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Old 13-08-2008, 09:40   #1
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Using an artifical horizon

Hi, folks!

Got a question concerning the use of an artifical horizon for practicing taking sun/moon shots.

The other day, I set up my ah (artificial horizon) on a steady level surface. I had a good view of ol' sol in the water. I then grabbed my sextant and tried to bring the mirror sun into coincidence with the sun in the ah.

Couldn't get them together at all. If I was able to see the mirror sun, I couldn't see the ah sun and vice versa.

I had previously adjusted my sextant by using a building about a mile away to take out the index error, both horizontal and vertical.

So, my question is, in order to use the artificial horizon, should I calibrate the sextant using the ah as my reference? In other words, adjust it so I don't have index error looking directly at the ah with both direct view and mirror view?

Or am I doing something else wrong?

Thanks in advance!

Nick
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Old 13-08-2008, 10:54   #2
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It's been a while since I used an ah but if I remember correctly your angle is double what it would be to the sea horizon. If you are trying to do a noon sight and you're near 90 degrees at noon it may not work. Take the sight later in the day or early in the morning.

Tim
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Old 13-08-2008, 11:00   #3
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Thanks, Tim! I appreciate the feedback.
What I'm really looking for is whether or not I need to adjust my sextant's mirros differently when using the artificial horizon. Reason being, I'm not able to see both the mirrored image of the sun and the reflected image of the sun at the same time.
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Old 13-08-2008, 14:16   #4
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No Mirror Adjustment Required.

Sextant mirrors must be perpendicular to the frame and arc of the Sextant and unless you have adjustment cyclinders and know what you're doing, I surely wouldn't fool with that at all. If you have reason to question the adjustment, go see an instrument maker.

As to practice sights, set the artificial horizon up in a manner that allows you to see the reflection of the sun as you are looking at it while standing erect. (Ensure that you are standing directly behind the AH with the sun in front of you. The shadow cast by the ridge of AH should be parallel to the bottom edge of the box for its full width and you should be standing directly in line with that shadow, otherwise you can't align the images.) Then set your sextant index (swing) arm to zero with the shades in place for the amount of sun and check your index error--on or off the arc--against the sun itself. Once you've recorded that, swing the index arm forward slowly as you rotate the instrument down toward the AH, maintaining the reflected image of the sun in the index mirror until you can see the AH (you may have to remove one or more of the shades) in the horizon glass (you shouldn't need the telescope for this exercise, just use the site tube.) Line the reflected image in the sextant mirror with the reflection of the sun on the surface of the AH--record your reading, deduct or add your index error and divide the result by two.

The only time you may have difficulty with the foregoing is if your location is quite far south--say south Florida-and you are trying to shoot the sun at mid-day near the summer solstice--i.e. when the sun is near 23* north, (or if you are south of 23* north in which case you'd have to face north!). Because readings taken from an AH are twice what one would get using the actual horizon, you may find that you cannot swing the index arm enough to be able to actually take a reading after mid-forenoon or mid-afternoon at that time of year.

FWIW...

s/v HyLyte
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Old 14-08-2008, 06:09   #5
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Well, THAT'S interesting.

I tried using the artificial horizon again yesterday by adjusting my index mirror. Didn't help, so I re-set it back to normal.

Strangely enough, I noticed the sun was reflecting in our swimming pool. The wind was light so I was able to sight both the reflected image and the mirror image.

Oddly, THAT worked, but I still can't get both the reflected image and mirror image in coincidence when using the artificial horizon.

Weird. Should I be closer to the AH, farther away? It appears my sextant isn't out of alignment but I'm doing SOMETHING wrong.
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Old 14-08-2008, 06:48   #6
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What kind of AH

What kind of artificial horizon are you using? Most consist of nothing more than a little tray that one fills with water which is then covered with an angular hood of clear or slightly shaded plastic to prevent wind disturbance of the water surface and the sun's reflection. A pan of water with a little mineral or cooking oil can also be used. Both are miniature versions of the surface of your pool absent the ripples.
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Old 14-08-2008, 06:59   #7
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Yeah, that's what's weird. Since it works with the pool, you'd think it'd work with the gizmo.

I'm using the Davis artificial horizon. I'm filling it up to the top of the tray and standing back about 6 foot or so. When I'm set at 0 degrees, my mirrored image of the AH is out of whack, which probably isn't a big deal. However, i just can't get both the reflected and mirrored image of the sun in the same view.
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Old 14-08-2008, 08:30   #8
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Swing the Arc

Are you sure you're holding the Sextant vertical? I have seen new users holding sextants at quite an angle without realizing they are doing so which will shift the reflections apart from side to side.

Also, the AH needn't be on the ground. You can set it on a table or chair seat so long as you are directly behind it and can see the reflection of the sun when you look down at it. Another thing you can try is working your sight in reverse. I notice that you are in Michigan so the Sun's elevation, even in summer, is not an issue. A noon Lat would be the easiest to work with. Just figure your local hour angle from your actual position on a given date and then use your tables to back into the Ho for that. Then set your Sextant on double that (because of the AH) and face directly into the sun, (dead south at your actual local noon) and you should be able to see both images which you can then align.

If that doesn't work, I'm stumped.

Good luck!

s/v HyLyte
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Old 14-08-2008, 08:34   #9
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Thanks, HyLyte.

I'm gonna try a few other things and see if I can't figure this out. Since it works with the pool I know it's got to work with the AH. Maybe I should sit rather than stand....I dunno. I'll let you know what I figure out in case anyone else runs into this.

Fair winds,

Nick
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