Sextant use on land question

[MM5AHO]

In practicing sextant use, used on land where no clear horizon exists, I've been using the technique of a bowl of water on a pedestal. Using the reflected image of the heavenly body, and the actual body, it's possible to measure the angle between the body and its reflection, and that should be double the elevation angle. Halving the meaured angle gives a useable elevation for that body. The reflected image can be a bit dim, so a bit of juggling the shades required.

But I'm wondering about corrections. I've been using the normal set of corrections - Index error, height of eye (count as zero in this reflection case), and atmospheric refraction correction. The question is, should any extra correction be used for the wider angle measured initially?
If I'm halving that angle first, then applying a standard set of corrections to half the measured angle is that enough? Or should I have any other correction for my artificial horizon?

 

[Gitane]

Not sure about the corrections, but instead of a bowl of water, used sump oil or treacle can give a clearer image of the sun. These also generate fewer waves in the bowl if the wind picks up.

Gitane

 

[Skylark]

From your observed angle, apply index correction. Then take half the resulting angle and apply the other corrections with the exception of dip (height of eye).

 

[Gadget257]

Geoff,

I use the Davis artificial horizon to reasonable effect in my garden. The link below is their instructions which should answer your question.

http://www.davisnet.com/product_documents/marine/manuals/0144_IM_ArtificialHorizon.pdf

Using a sextant is a skill that requires practice, I usually take my plastic sextant on holiday with me, where I do get some strange looks and comments. But what else is there to do on a beach other than take the Noon sight?

Fair winds

Chris

 

[pcatterall]

Geoff,

I use the Davis artificial horizon to reasonable effect in my garden. The link below is their instructions which should answer your question.

http://www.davisnet.com/product_documents/marine/manuals/0144_IM_ArtificialHorizon.pdf

Using a sextant is a skill that requires practice, I usually take my plastic sextant on holiday with me, where I do get some strange looks and comments. But what else is there to do on a beach other than take the Noon sight?

Fair winds

Chris

We know your sort! ogling the ladies while apparently looking out to sea!

 

[Gadget257]

We know your sort! ogling the ladies while apparently looking out to sea!

Damn, I've been rumbled! I may have change my chat up line of "do you want to come at see my vernier"!

 

[Porthandbuoy]

I find a spirit level and an iPad or Kindle Fire makes a pretty good artificial horizon.

If you're near a river, pond, lake (or loch) you can use a 'short dip' table. There's one in Bowditch, table 14 if memory serves.

 

[jwilson]

I find a spirit level and an iPad or Kindle Fire makes a pretty good artificial horizon.
snipped --

I doubt a typical DIY spirit level can give an accurate enough level. water or oil definitely does as long as it's not windy. Never tried treacle!

BTW davidjackson is correct about the corrections. Practicing on land is a good way of checking your calculations, as long as you can accurately locate your position on a chart. When I was first practicing I got a very accurate land position (5 miles inland) on a chart from horizontal sextant angles of two church spires and a radio mast. After a while I could get a plastic Ebbco sextant to give me position lines within under a mile when ashore by averaging sight angles and times plotted on graph paper. Taking the same sextant to sea accuracy degraded, but in good conditions and again by averaging I could usually get well within 2 miles.

 

[Porthandbuoy]

I doubt a typical DIY spirit level can give an accurate enough level. water or oil definitely does as long as it's not windy. Never tried treacle!

True. Some self-levelling liquid is best, unless it's windy, then use mercury.

 

[MM5AHO]

Mercury would be a great reflector, but horrible stuff to handle.
The reason I wondered about the extra correction, was that the correction increases with elevation measured. So measuring a reflected angle, and having twice the angle that is true, I wondered about some extra correction, but see that just halving the measured angle immediately after the IE correction, takes care of that, so that the correction only applies to the half measurement.

I'm normally getting with in a mile or so on land this way. Jupiter is convenient in the evenings at the moment.

 

[Graham_Wright]

True. Some self-levelling liquid is best, unless it's windy, then use mercury.

Apart from other trivial (!) considerations, mercury has quite low viscosity.

 

[Skylark]

If you're near a river, pond, lake (or loch) you can use a 'short dip' table. There's one in Bowditch, table 14 if memory serves.

I'm always keen to learn more about Celestial Navigation so I've looked at Table 14 in my Bowditch "Dip of the sea short of the horizon".

Can you please either give an example of use or refer to the relevant section in Bowditch? Thanks

The arguments are Distance in miles (0.2 to 10.0) and Height of eye above the sea. (5 to 50 feet). Tabulated values appear to be in minutes of arc.

 

[Porthandbuoy]

Table 14. Dip of the Sea Short of the Horizon [p559 in my copy]

If land, another vessel, or other obstruction is between the observer and the sea horizon, use the waterline of the obstruction as the horizontal reference for altitude measurements, and substitute dip from this table for the dip of the horizon (height of eye correction) given in the Nautical Almanac. The values below the bold rules are for normal dip, the visible horizon being between the observer and the obstruction.