How to use a sextant on dry land

[spillemw]

I am trying to exercise with my sextant away from the sea. I know I am supposed to create a artificial horizon by using for instance a pool in which the sun reflects. However, I don't quite understand from the manuals what I have to aim the telescope at. Do I aim the telescope at the reflection of the sun in the pool? It does not seem to work for me that way.

 

[Dave_Seager]

As I understand it, you need to divide the angle between the sun and its reflection by 2 to get the equivalent angle between the sun annd horizon. To put it simply, the reflection of the sun appears as far below the horizon and the real sun is above the horizon.

 

[Litotes]

A washing up bowl of water with a film of oil on top makes a good artificial horizon. Vegetable oil is fine. You need to get your eyes as nearly level with the bowl as possible.

Halving the resulting altitude is right and remember that you will be bringing the sun's image down onto itself. At sea, you need to correct for the fact that you are sitting the sun (or whatever you are using - usually the sun under these circumstances) on the horizon. This correction isn't needed when the image is overlaid in this way.

 

[dt4134]

An alternative is to find a flat surface. There's a local office roof that I can use for a noon sight that only requires a small correction. You've got to be very careful to sit or stand in exactly the same place and in the same posture each time, but I found I could consistently get latitude accurate to within one or two miles this way.

 

[spillemw]

So I have to divide by two the reading I take from the sextant. That may have been my problem indeed.

 

[savageseadog]

[ QUOTE ]
So I have to divide by two the reading I take from the sextant. That may have been my problem indeed.

[/ QUOTE ]I assume you've been plotting your home in Africa somewhere. /forums/images/graemlins/grin.gif

 

[Starfishbooks]

I used to do this many moons ago as a deck officer cadet. We used the compass bowl on the bridge wing as a reflective surface as it was gimballed and should therefore be horizontal. Can't quite remember which corrections to apply now, but you do need to halve the angle once the corrections have been applied. I should imagine index error will only need to be applied once, but as for parrallax etc., i need to sctatch the brain a bit and remember all the theory! I think you have to leave out the correction for either the upper or lower limb (sun) as you are measuring twice the actual angle and no adjustment for this needed.

It wasn't that accurate, but on a couple of times (pre GPS days) it got us a much needed position in poor vis with no horizon to use.

 

[Starfishbooks]

Brain scratched a bit - If you shoot close to the reflective surface you won't need the height of eye correction either.

 

[actionoptics]

I've got a Davis Artificial Horizon with instructions. I could scan and e-mail you a copy if you want.

 

[james c]

Using and artificial horizon

In answer to your question, you aim your telescope at the reflection of the obect in the artificial horizon. You then adjust your index so that the image from the index mirror is superimposed on the reflection. That way you are measuring exactly double the angle from the horizon to the object. You will see that by rocking the sextant you can take out the vertical error. You could get the same effect by laying a mirror on a table if you could guarantee that the mirror was perfectly horizontal. But you can't - so you use a liquid mirror, which is perfectly horizontal.

 

[Neil_Y]

That's very useful, thanks. I've had sun and no horizon a few times and been stuck for a fix...now all I need is a bowl of water on the coach roof!

 

[Lady Campanula]

.....now all I need is a bowl of water on the coach roof!

Be most wary of using any form of oil-on-water in such circumstances. It makes one helluva mess when it spills...

 

[jwilson]

Brain scratched a bit - If you shoot close to the reflective surface you won't need the height of eye correction either.

Height of eye correction zero and irrelevant: you get the same angle between sun and reflection if you are sixty cm or sixty meters from the reflecting surface.