Sun sights and misty horizons

[zvidoron]

In weather conditions like we had in the last few weeks - bright sunshine but misty horizons - what can be done to improve the accuracy of sun sights? It appears that the mist causes the horizon to appear closer and lower than the true horizon, the correction for dip is based on height of eye only and therefore the measured angle is too big causing an intercept error of 1-4 miles towards the sun. It makes sense to take the sights from as low a height as possible to counteract this but is there a more formal method to take "short" horixons into account when reducing a sight? I heard that you can take a sight against a shore of a lake for example which is much closer than your true horizon. Any ideas?

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[Guest]

Post deleted by danfoley

 

[zvidoron]

Thanks for that. The edge of lake, or a shoreline from a boat for that matter, was just an example of being able to take a sight knowing that the water line you bring the sun down to is closer than the natural horizon would be for your height of eye. In misty conditions I think if you take the sight from as low as possible over the water your natural horizon would be close enough not to be affected by the reduction in visibility in the way I described in the previous message. One problem I encountered before is that unless there are objects around you at known distances it is very difficult to tell what the range of visibility actually is.

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[richardandtracy]

This'd require a bit of contortion but it may work.

You're really wanting to find out how inaccurate the apparent horizon angle is. If you know that you always take the sight from roughly the same position on the boat, you could measure the angle between a known angle (eg vertical down or up) and the true horizon (mark vertical down with a plum bob maybe, haven't fully thought the practicalities through). Then on a murky day, measure the angle between vertical down and the apparent horizon. Subtract the difference from the sun angle, and you should have a better sight than you're getting without the correction. It won't be perfect as you've now got twice the chance of making an error.

Regards

Richard

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[AndrewB]

Using close horizons.

The HE correction to a near horizon like the far shore of a lake is a reasonably straighforward piece of trig if you know the approximate distance and the height of your eye above the far shore. (The latter is easy if you are standing on the near shore).

Take r = 3440.1nm as the radius of the earth, d = the distance to the far lake shore (in nm), and h = the height of eye in metres, then the angle of dip is (90 - the angle opposite side A) of a triangle whose sides are:

A = r
B = r &#43 h/1852
C = d

Incidentally, the same method can be used to illustrate the effect of a 'near' horizon caused by poor visibility, as follows:

If the height of eye is 4m, then the distance to the horizon is 3.9nm and the angle of dip would be 3.9'. Suppose that due to decreased visibility, the apparent horizon is just 1nm. Then, by the above method, the angle of dip is actually 8.1', i.e. 4.2' different, and your sight reductions will be approx 4 miles out towards the sun.

Get your height of eye down to 2m, then the distance to the horizon is 2.8nm and the angle of dip is 2.8'. If the apparent horizon is still just 1nm, then the actual angle of dip is 4.3', i.e. 1.5' different and you will be only 1½ miles out.

So you are quite right about keeping your head down when taking sights, and not just for the sake of accidental gybes!

(Note that I have not allowed for refraction in the above calculations which is why the angles of dip to the true horizon given above are slightly larger than the HE corrections in the standard tables. But they are close enough for practical purposes).

 

[alex_rogers]

During his sailing trips to Greenland, Bill Tilman would sometimes take sights from the dinghy in order to shorten the distance to the horizon when it was misty. Needs a calm sea though!

Another trick he used when the horizon was totally obscured was to use a pool of oil in the lid of a small tin as an artificial horizon. The viscosity of the oil ensures that the surface remains horizontal. You use the horizon mirror to view the reflection of the sun from the surface of the oil, divide the angle measured by two and forget about the height of eye correction. I've used this method at home but I suspect you'd need to be becalmed to use it at sea.

<hr width=100% size=1>Alex Rogers
<A target="_blank" HREF=http://www.YachtsAtSea.com> www.YachtsAtSea.com </A>