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28-10-2011, 18:39
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#1
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Registered User
Join Date: Sep 2011
Posts: 2
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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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28-10-2011, 19:24
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#2
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Moderator Emeritus
Join Date: Aug 2005
Location: Carlsbad, CA
Boat: 1976 Sabre 28-2
Posts: 7,505
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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.
__________________
Peter O.
'Ae'a, Pearson 35
'Ms American Pie', Sabre 28 Mark II
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28-10-2011, 21:48
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#3
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Registered User
Join Date: May 2010
Location: some ocean down under
Boat: Kelsall Suncat 40
Posts: 1,248
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Three.
__________________
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28-10-2011, 22:31
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#4
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Registered User
Join Date: Oct 2008
Location: Northern British Columbia, part of the time in Prince Rupert and part of the time on Moresby Island.
Boat: 50-ft steel Ketch
Posts: 1,884
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Re: Taking Multiple Sun Shots
Generally one per noon sight per day, when the sun shines at any rate.
__________________
'Tis evening on the moorland free,The starlit wave is still: Home is the sailor from the sea, The hunter from the hill.
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28-10-2011, 22:44
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#5
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Registered User
Join Date: May 2008
Location: Nova Scotia until Spring 2021
Boat: Custom 41' Steel Pilothouse Cutter
Posts: 4,976
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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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28-10-2011, 22:56
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#6
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Registered User
Join Date: May 2010
Location: some ocean down under
Boat: Kelsall Suncat 40
Posts: 1,248
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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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28-10-2011, 23:05
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#7
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Moderator Emeritus
Join Date: Aug 2005
Location: Carlsbad, CA
Boat: 1976 Sabre 28-2
Posts: 7,505
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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.
__________________
Peter O.
'Ae'a, Pearson 35
'Ms American Pie', Sabre 28 Mark II
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29-10-2011, 06:14
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#8
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Registered User
Join Date: Dec 2010
Location: PNW
Boat: Tayana 37
Posts: 42
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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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29-10-2011, 06:16
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#9
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Senior Cruiser
Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 49,139
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Re: Taking Multiple Sun Shots
Greetings and welcome aboard the CF, bajardine.
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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29-10-2011, 06:55
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#10
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Registered User
Join Date: Feb 2011
Location: Virginia
Boat: Shopping for a 30-ish cat
Posts: 418
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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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29-10-2011, 07:20
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#11
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Registered User
Join Date: Nov 2008
Location: Washington, DC
Boat: Columbia 41
Posts: 522
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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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29-10-2011, 07:26
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#12
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Moderator Emeritus
Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,406
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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.
__________________
David
Life begins where land ends.
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29-10-2011, 07:44
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#13
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Guest
Join Date: Nov 2010
Posts: 433
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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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29-10-2011, 08:04
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#14
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Registered User
Join Date: Jan 2011
Location: Now based on Florida's West coast
Boat: Pearson 34-II
Posts: 2,541
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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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29-10-2011, 09:06
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#15
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Registered User
Join Date: May 2008
Location: Nova Scotia until Spring 2021
Boat: Custom 41' Steel Pilothouse Cutter
Posts: 4,976
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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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