Celestial Nav Question for Expert

[michaelratinter]

Altitude does make a difference but, as stated above, it will be only a few miles, tan (ho) = 5000' (elevation)/(error in feet). I live at 5000 feet and use a pan of water for reflection. The only difficulty is ripples caused by light breezes. But what you need to practice is the calculations. If you are within 100 miles you probably did it right. Early Antarctic expeditions used a pan of Mercury. Davis sextant is fine. Have fun!

[roland stockham]

Given that you know your position accurately from a map (and GPS) you can answer you own question by comparing a celestial fix. If you are miles out its probably a error in fixing your position but if you get a consistent small deviation it may be an altitude error. Don't worry as it will be less than you get on a pitching deck!

[El Pinguino]

Originally Posted by Snowpetrel

Thanks Ping. Interesting. Now just got to work out what it means!

What it would appear to mean is that if you only take 'sights' of objects more than 20* above the horizon the change in the correction for refraction is negligible and can be ignored.

[knockabout]

you will need an altometer on a mountain for your height measurement so you put the right dip angle into your azimuth table.
you have already mentioned you altitude that's your height above the water = your dip angle. one of the best celestial navigators I have met so far is a retired gunnery sargent / had trouble understanding his formula's for working out his position / he mentioned it was easy just assume the boats the target from a known position

[El Pinguino]

Originally Posted by knockabout

you will need an altometer on a mountain for your height measurement so you put the right dip angle into your azimuth table.
you have already mentioned you altitude that's your height above the water = your dip angle. one of the best celestial navigators I have met so far is a retired gunnery sargent / had trouble understanding his formula's for working out his position / he mentioned it was easy just assume the boats the target from a known position

That won't work in High Peru ( or Colorado).... unless you factor on the altitude of the surrounding Alto Plano.....

[charliehows]

The great thing about cheap available gps is that you can experiment with celestial and have a ready and accurate check of the results. People like St. Hilaire came up with their nav methods by having such an intimate knowledge of their subject that they were able to play with the maths, a kind of virtual gps in their heads.
as for cheap davis sextants - I'd trust mine anywhere in the world, a really nice simple elegant piece of equipment - slocum's timepiece was a $1 alarm clock...

[rhubstuff]

I have a Celesticomp V for sale if anyone is interested....PM me.

[bletso]

With the Davis Mk 15 or 25 and an AH you should be able to get within a mile or two. If you take the time to zero everything out, even better than that.

[charliehows]

if you arent one of the lucky few who spend considerable amounts of time bobbing about on a wine dark sea, you're going to have trouble finding a convenient horizon to practice with. Thing is - the sextant is the least important, easiest part of the nav process anyway. Excuse me if this sounds like sacrilege but i'd recommend skipping that bit altogether in favour of getting a decent grasp of the math and calculation process - far and away the most complex part of the process, but easy to practice using invented readings. Also - using maths to reverse the processes is a great way to get a decent understanding of the whole thing - one of the saddest things about Donald Crowhursts misadventure was the incredible mathematical feat he pulled off by reversing the maths to load a whole logbook with fake sights to make it seem he'd circumnavigated to get to his final position before he went overboard.

[reed1v]

Originally Posted by charliehows

if you arent one of the lucky few who spend considerable amounts of time bobbing about on a wine dark sea, you're going to have trouble finding a convenient horizon to practice with. Thing is - the sextant is the least important, easiest part of the nav process anyway. Excuse me if this sounds like sacrilege but i'd recommend skipping that bit altogether in favour of getting a decent grasp of the math and calculation process - far and away the most complex part of the process, but easy to practice using invented readings. Also - using maths to reverse the processes is a great way to get a decent understanding of the whole thing - one of the saddest things about Donald Crowhursts misadventure was the incredible mathematical feat he pulled off by reversing the maths to load a whole logbook with fake sights to make it seem he'd circumnavigated to get to his final position before he went overboard.

Which is why the new crop of navigational software is so attractive. Look, no numbers to crunch!! In theory one could put together a computerized sextant that automatically calculates your position from star sights. Still have the old A.O. tables. What a pain. And nothing like doing one's lines of position to have then intersect someplace out in the desert while you are still surrounded by water. Happened to us off Morocco. Hey, honey, we are in the Atlas Mountains!!!

[charliehows]

have to agree, unfortunately. It's a moot point which level of magic black box you decide to accept. What are the odds of being marooned with (a) just a sextant; (b) just a sextant and watch; (c) sextant watch and almanac; (d) just a handheld gps; (e) just a satphone ; (f) all of the above, no batteries and no paddle.
Personally i love the exercise of mathematics, mechanics and the excuse to just gaze at the sky in wonder, as well as being master of the solution to a problem people struggled with for thousands of years. Like almost everything connected with this sailing addiction, i do it for fun.

[reed1v]

Originally Posted by charliehows

have to agree, unfortunately. It's a moot point which level of magic black box you decide to accept. What are the odds of being marooned with (a) just a sextant; (b) just a sextant and watch; (c) sextant watch and almanac; (d) just a handheld gps; (e) just a satphone ; (f) all of the above, no batteries and no paddle.
Personally i love the exercise of mathematics, mechanics and the excuse to just gaze at the sky in wonder, as well as being master of the solution to a problem people struggled with for thousands of years. Like almost everything connected with this sailing addiction, i do it for fun.

Well, you only need a good memory to navigate around the world. Star charts rule. Even a blind person decades ago crossed the atlantic ok, using wave swells as a guide. Another person sailed around the world without any navigation aids. Simple astronomy will guide you anywhere, clouds permitting. Or just get the old Raft Book and learn how to navigate by swells, cloud colors, and bird identifications.

[Adelie]

Originally Posted by knockabout

you will need an altometer on a mountain for your height measurement so you put the right dip angle into your azimuth table.
you have already mentioned you altitude that's your height above the water = your dip angle. one of the best celestial navigators I have met so far is a retired gunnery sargent / had trouble understanding his formula's for working out his position / he mentioned it was easy just assume the boats the target from a known position

Dip is not dependant on your height of eye above sea level, it is dependent upon the height of eye above the local water surface you are using for a horizon. In Colorado that would be a lake.

When using artificial horizons there is no dip correction.

Being at altitude will affect you final result (you have changed the radius of the theoretical sphere you are measuring angles for but the ratio of radii is like 4000:4001) but the error will be one the order of 10s or 100s of feet and the best accuracy you can count on is 0.5-1.0 nm with premium equipment, extra accuracy almanac and sight reduction using electronic calculation

Basically the error is insignificant.


A house is but a boat so poorly built and so firmly run aground you would never try to refloat it.

[Snowpetrel]

Originally Posted by El Pinguino

What it would appear to mean is that if you only take 'sights' of objects more than 20* above the horizon the change in the correction for refraction is negligible and can be ignored.

:thumbup: Thanks Ping, just took the time to study the file you linked to. Interesting, that coriolis correction had me beat for a while, but I guess a bubble sextant reads from the apparent direction of gravity, not the true direction. I enjoyed this http://www.oceannavigator.com/May-Ju...up-in-the-air/ those air navigators sure worked fast..

[jamhass]

Hears what I did to learn

1 take sights at a known location, use software to reduce the sights. This teaches you to use the sextant and sun/stars

Repeat until you get reliable results consistently

2. Now do your reductions using the almanac and tables
Again repeat until comfortable with the process.