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Old 10-09-2016, 15:59   #1
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Celestial Nav Question for Expert

Hi,
This is a question that I think can only be answered by somebody who is an expert at celestial navigation.

I'm currently on a consulting gig in Colorado for a couple months. During my spare time away from home I thought I'd teach myself celestial navigation as a hobby/interest and eventual backup to GPS. Also, I think it would be a fun thing to occupy time while at sea for days. I got a good book, Celestial Navigation by Burch. It's doing the trick at getting me started and I'm now eager to get a sextant and practice.

However, I'm currently in Colorado studying. My current elevation is 5400ft, just outside Denver. I am wondering if it is possible to take sights in practice at that elevation above sea level. Nowhere can I find if being over a mile above sea level will negate land based practice with an artificial horizon. There is dip correction, which is a correction for sextant feet above tangent line, which all books and internet reading I find always assume one is at sea or on the coast.

So, my novice question does land altitude using an artificial horizon matter in the calculations? if so, is there a way to correct? It would seem to me by basic geometry common sense (if there is such a thing) that yes it would matter. But would a mile above sea level introduce significant error?

BBB3
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Old 10-09-2016, 16:13   #2
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Re: Celestial Nav Question for Expert

A very interesting question posed here. My assumption would be if you were on a mountain and your horizons was level with you that the bearings would be no different than being seven foot above the water shooting the horizon. If you were on a mountaintop shooting The Horizon down in the valley then your Sextant would obviously not be level and your degrees on the 6th it would be screwed. The same would hold true if you were in the valley looking up. So my thought is as long as your horizon is it the same elevation as you it should not make any difference. Those are my thoughts.

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Old 10-09-2016, 16:32   #3
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Re: Celestial Nav Question for Expert

If using an artificial horizon, you should be just fine. Consider that airmen use(d) bubble sextants in place of true/artificial horizons. Calcs are all the same. The air reduction tables are merely simplifications of the naval ones, allowing for slightly simplified (and slightly less accurate) calculations.

Finally, the sight reduction tables (and spherical trig itself) assumes the globe to be spherical. One mile altitude difference is nothing compared to the inaccuracies inherent in the spherical globe assumption. As you noted, the eye height is an angle correction, not an "altitude" correction in the sense of being farther from the earth center.

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Old 10-09-2016, 16:43   #4
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Re: Celestial Nav Question for Expert

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So, my novice question does land altitude using an artificial horizon matter in the calculations? if so, is there a way to correct? It would seem to me by basic geometry common sense (if there is such a thing) that yes it would matter. But would a mile above sea level introduce significant error?

BBB3
No altitude does not matter. There is no dip correction. All you need to do is divide the sextant reading in half.
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Old 10-09-2016, 16:58   #5
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Re: Celestial Nav Question for Expert

I wonder if their would be any difference in refraction due to the higer altitudes. Probably not, since its only a small fraction of the atmosphere we are talking about, and probably no where near as bad as the high latitude effects? At any rate it is a small order correction.
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Old 10-09-2016, 18:24   #6
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Re: Celestial Nav Question for Expert

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I wonder if their would be any difference in refraction due to the higer altitudes. Probably not, since its only a small fraction of the atmosphere we are talking about, and probably no where near as bad as the high latitude effects? At any rate it is a small order correction.
I'm away from my stuff just now but I am pretty sure there is a supplementary table in both the Almanac and the Air Tables that allows for 'non standard temperature and pressure' and gives a small ( additional ) correction for refraction.

Also just found this.. http://msi.nga.mil/MSISiteContent/St.../Backcover.pdf
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Old 10-09-2016, 18:54   #7
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Re: Celestial Nav Question for Expert

Altitude doesn't matter when using an artificial horizon. Interestingly, the number of interest isn't the distance of the sun above the horizon, but it's distance from your personal zenith (known as "zenith distance"). Somewhere in the calculations the cipher gets converted to that, which is why the air tables, for reducing sights done with a bubble sextant, are the same in substance. But that's something you should not worry about until you get to trying to figure out WHY it all works--useful to know for troubleshooting in future, but too much to absorb when simply learning to reduce sights.
Have fun--celestial is a wonderful, rewarding thing.
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Old 11-09-2016, 01:03   #8
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Re: Celestial Nav Question for Expert

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Thanks Ping. Interesting. Now just got to work out what it means!
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Old 11-09-2016, 04:30   #9
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Re: Celestial Nav Question for Expert

Greetings and welcome aboard the CF, BBB
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Old 11-09-2016, 05:42   #10
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Re: Celestial Nav Question for Expert

If you are interested in celestial, here is a forum for that. fer3.com: NavList celestial navigation community There are folks in that community that are way, way out there. I have picked up a few tricks from them.

