Using the plastic practice horizon bubble sextant, you do have to make a correction value. There are Two ways of doing it, one with the natural ocean horizon the other is with a known position. Make sure your sextant is first adjusted correctly and do a few Solar IC adjustment or calculation. Keep that figure / value written down. After everything is adjusted correctly put the practice horizon on. Bring the suns Lower Limb to the line. Now take about 30 sites from a known position. Then adjust the error with a bubble correction. Remember the correction has that I.C. Error already added to the correction. This is why you need to know the error because your BUBBLE CORRECTION will change, it will change with the I.C. Error. Plus it gets you into the practice of knowing your sextant intimately. Ok always, Keep doing this until you can draw our a slope line and visually see the errors. Then average out the errors. Once you have an average, apply them to your sites. See if your sites begin to line up with the slope line. When it’s does you got your bubble correction plus your IC error. Now write down your bubble correction SUBTRACT out the I.C. And you have your base error of the unit itself. Each practice horizon has its own error value. Mine was about 13.72 as an average.
Now as a side note, that practice horizon is a pain in the ass to use. If your careful and everything is done perfect and your average is on target, maybe you will get a LOP about 1.0 to 3.0 miles from your DR. But realistically count on 2 to 7 miles. If you move your sextant a nano meter you will be wondering what you did wrong. Answer nothing, it’s just for practice. It’s to get you used to holding the sextant, understanding how to hold it steady and to take the shot. You will be amazed when you go out to sea or from the shore line and you take the shot and you end up with awesome results. Now For better results try an artificial horizon. It gets you to about 0.5 to 2.0 miles from your known DR. Or better yet build artificial horizon in the backyard with string and stakes. If done right you will be about 0.3 to 1.0 from your Known DR.
The main thing is to practice daily. Use the moon, stars, planets, the sun. practice daily checking your side and index error. Practice daily solar IC correction or if you have a ocean horizon and know your Height of Eye using the natural horizon for IC error.
The more you practice the better you will be using your sextant. Then soon you will move up to doing lunar sites... when you can do lunars correctly and accurately your now the master of the sextant.
Hope that help, happy holidays
Sent from my iPhone
On Dec 24, 2019, at 14:03, Huub Robroek <NoReply_Robroek@fer3.com> wrote:
Thank you for the explanations. This helps.
I am testing with my (newly acquired) Astra 3B with bubble horizon on land. Do you know where the difference between the Horizon "Bubble" and "Sea" settings is coming from (for Sun LL about 12')? Thank you for your feedback.
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