Sextant sun sight eye protection

[zvidoron]

Most books on the use of sextants for sun sights recommend setting the sextant to 0 degrees, dropping the darkest index mirror shade on, aiming the sextant at the sun and bringing the sun down to the horizon. Some books also mention dropping the lightest shade over the split view horizon mirror, others not even that. It appears to me that although the reflected image of the sun is greatly reduced in intensity by the dark index mirror shade, the direct image of the sun seen through the clear (left) half of the split horizon mirror, even for a short time, can damage the eye. Shouldn't the darkest horizon mirror shade be used as well, at least until the sextant is brought down and away from the sun? or am I missing something?

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[alex_rogers]

If you set the sextant to zero, only use an index mirror shade and point it at the sun you'll certainly end up seeing a very bright image of the sun. Probably for the next few days as well!

I've never used this approach. In general you can always guess the angle of the sun to within 5 degree so it doesn't take long to point the sextant at the horizon below the sun and wiggle the index arm until the sun appears in view. Start with the darkest shade but after a couple of times, you know which shades to use as well.

The technique for slowly bringing the sextant down to the horizon is useful for star sights where you don't know the approximate altitude of the star. However in this case you won't want any shades.

I think they may suggest this technique for sun sights as it is an effective way to check the index error of the sextant. If the error is zero, you should have the two images of the sun perfectly over-laying. In this case, you should definately use a shade in fron of the horizon mirror. I find it easier to check the index error using the horizon.


Alex


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

 

[wpsalm]

been a few years since had the sextant out of the box or even looked in the box for that matter....fan out the shades hold the sextant up to the sun pick the one you,ll use put it in place set the index arm to the approx alt aim in the approx direction swing the index arm back and forth till you find the sun...it gets easier with practice....setting the index to 0 is fine for bringing a star down to the horizon also remember holding the sextant up side down to bring the horizon up to the star....could be wrong about that part its been awhile... in practice its very rare to be able to take usefull star sights from the deck of a small boat ...as for eye damage yes indeed I remember reading an article on the subject develope a spot on the retina..probably got one myself but don,t notice it...estimate have taken some where in the vicinity of 10,000 sun sights... good luck to you

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[snowleopard]

i soon gave up that technique. now i face the sun, look at the horizon and move the arm up and down the arc till i see brightness in the mirror, then home in on the sun. with a bit of practice it only takes a few seconds and no fried eyeballs.

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[AndrewB]

Books written by blind men, methinks?

Nothing could be so foolish as looking direct at the sun through an unshaded sextant, specially with the telescope fitted. Even with careful use of the shades, eventually some damage will be done to the eye. Bill Cooper, who was a professional chart surveyor, wrote on this forum that the sight in his right eye had been damaged that way.

Anyway, the method sounds wrong. Starting with the sextant set to zero is appropriate for an IE check, when you aren't looking at the sun, or with a sunrise/sunset compass check, when the sun's light is least strong and a light horizon shade is all that is necessary. But do it the way you describe for a normal sun-sight and you will immediately lose the sun again as you open up the angle. 'The book' doesn't propose to wind all the way on the micrometer drum, surely?

The normal procedure for taking a sun-sight is to set the sextant angle close to that expected, even working the calculation backwards from the EP to give a rough estimate of the expected angle, and then pointing the sextant at the horizon beneath the sun. Judging the sun's height to within a few degrees soon becomes automatic.

If you are taking a star sight, then of course you won't be using shades. Because its easy to confuse stars through the sextant telescope, it is sometimes recommended to turn the sextant upside down, point direct at the star you want, and bring the horizon up to the star.

With an unfamiliar sextant being used for the first time, it may be helpful to set the sextant to zero and look at the sun with all shades on, just to get an idea of what it looks like through that particular sextant, and to judge how much shading is needed.

 

[zvidoron]

Thanks very much for that and all other comments.

well Tom Cunliff in Celestial Navigation (2nd edition) clearly says on page 19 Para 5 :"With the instrument still at zero, drop a shade or two over the index mirror and aim the telescope at the sun. It may be advisable to put the lightest shade over the horizon mirror as well before you do this, otherwise you could end up with a sore eye."

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[zvidoron]

Re: Books written by blind men, methinks?

Thanks Andrew please see my response above your entry

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[jleaworthy]

When I used to do this in anger many years ago I used the star telescope (wider field of view) with a dark lens (there's usually a choice of three or four) fitted to the eyepiece of the telescope - dark enough to take the brightness out of the sun but not so dark that you can't see the horizon. It was far simpler than fiddling with all those pivoting shades with the real possibility of serious accidents. This method worked even for Dartmouth cadets who only experienced problems when they forgot to remove the dark lens for evening stars!

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[ponapay]

It should not be necessary

when taking star sights to use anything other than a pre-calculated altitude and therefore unnecessary to start at 0 degrees.

With the sun it is much easier to check the horizon requirements first with the sextant at 0 then set the rough altitude and use the darkest shade first (reducing as necessary) while adjusting the altitude to get an accurate altitude.

It certainly saves the eyesight.

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