Taking Multiple Sun Shots

[barnakiel]

Originally Posted by sneuman

How can you take only one noonsight? You need several to determine local noon - which is the whole point, is it not? Or do you just take one sight, get an LOP and cross it with your DR position?

When the sun approaches the peak I simply follow it and adjust the sextant. When it starts falling I simply stop adjusting and this max value is how high the sun got at noon. Sure - you can get unlucky and there may be a cloud. Taking sights before and after is OK, it will tell you when the noon was but not how high the sun got (which is what you actually want to learn at noon).

You would be very lucky to have the LOP cross with the DR. The DR is a point and it will be on your LOP only if the intercept is zero. This is not very often.

The noon LOP gives you your latitude and you do not need the DR to calculate this LOP, and that's the beauty of it. This noon LOP you can use in conjunction with any other LOP to get the fix - you can use another celestial LOP (e.g. a morning or evening one) or you can use a transit or really just about anything else, including say a line of soundings. Or else you can elect to sail the latitude till you get to your destination.

Basic celestial is very, very simple and once you understand why and what you are looking for, you can toss and turn the possibilities to get what you want.

b.

[alan2]

Originally Posted by sneuman

How can you take only one noonsight? You need several to determine local noon - which is the whole point, is it not? Or do you just take one sight, get an LOP and cross it with your DR position?

I'm an amateur at celestial, so correct me if I'm wrong - but when I took noonsights in practice, I began taking a series just before and just after my estimated local noon to determine the apogee of the sun and therefore your noon fix.

Re the above and other posts concerning taking one or multiple sun shots for the noon sight (meridian transit??), the following might be of interest, assuming that I'm not misunderstanding the entire business. BTW, re taking a series of shots, before and after meridian passage and averaging, I have yet to get that to work, possibly because I’ve found something to do wrong, and continue doing it.
Anyhow, looking at the daily pages of the Nautical Almanac, a small box in the lower right hand corner, note part called SUN, and 3 days mentioned. Taking the date of the day you use. note Mer. Passage listed, this is time at "standard meridian". Correct this for your longitude, let's say that the "standard meridian is 75 degrees, your longitude is 77 degrees West, you would add 8 minutes to the indicated time, see Conversion of Arc to Time table in NA. Then shoot the sun at that time.

You might try using the USPS meridian transit form, downloadable from their web site, usps.org, as I recall.

[sneuman]

I was conflating "multiple sights" with multiple shots, or as has been mentioned simply keeping the sun on the horizon until its max alt.

[boasun]

For each sun line I only take the sight once... Same for the LAN. Two to three sun lines in the AM, the LAN and two to three Sun lines in the PM.... A round of stars in the early twilight of the Morn & evening.
Don't have a cloud splitter for my sextant, so can't take the sights on over casted days... Do a lot of DR'n during those days.

Of course with the modern electronics, people seem to think that they are Magellan...But when those battery powered toys go belly up... Do they have a sextant? Can they make an emergency sextant? Or are they Royally Screwed??

[nigel1]

Originally Posted by boasun

Of course with the modern electronics, people seem to think that they are Magellan...But when those battery powered toys go belly up... Do they have a sextant? Can they make an emergency sextant? Or are they Royally Screwed??

Pass the popcorn over

[alan2]

seaduction wrote:

I think you meant: "non illegitimi carborundum." Sorry but I had to sit through 3 years of the stuff in jr. high school.
__________________

I think it's about 6 of one, a half dozen of the other.

[Tia Bu]

Originally Posted by sneuman

I was conflating "multiple sights" with multiple shots, or as has been mentioned simply keeping the sun on the horizon until its max alt.

We're really talking about two different things here. For a noon site (or "meridian passage"), you keep taking sights starting, say, 15 minutes before the time you calculate the sun will pass your meridian. When the sun stops rising and sort of hangs at one altitude for a while before beginning to drop, you record that highest altitude. Since this involves several sights, you usually get one good, consistent reading for the highest altitude.

