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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 04-25-2010
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practicing with sextant on land

So I've been having a great time learning celestial navigation with my new Astra IIIb. Since I live far inland and at an elevation of 3500ft I've been practicing my sun sights with dish of water and oil. I believe that there is no dip correction when you practice this way but what effect if any will the altitude have on the final result??
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Old 04-25-2010
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Your height of eye may be interesting! I'm more used to using from six to eight feet, depending upon whether I'm standing in the companionway or up by the shrouds.
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Old 04-25-2010
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I'm sure there's some effect but never having used a sextant "uphill" I have no idea.

But atmospheric effects, mainly localized heating and flows of air masses, may prevent you from getting a fix better than 2 miles. So if you get that close--don't bang your head, it may simply be as good as it will get. (The same problem applies in coastal use of a sextant.)
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Old 04-25-2010
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As you say, there is no dip correction when you're using an artificial horizon, as you are. Height of eye is only important for calculating dip correction.

You probably already know that the height of eye/dip correction comes from the fact that an ideal horizon is truly horizontal, whereas your real horizon at sea is somewhat (dipped) below an imaginary horizontal plane tangent to the Earth at your feet. So if you're measuring the angle from the apparent horizon to a star, it's the angle from the invisible true horizon to the star, plus the angle from the apparent horizon to the invisible true horizon. Hence the dip correction is always subtracted from a sight off the apparent horizon.

On the other hand, regardless of your altitude, the artificial horizon is parallel to the ideal horizon, which means that they meet at the horizon So they're really the same thing when you put it all into perspective.

Back to altitude, the difference is too small to create a parallax effect, which is the only other geometrical issue arising from changing your position on the Earth. One possible problem would be atmospheric refraction of low-altitude bodies, but only at very very low altitudes that you probably cannot get in your artificial horizon anyway.

I would not worry about your height of eye.

Your main problem is that the more you practice, the more frustrating it will be for you to be landlocked.
SVTatia and acunningham like this.
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Old 04-25-2010
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Thanks guys for your input. Good explanation Adam. Your explanation was what I hoped was the case. BTW, I notice that you are in Coquitlam. You too have a ways to go to get an unobstructed horizon n'est pas? Thanks again for the insight.
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Old 04-26-2010
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Quote:
Originally Posted by RandyBC View Post
unobstructed horizon
I use an artificial horizon at home. I've had very little at-sea practice, but from the waters just outside my marina I've got a clear view to the southern horizon. Note that there are plenty of islands and things, but their shores are beyond the horizon, whose distance is less than three miles away (according to 1.169*sqrt(height of eye)). I just bring the sun's lower limb down to the line separating trees from water and that has worked pretty well.
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Old 04-27-2010
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I thought that the horizon would be much further away but I looked it up and you are quite correct. That is great because when I'm sailing in the Strait I can practice my sights.
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Old 05-03-2010
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I was "playing" with my sextant in Georgia Strait last week. It works well.

Jack
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Old 04-05-2012
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Re: practicing with sextant on land

Hi,
I'm new to the forum. Thanks everyone, especially AdamLein for the explanations. I use GPS for work and travel but am interested in navigation using basic skills. I just bought a second hand Davis 15 sextant and am learning how to use it and do the calculations. I'm landlocked in Burnaby BC so have been using a pan of water (just water, no oil, pan placed in a dark place in the backyard). It worked great last night with the full moon. However my latitude came out somewhere in the lower USA!

Am I correct that for the sextant reading (assuming I've zeroed out all index error in the sextant) that I superimpose the 2 images of the moon (direct and reflected) and divide the angle by two as my starting figure? My Ha came out at 31.5 deg however. I made sure I had the time relative to GMT and even cheated and entered GPS coordinates for the Dr.

To get used to the sextant first I'm using the Celestial app for Android for the calculations. It also gives almanac figures for when I move to doing the calculations myself.

So my question is whether I do the reflected moon shots the same way as described for sun shots using a reflecting surface and taking half the angle off the sextant as my starting number for the sight reduction?

This is great, thanks everyone,
-Tony

Last edited by tony9; 04-05-2012 at 12:53 PM.
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Old 04-05-2012
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Re: practicing with sextant on land

Any sights using an artificial horizon of the kind you describe need to be halved. This is because you're measuring from the real object down to the horizon + from the horizon down to the reflection of the object. Those angles are the same, so the total angle is twice either of them.

As for your sight last night, let me guess: you did your sight around 9:45pm?

One way to check your sights is by looking up the altitude you should get in the Sight Reduction Tables (Maritime Safety Information). They're a little tricky to use. To check your sight, I went to the Latitude=49°/Declination 0-14° contrary name table (contrary name because we are N and the moon is S 2°). At meridian passage, the moon's LHA is 0, so find 0 in the left-hand column, and look for what it says in the Declination column (2°) in your case.

There I see:

Hc = 39°00', d = -60', Z = 180°.

The d is for interpolating, which we'll just skip for now. Z is the azimuth angle, which in this case is the azimuth. Hc is the altitude you should get if you're at the given latitude and the body's LHA and Dec are what you used to index the table. However, you saw a much smaller altitude, around 31°. Assuming you read your sextant correctly, and didn't make a mistake with the arithmetic, you probably didn't shoot the moon when it was on the meridian.

According to the same table, the moon's altitude was around 31.5° some time when its LHA was +/- 33°, i.e. GHA = 90° or 156°, which (according to the almanac), should have happened roughly around 21:45 last night and 02:00 this morning. The moon's meridian passage, on the other hand, happened when its GHA equaled our longitude, a bit after midnight (which makes sense for a full moon).

Had you gotten an altitude of 39°, the zenith distance would be 51°, and since Lat = ZD + Dec at meridian passage, we get 51°-2°, which is just about right for our latitude.

So the issue here is that there are basically two kinds of celestial navigation with a sextant (that I know about): meridian passage and LOP or intercept method. The meridian passage can be done without a chart but requires knowing the altitude when the body is due south. The intercept method works regardless of the body's position in the sky, but requires drawing LOPs on a chart.

The meridian passage method can be done entirely in your head, or with a little bit of paper arithmetic for a more accurate answer. You shoot the object and note the time. Subtract the altitude you got from 90 to get its zenith distance. Look up the object's declination in the almanac and add that to the zenith distance to get your latitude.

The reason this works is that, when the object is on the meridian, the north star, the body, and the zenith are all lined up. If you know the angular distance between any two, therefore, you can get the third. In general, this is not the case: the three points will form a triangle on the sky, in which case all bets are off. You can't just add angular distances anymore: you need trigonometry. So I'm guessing you tried to use that method, but when it was not applicable.
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