Not a war, just a sextant question.

[chrisncate]

Originally Posted by hellosailor

chrisncate-
I'm not sure if you got the direct answer to this along the way...but actually "how the heck does one do celestial nav by shooting sites of other bodies - stars, the moon... ?" Shooting anything with the sextant doesn't really do anything by itself. The sextant would be a lousy nutcracker, without "sight reduction tables".
The easiest way to see that is with a real globe and a piece of string. The tables tell you exactly where some object (planet, moon, star) is above the surface of the earth as some specific time. Sort of a "local noon" for each object at every possible time. If you were at that location, the object would be dead above you, 90 degrees above the horizon. If it is at any other angle...that just means you are not directly under it. You are offset from that position by xxx miles, and by using spherical trigonometry (which seems perfeclty useless back in grade school) you can work backwards saying "if the angle is x, then I am y miles away from that spot." Now you draw a big cicrle on your globe, you know you are somewhere on that circle y miles away from that spot.
Do it again with a second body, and the two circles should intersect in two places. Do it a third time, and all three should intersect more or less in only one place. (Realistically, a sloppy triangular splotch not a pure intersection.)
Which is how a navigator says "We're here-ish."

If you don't mind a little slop, really, you can navigate with a globe. But it really helps put the whole thing in perspective.

And if you don't mind a little more "electronic reliance" a sight reduction calculator or program makes for much faster work than sight reduction tables, with possibly less chance for error. It all comes down to measuring angles, then marking them on charts. And without the sight reduction tables (based on incredible amounts of calculations down by professionals, one hopes) it doesn't work well at all.

Between all the great replies/resources posted here, I THINK (think) I might basically understand how it works now, and I'm really looking forward to trying it "in the field".. Regarding more electronics... I'm open to the calculator, as I'll probably need one no matter what (I am just plain horrible at math, and I'm not sure I could do the calculations without on in fact). I do plan on trying to learn it without the aid of a calculator though, so we'll see..

[PaulfromNWOnt]

I don't know if this older thread will pick up any traffic, but a friend directed me to an iphone/ipad app that contains a "sextant", as well as a multitude of nav tools. It's called "Spyglass", and a google search for spyglass sextant will turn up information on this app.

It's a 2 dollar app, but lacking an ianything I can't try it out myself. The youtube videos make it seem like a very versatile little app.

Anyone want to try it out and get back to us?

[AdamLein]

Originally Posted by PaulfromNWOnt

I don't know if this older thread will pick up any traffic, but a friend directed me to an iphone/ipad app that contains a "sextant", as well as a multitude of nav tools. It's called "Spyglass", and a google search for spyglass sextant will turn up information on this app.

It's a 2 dollar app, but lacking an ianything I can't try it out myself. The youtube videos make it seem like a very versatile little app.

Anyone want to try it out and get back to us?

I'm a total sucker for this sort of thing but the app doesn't seem to be available on Android.

However I have played with an Android app, StarStruck Navigation, that I thought did a pretty good job. I got in touch with the developer to report a bug and request a couple of features and he was pretty quick in responding.

With StarStruck, you point your phone at (either align normal vector with, or much easier, sight along the edge) a star and click "record". Do it several times and you can then ask it do plot a fix on the map using the intercept method. Pretty straightforward.

Alternatively you can manually enter in altitudes.

Finally it acts as a crude starfinder for your location.

I have no commercial interest.

[Minnewaska]

I may give it a try for giggles. Since my iPad has an internal gps, I have to admit to being suspicious of the app. I guess I would feel better if it was wrong, but then........

[AdamLein]

M: fwiw Star Struck was pretty wrong. About as wrong as you'd expect, sighting along the edge of a phone at a star.

[Minnewaska]

Originally Posted by AdamLein

M: fwiw Star Struck was pretty wrong. About as wrong as you'd expect, sighting along the edge of a phone at a star.

Ok then, great. I think.....

I went to the app store and found several CN apps. Spyglass really isn't a CN app per se, but a nav app that has a CN feature. There were a few others, such as one from Navimatics (maker of Charts and Tides), that looked liked they could be more robust, but I lost interest in doing the research on which to try. It is cool to have an app that has perpetual lifetime sight tables, even if you took your own sightings.

[sneuman]

I'd be more surprised if it worked and was anywhere in the ballpark. A sextant is a very precise instrument for a reason. You can't just substitute sighting down the edge of a cell phone, no matter how "smart" that phone is.

[svzephyr44]

Suggestions:

1. Don't try and figure out the math. Just get a book that gives you a step by step worksheet that you can understand. You are trying to get a position. You don't need to understand celestial mechanics - just how to read the tables and plug in the numbers. If you do that often enough you might have an "ah ha" moment.
2. Use your GPS position as the DR (dead reckoning) start position at first. This eliminates the steps of using your log to figure out where you estimate you are from your last fix.
3. If possible find someone who knows how to do the math and have them walk you through the steps a few times.
4. Consider getting a celestial navigation computer. This solves all of your problems. I would bet, although I have not looked, that there might even be a web site that would take your sights and reduce them to Lines of Position (LOP's)
5. Relax - relax - relax. Those of us that do this do it for the psychic benefits. We actually enjoy it (hard to believe but true.) Don't go overboard (not an intended pun but until I remembered my harness I almost did once.) Some of the old navigation systems, like staring at the sun through a forked stick, are hazardous to your health, but this one is pretty safe.

A hint - all you are doing is taking three bearings on stars. Think of it as if you were in an enclosed harbor with a hand held compass and you were taking bearings on three landmarks. You would then transfer the reciprocal bearings to lines on your chart and where they cross is where you are. For reasons that are unimportant it is very difficult to measure the angle from 000 degrees of a star. Celestial navigation is actually a work around for that problem.

Don't worry about the quality of your sextant unless you actually get good at this and do it every day. Taking the sights is what introduces the errors. I don't know anyone who can take decent sights without doing it very frequently. If possible start on land or when you are at anchor to minimize the impact of trying to take a sight from a pitching boat. It also helps if you have a scribe to record the time and sight angles.

Most important have fun.

[PaulfromNWOnt]

Zephyr, I love the Zen approach to CN. I figure as long as I'm getting a consistent error I'm doing OK.

Minne, I also like the idea of perpetual tables. I suppose if the electronics go then we're in serious trouble by not having paper tables, but hey.... I'd be in trouble anyway. heh heh

[hellosailor]

Anyone who has used Google Sky Map can tell you that using a smartphone for anything astronmical can be a rude surprise. Mine kept saying "West" when I was looking SOUTH.

It turns out that this is a common error, the magnetic sensors in the smartphones need to be regularly recalibrated (you make voodoo figure 8 passes with the phone) in order to get anything near reality with them.