For all other sun lines, a small boat (really anything smaller than a ship) is moving around so much in a seaway that it's hard to get accurate measurements. So most navigators take several sights one after the other (over a period of 5-10 minutes or so) and then analyze them or average them to choose the most accurate measurement they can use to calculate one line of position.

Because of that motion, I found it impossible to take reliable star or planet sights on a small boat. But some bad mamba jambas apparently can do it.

[NevP]

Originally Posted by boasun

Can they make an emergency sextant?

On that topic. I came across an article some time ago about how to make a prism out of 3 small pieces of glass glued together in a wedge that would allow you to take 3 timed sights at fixed elevations as the sun was setting or rising. Was an interesting read but it was a couple of years ago and can't for the life of me find it again now.
Does anyone know what I'm talking about or if there are any articles online describing it?
From memory the guy that develloped the idea was into minimalist sailing across oceans in very tiny boats.

[NevP]

I have just answered my own question after some extended searching.
Sven Yrvind of Sweden developed the "Bris Sextant". Not a true sextant but an elegantly simple device for taking sun sights.
http://www.yrvind.com/various.html (bottom of page)
It appears that they were manufactured and sold at some stage but are no longer in production. Might be an interesting exercise to have a go at making one. Has anyone seen or used one?

Bris minisextant - YouTube

[gjordan]

I learned Celestial sitting on the deck of a trimaran in Mexico, from an ex Navy Quartermaster. I had tried a course before we left and was baffled. Once the ex Navy guy said, Hey, its not that tough to do, I was shown how to do a LOP and it wasnt that tough. I had a little book called Ten Easy Steps to Celestial Navigation, which showed me how to do the rest. We left Mazatlan for the Revillagigedos Island (about 300 miles) with the idea that if we found them we were doing alright. The islands appeared just where they should have, so we spent 3 days there and then off for the Marquesas. On the passage we usually did a round of 3 stars in the evening with 3 shots for each star and averaged them. The triangles got smaller as we got better, and we tried other shots. By the end of the passage we were doing sun, stars and the moon. The moon was said to be so hard, but in reality it was only a step or 2 more than a star. About 1/3 of the month you can cross the sun and the moon in the mid morning or afternoon. This was done with a $25 plastic sextant that I ordered from England. The $15 Davis was a back up but didnt have a scope so it was only useful for sun shots. We didnt have an electrical system in our boat so we used a Radio Shack Time Cube and a stop watch for time. We also had a very reliable battery powered depth sounder for land falls. I never did take a noon site. I tried a few times and found them difficult and more guess work than precision. I later bought a nice 3/4 size Tamaya, but I was doing deliveries by then and they all had Satnav or Loran(yes this was many years ago). I still carried the sextant but almost never used it. When I get free of the land in a year or so, I will brush up on the Celestial and always have it available. As I said in another thread(if it is electrical, it will sooner or later break down for you)._____Grant.

[James S]

Originally Posted by NevP

I have just answered my own question after some extended searching.
Sven Yrvind of Sweden developed the "Bris Sextant". Not a true sextant but an elegantly simple device for taking sun sights.
http://www.yrvind.com/various.html (bottom of page)
It appears that they were manufactured and sold at some stage but are no longer in production. Might be an interesting exercise to have a go at making one. Has anyone seen or used one?

Its an interesting device...I've never heard of it.
The "fixed angle" idea makes me wonder what might be done with a couple prisms.

[alan2]

gjordan's comments make interesting reading, and have that ring of practically re the ever present possibility of "electrical mishaps".

That having been said, using a sextant, reducing and plotting the sights strikes me as a whole lot more rewarding than simply reading numbers off some piece of electronic magic, that might well to "tits up" at the most inopportune moment.

Answere "from the box" are more accurate, as some will no doubt claim, however this is not to say that answers obtained other ways are unreliable.

[Adelie]

Originally Posted by James S

Its an interesting device...I've never heard of it.
The "fixed angle" idea makes me wonder what might be done with a couple prisms.

What are couple prisms?