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Old 11-03-2013, 09:18   #16
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Re: Sextants and celestial navigation

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You do not have to be nuclear scientist to figure out that you can not take sights in pitch black when you can not se even your own nose let alone horizon.
There is a technique which can be used around moonrise or moonset with stars/planets that are on nearly the same azimuth as the moon. As the moon sets it gets bisected by the horizon, and you can use that to define your horizon for the shot. The brighter the moon the more difficult it is, so works best when the moon is actually a small crescent. Not something I'd recommend for a newbie, and not something I'd recommend for accuracy, but it can be an interesting exercise on a night watch.

I second the comment above about pre-working star sights. Whenever I do star sights (whether the most common in HO 249 or others) I always calculate the estimated azimuth and altitude ahead of time from our DR position. Then you can set the sextant, use the compass for the azimuth, and bring down the star and move on. When you are trying to get a true fix by pulling in three (or more) sights nearly simultaneously in a period of 5 or so minutes this is the only way to go. One other thing to remember, in the evening take the sites to the East first, the sky gets darker here sooner, and the horizon disappears a little sooner. Converse in the morning.
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Old 11-03-2013, 09:50   #17
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Re: Sextants and celestial navigation

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There is a technique which can be used around moonrise or moonset with stars/planets that are on nearly the same azimuth as the moon. As the moon sets it gets bisected by the horizon, and you can use that to define your horizon for the shot.
Yeas, i know that, and of course you can do that, but it is not worth even mentioning it, let alone to do it. I do not know even any book writing about that "procedure" taking sights . In my almost 40 years of navigation as a mate/master mariner (unlimited) to the cruiser I never met anyone using that "method" for star sights.
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Old 11-03-2013, 10:11   #18
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Re: Sextants and celestial navigation

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In my almost 40 years of navigation as a mate/master mariner (unlimited) to the cruiser I never met anyone using that "method" for star sights.
Well now you have.
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Old 11-03-2013, 10:11   #19
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Re: Sextants and celestial navigation

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"One issue not yet mentioned is the ability to see the horizon at night...is this only possible on well moon light nights"

---------------------------------------------


I was already ben warned. to be polite, but i can not resist.

You do not have to be nuclear scientist to figure out that you can not take sights in pitch black when you can not se even your own nose let alone horizon

how many sights you would be able to take waiting for moonlight to "light" your horizon. probably two a month
Firstly, was able to see the horizon with the naked eye. Secondly, my preconcieved notions had by me were that celestial navigation was done strictly at night so I waited for night. Lastly, next time head the warning.
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Old 11-03-2013, 10:16   #20
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Re: Sextants and celestial navigation

As always, this is the site to count on for great information from
knowledgeable folks eager to help. There is lots to go over from what's been said, referenced, etc. Discouragement gone, ready to get back at it. Thanks everyone.
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Old 11-03-2013, 10:39   #21
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Re: Sextants and celestial navigation

Here is a copy of the correction tables from the UK Nautical Almanac. Using this, you only need to apply height of eye (right hand column) and the totral correction from the left hand column (for sun sights). Make sure to note upper or lower limb, and the date period, (October to March and April to September)
Good luck, nice to see people taking up the black arts
Attached Files
File Type: pdf Sight Corrections.pdf (61.9 KB, 42 views)
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Old 11-03-2013, 11:45   #22
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Re: Sextants and celestial navigation

The refraction tables are the first few pages of the almanac. There's one ordinal for temperature, another for barometric pressure. Cross reference them for the correct latitude and you're all set. Actually, the correction is usually quite small and you could probably plot an LOP in the ballpark without it.

Grain of salt, however, since I am still learning this myself.
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Old 11-03-2013, 13:38   #23
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Re: Sextants and celestial navigation

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Actually, the correction is usually quite small and you could probably plot an LOP in the ballpark without it.
For the most part true. Be careful when the body you are observing is close to the horizon (less than 15 degrees) as refraction starts to increase rapidly. When the body is at the apparent horizon the refraction correction is ~34', which is significant.
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Old 11-03-2013, 14:03   #24
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Re: Sextants and celestial navigation

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Here is a copy of the correction tables from the UK Nautical Almanac.
Hi Nigel


Have you got Altitude correction tables for Moon and Polaris correction tables.

Thanks.
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Old 11-03-2013, 19:13   #25
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Re: Sextants and celestial navigation

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Hi Nigel


Have you got Altitude correction tables for Moon and Polaris correction tables.

Thanks.
If you can wait a few hours I'll scan them later in the day
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Old 11-03-2013, 19:30   #26
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Re: Sextants and celestial navigation

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If you can wait a few hours I'll scan them later in the day
thanks I would like to have it. I was/am looking/searching everywhere on the net, can not find. I need only those few pages but thought to buy almanac on the end if I can not get it for this summer cruising.

thanks again.
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Old 11-03-2013, 20:04   #27
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Re: Sextants and celestial navigation

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thanks I would like to have it. I was/am looking/searching everywhere on the net, can not find. I need only those few pages but thought to buy almanac on the end if I can not get it for this summer cruising.

thanks again.

For Polaris, you can get correction tables from the publication "Sight Reduction tables for Air navigation (selected stars), available for download here
Maritime Safety Information

The correction table is not as accurate as those published in the annual almanac, but close enough, remember to apply the year correction for precession and nutation (also in the reduction tables)
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Old 11-03-2013, 20:14   #28
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You may also want to check out this video from Jim Cook:

Good video, thanks
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Old 11-03-2013, 20:50   #29
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Re: Sextants and celestial navigation

I'm curious if anyone has a suggestion on celestial navigation software. My only experience with a sextant is finding Hawaii and home again a few years ago, and exclusively used dawn and dusk star sites reducing them with Merlin II on a Sharp programmable calculator. This was so simple and accurate I really regret losing the Sharp. We'll be heading back to Hawaii, hopefully year after next and would prefer to continue to use my sextant for navigation. Yes, I have GPS now, but you need something to do on long passages and celestial nav is fun. So, any recommendations from the experienced crew on this thread for software that will work as well as the Merlin software did?
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Old 11-03-2013, 23:58   #30
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Re: Sextants and celestial navigation

You can probably still get the calculator on eBay.
The software, if you no longer have it, you can conceivably download.

I just bought an even older calc - a Casio CQ-81 - that way, simply because its predecessor had been the BEST alarm / timer I'd ever owned.

I once snagged a berth as paid navigator on a big cruising yacht in the Pacific because they'd been relying on satnav, backed up by celestial, with a Sharp printing calculator and Merlin II software. (No sight reduction tables were on board - a fairly radical step, in the mid eighties - and if they had been, nobody knew how to use them)

Somewhere between NZ and Tahiti, the satnav went on the blink, and they ran out of thermal paper for the calculator. Owing to a feeble-minded program spec, it was not possible to redirect the output to the display, so although the calculator knew where it was, it was no longer able to share that information with the crew.

Then they got monstered by a tropical revolving storm .... on their eventual arrival, which was more by a mixture of good fortune and following contrails than anything else, phone calls were made, and the next flight to Papeete included me, my tables, my Polaroid sunnies, and my swimming togs (trunks).
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