refers to the knowledge of stars and their azimuths which make steering on a dark, clear night much easier than watching a compass. It certainly explains why sunrise in June is much earlier on the Chesapeake Bay than it is in Florida.
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Old 29-10-2011, 09:04   #14
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Re: Taking Multiple Sun Shots

Bajardine, Celestial navigation is still a valuable skill when voyaging offshore. A noon site and an evening site are sufficient. The more you practice, the better you'll get. A good celestial fix should be within 2-3 miles of your actual postion which is why it is not useful for coastal navigation. An excellent study book I used in the 90's was Susan Howell's "Practical Celestial Navigation. It is a well-written, understandable study with good exercises and based upon cumulative knowledge from chapter to chapter. I always enjoyed Buckley's political commentaries but found his tome on celestial navigation unreadable. In today's world of GPS and radar, celestial navigation is just another complement to your navigational bag of tricks. Good luck and good sailing, Ron
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Old 29-10-2011, 10:06   #15
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Re: Taking Multiple Sun Shots

Celestial's valuable as long as batteries die.

It is also valuable because there's a load of math (although this can be greatly simplified to just addition and subtraction) and good technique to master. It is a routine activity, like morning sit-ups, that yields better results over time, and it is also a mental discipline that can be an important part of keeping your marbles rolling right on long trade-wind passages.*

*Ever notice how everyone stops chatting about a week in?
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