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Discussion Starter #1 (Edited)
Im teaching myself Celestial Navigation. I got a nice deal on a Astra III sextant on ebay and have been reading up on the subject. Anyway, I am currently land locked in Kentucky. I purchased a bubble horizon to practice with, since its pretty much impossible to get a good horizon line here.

I started taking some sun shots to get the feel for it and I wanted to make some LOPs for practice. Do I make "height of eye" corrections for my height off the ground (6 ft) or do I use my eye height above sea level (about 560 ft)? Seems like it should be from sea level. But then using the bubble I would get a different angle to the "real" horizon if I could actually see it compared to the bubble, wouldn't I? Confused.

Thanks in advance!
 

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I think since you are using the bubble, there is no DIP correction for your sextant altitude (Hs->Ha). Your "bubble horizon" just run parallel to the real horizon or sea horizon.
I have never taking sight with a bubble, maybe others can share their thoughts.

Best
 

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Your height of eye correction is height above sea level. Correction tables likely don't extend to 560 ft above sea level, so you will likely have to either extrspolate or accept the error since its just for learning purposes any way.
 

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Discussion Starter #4 (Edited)
Well, I did a test shot and reduction. Since Im sort of on my own I use a pretty nice excel sight reduction calculator to check my work (free at Interactive Spreadsheets for Celestial Navigation ).

What it told me was I need to use height above sea level. I was about 3.1 miles off of actual GPS position using height above sea level dip correction, and about 17 nm off using height above ground. Ive just done it once tho. It could be that I was horribly off and just dumb luck that made the results work that way.
 

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Love the elegance of celestial. Like you when starting out practiced on land. Being near the ocean didn’t need to use an artificial horizon.
Still I learned celestial to not take the time penalty on Marion Bermuda races. So practiced on the boat. Boy was that a huge wake up call. It’s so much harder to do on a boat. Even simple things like getting local noon or lower limb of the moon.
Really glad you’re learning this skill. But if you can practice on a boat. Even if it’s just on a big lake with a chop. See how you do getting both your sight done and recording the time.
Think the mechanics of a good sight is harder than the rest of it. Personally never got better than magnitude 1 stars, moon, planets and sun on the boat.
 

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Well, I did a test shot and reduction. Since Im sort of on my own I use a pretty nice excel sight reduction calculator to check my work (free at Interactive Spreadsheets for Celestial Navigation ).

What it told me was I need to use height above sea level. I was about 3.1 miles off of actual GPS position using height above sea level dip correction, and about 17 nm off using height above ground. Ive just done it once tho. It could be that I was horribly off and just dumb luck that made the results work that way.
Hi,

I have been thinking about it, and still I think there should be no Dip correction in your case using the bubble horizon. Let get practical; I heard about the excel sheet you mentioned. Looking into it, I don't see (check settings tab) the option of "bubble horizon", just artificial horizon (which is totally different). When I want to check my sights, I normally use two programs (free):
Celestial Tools by USP https://celestialtools.webs.com/
or
TeaCup Navigation Teacup Navigation Home

In both, they DO include the option of "bubble horizon". If you run Celestial Tools, when you click on bubble horizon, it remove the option of HE (height of eye).
Maybe you (we) need to ask the experts like in the forum NavList (NavList: for the Preservation and Practice of Celestial Navigation by Sextant), which I think is run by Frank Reed from RI. There is a couple of facebook sites about it too.

As outbound mentioned, one you get your sights on land (with whatever horizon) with accurate calculations, the real challenge is to do it on a boat.
 

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First thing you are going to want to do, if you have a fine nautical sextant (not plastic) is to send your sextant to a nautical instruments technician for calibration. When I used a sextant as my only form of navigation (electronic navigation hadn't been invented for deep sea) I did this every year or when I reached a city large enough to have one of these shops. Sextants can get out of adjustment.
 

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Discussion Starter #9 (Edited)
Capta, you are right about the calibration. Ive calibrated it and it was pretty far off.

As for the debate of dip correction, I have answers. I was able to find and download the directions for this Bubble Horizon from the manufacturer. Turns out -drum roll- "A bubble horizon does not require a height of eye correction". Good and bad news for me. It means my first sight was total dog doo. But semi good news, I need to figure out a Bubble Correction which I did not factor in. It says the BC is normally less than 15 minutes of correction. The process looks like kind of a PITA but I dont have much choice.

EDIT: The Bubble Correction involves taking a few shots from a known position and averaging the error. Or, sighting the actual horizon, subtracting dip, and reading the index. Thats the Bubble Correction. I wish I had a real horizon :-(

Ill update later. Thanks all for the comments and links. And keep'em coming.
 

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Discussion Starter #11
Well, I did 3 shots of the sun yesterday and 8 shots of the moon last night using the bubble horizon. I threw out the results of my sun shots because they varied quite a bit on accuracy. The 8 moon sights were fairly consistent. I think this is due to becoming more proficient at taking a sight. I soon realized how sensitive to error these readings are. It amazes me that a second or two of time error, or just a few minutes of arc error can make a difference of miles. Once I realized that I became much more careful. Taking sights on a moving boat will be a challenge. Although this bubble apparatus has its challenges as well.

So in the end, my bubble error (BE) was averaged to -19.3'. So in practice with a bubble horizon Ill not use dip correction and just take the 19.3' off of the sextant reading. According to the directions, once the error is calibrated it remains consistent even after removing and installing the bubble horizon. It should be good enough for practice.

Thanks Cascoamarillo for the Navlist link. It had a useful discussion on using a bubble horizon. And its a great resource. And thanks to everyone for discussing this topic with a celestial nav noob.

PS: Im a Apple/Mac guy. Last night I probably spent too much for a Cel Nav app called StarPilot. Jury is still out on what I think of it. For the money it did the job but Im a little disappointed. Ill mess with it some more.
 
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