As far as an AH, (Artificial Horizon), I have found a 1st surface mirror to be far superior to the pan of liquid types. You can take sights in the dead of night with no problem. The only problem with the mirror is to get it absolutely level. That said, if it isn't and your error is the same, you are probably doing the sextant correct.

BTW, there is a United States Power Squadron in the Denver area. http://www.bvsps.org They offer courses from Piloting through Navigator. http://www.usps.org
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Old 11-09-2016, 09:57   #11
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Re: Celestial Nav Question for Expert

I took a course using a large lake in Tulsa OK using the USPS
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Old 11-09-2016, 10:12   #12
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Re: Celestial Nav Question for Expert

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Originally Posted by bbb3 View Post
Hi,
This is a question that I think can only be answered by somebody who is an expert at celestial navigation.

I'm currently on a consulting gig in Colorado for a couple months. During my spare time away from home I thought I'd teach myself celestial navigation as a hobby/interest and eventual backup to GPS. Also, I think it would be a fun thing to occupy time while at sea for days. I got a good book, Celestial Navigation by Burch. It's doing the trick at getting me started and I'm now eager to get a sextant and practice.

However, I'm currently in Colorado studying. My current elevation is 5400ft, just outside Denver. I am wondering if it is possible to take sights in practice at that elevation above sea level. Nowhere can I find if being over a mile above sea level will negate land based practice with an artificial horizon. There is dip correction, which is a correction for sextant feet above tangent line, which all books and internet reading I find always assume one is at sea or on the coast.

So, my novice question does land altitude using an artificial horizon matter in the calculations? if so, is there a way to correct? It would seem to me by basic geometry common sense (if there is such a thing) that yes it would matter. But would a mile above sea level introduce significant error?

BBB3
Pilots flying airplanes used celestial navigation for decades. Their altitude? guess !! If you see an old DC3 you will notice a dome above the cockpit- It was used from the navigator to get sights !

So there is nothing wrong to practice celestial navigation at this height, as a Pilot my self I can assure you !

A good size pot with water in your back yard will do the job for the horizon purpose

Good luck - It is the most interesting hobby of all, if I may add
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Old 11-09-2016, 10:41   #13
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Re: Celestial Nav Question for Expert

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A good size pot with water in your back yard will do the job for the horizon purpose

Good luck - It is the most interesting hobby of all, if I may add
I've had better luck with used motor oil--it reflects better.
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Old 11-09-2016, 11:18   #14
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Re: Celestial Nav Question for Expert

I want to thank everybody for the awesome replies. And the welcome to the forum.

So, seems altitude is not a factor, or at worst an insignificant factor.

I just noticed after I made the post that the sight reduction tables referenced in my training book, Pub 249, is titled 'Sight Reduction Tables for Air Navigation'. After a little research, it appears Pub 249 was originally developed for air navigation, but today is mostly used for ocean nav.

I'm going ahead and buying a sextant and I think I'll initially go with an inexpensive plastic Davis sextant.

I'm finding celestial nav is seriously fun and interesting stuff.

Bernie (bbb3)
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Old 11-09-2016, 12:05   #15
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Re: Celestial Nav Question for Expert

There are many sailors who learned their celestial navigation once they were at sea. Easier to learn when you are actually doing it for real. Keep up the work; it will make actually doing it at sea lot easier since you will know the basics and the techniques. PS: we found a whole horizon sextant easier to use. And while a Plath is heavy, the weight is actually an advantage since it reduces the device from jumping around when in your hands. Also get to know your stars by sight. You can almost navigate as well just by knowing a star's transit through the night and across the seasons. There are some fine books that will teach how to navigate by the stars without a sextant. BTW: if you can get a copy, buy The Raft Book. Was standard issue during WW2 for lifeboat navigation. Get the complete version with the maps and cloth tape aids in the back folder. Ideally if you can find it, get the oil skin version. We used the paper version which worked well over the past fifty years.
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