Celestial Navigation...Tough to learn?

[Ross]

Craig, in a sailboat you are almost not moving most of the time

As another way of considering your noon fix. If you start taking altitude sights before local apparent noon(LAN) and note the time of each sight and continue after LAN when you get the same altitude AM and PM then LAN was half way between the times. The time of greatest altitude is a little difficult to get directly because the curve is rather flat for a few minutes. Greatest altitude will give Latitude, LAN will allow you to find you local hour angle and worked from Greenwich will yield Longitude.A noon fix will tell you if you are at the entrance to Miami or Daytona. It won't tell you if you are on the right hand edge of the channel.

 

[Peter]

Didereaux, you're right, I mis-stated time logic

I mis-wrote the explanation of the logic of the process. As others here have stated, the need is for the correct GMT time. Then you can figure the difference of the GMT time of your sight of Local Apparent Noon to detemine the the longitude.The link explains in some more detail.And you're right, I haven't done this myself, and I misstated the process. We all do that on occasion. Mea Culpa!

 

[Didereaux]

Craig, as to your ?...

Didereaux - a questionCraig,  See Ross' post just below this one, it pretty much sums up the situation. I don't mean to seem to be attacking anyone in particular, but on several things and cel-nav in particular, there is just so much uninformed BS that gets posted that any newcomer asking for legitimate information has only a slim chance of picking out the true stuff from the chaff.I fully understand that this is more or less unavoidable in this media, but arthritic temperment seems not to be moderating with age. I have no problems with people posting second hand stuff, I just wish they would thoroughly research what they say first, with the fact that they may be endangering or at the least causing someone else to forego something that could enhance their sailing.Top O' the Season to you and yours,didereauxCraig said:>Now don't attack me for this, since I am admittedly a novice at celestial navigation, and I'm just asking. I'm wondering if it is possible to determine both lat and long with a noonsite in following way:1. assume you're more or less stationary (you know you're near noon, as you can tell the Sun is near it's peak in the sky, so you heave to, or whatever)2. take a series of Sun altitude sights at 5 minute intervals, and note the altitudes along with the GMT from your chronometer.3. By looking at the data, you can see when the Sun's altitude was at it's peak and thus when it was "local noon". Since you wrote it down, you know the GMT time at that local noon.3. Couldn't you use the time differential between local noon and GMT to get the east/west location, and the Sun's peak altitude to get the north/south location?I understand this wouldn't be very practical for coastal navigation, but wouldn't it work to figure out generally where you are on the ocean in a pinch?Craig

 

[Peter]

D. -that's why its a "Forum"

If we all "thoroughly research what we say first", then, we'd never have any time to sail, cause with as many questions as we see on sailboatowners.com, we'd all be nothing but a bunch of librarians with our noses in the books 24/7.The purpose of a forum is to allow lots of people to ask ??s and to express their views, some mostly right, some occasionally wrong. The challenge to the questioner is exactly what you say - to separate the good info from the bad. (Maybe by the weight & logic of the responses?) But that's part of the challenge of the sea and of life in general. We all have opinions (Bruce or CQR? Chain or rope? Sea anchor or heave to?) based on own experiences (or lack thereof). The one who has to decide is the one who actually has to do it, and by sifting thru opinions and references with a critical attitude decide what (along with their own independent research) makes sense for them to use.Anyway, as to CelNav, I'll stick with my GPS for most day-to-day uses, but learn it in class, and practice skills for the context of tradition and historical interest.Now if I can only find a publication with modern tables of Lunar Distances...

 

[Ross]

Peter, not a chance

of finding lunar distances. But I did find an explanation of them the other day. Quite often you will find that I answer a question with several questions. I have seen posts that ask, "how long should my jib sheets be?" How in the hell do you write an answer to that kind of a question? So I ask a lot of questions for clarifiaction. Sometimes we get a good discussion going and sometimes they just go away. By the By, You can find lunar distance by GHA of the Moon and GHA of the planets for any minute of the day. But you have to do your own arithmatic.;o)

 

[Garry @ S/V TASHTEGO]

Lunar Distance Tables

You can get tables for the Lunar Distance method from Celestaire at http://www.celestaire.com/catalog/products/5510.html

 

[Peter]

Lunar distances-a joke!

C'mon guys!No matter what Celestaire says about getting screwed up timekeeping from lightning strikes, other than for the historical or astronomical interest, I can't imagine actually using LD calcs for a practical purpose in real life!But thanks for the link, Garry, it is an interesting concept!

 

[Craig522]

Thanks for the replies

Thanks for the info guys.As a history buff I really like trying to gain some understanding of what people used to have to go through in daily life. I also enjoy trying to be self-sufficient (but not to a "wacko" extreme). So I'll try to learn celetial nav for fun and use GPS as a primary system.As a fun experiment I may try to do a whole trip by celestial nav, while having my wife keep a separate log of where we REALLY are via GPS. So if a problem arises or I'm about to drive us onto an island in the middle of the night, she can step in!Any ideas where I could get a reasonably cheap sextant?Thanks,Craig

 

[Garry @ S/V TASHTEGO]

Lunatics

Of course lunars are not really practical but then again, the practical thing is to stay in home waters. When Joshua Slocum was sailing around the world alone his only time piece was a rusty tin clock with only an hour hand. When he was sailing toward Juan Fernandez (Robinson Crusoe's island) he took a lunar distance measurement to make sure he didn't miss the island and he was spot on. In the 19th century a chronometer was extraordinarily expensive and three (if one goes wrong the other two will show it) beyond the reach of private sailors and even many merchant ships. In those days every navigator could and did take lunars and the necessary tables were published in the almanac.I don't expect to ever be stuck with nothing but a one-handed tin alarm clock and a sextant but it sure is a lot of fun to try the old methods. I have two GPSs and eight sextants (four of them full size metal instruments, two plastic ones and two toys) but that's because I'm a celestial nut case.

 

[Ross]

Davis mark 3

sextant doesn't look like much but it is adequate for sun sights and only costs about thirty bucks. Just treat it kindly and it will serve you well.

 

[Scott]

Lunar Tables

I recall that Slocum claimed that he discovered errors in the lunar tables that he was somehow able to verify through mathematics. He was an accomplished sailor!

 

[Ross]

We must remember

In Slocums day all calculations were done in longhand on paper. The hand held calculator is newer than many of us. I saw my first one in 1973 it was four functions, no memory, LED display, and cost 120 USD. at a time when 4 dollars per hour was good pay.

 

[Didereaux]

So just what were lunars used for?... a link

For those uncertain of just what is being discussed in regards to lunars, and perhaps the one in a thousand who might ask themselves could I someday have a use for this...heres a link with a rather good overall commentary on lunars.http://www.irbs.com/lists/navigation/0212/0116.html