NavList:

   

Reply

Kieran,

Excellent case report; thank you.  I made the stupid mistake (once) of
starting to view the sun to check index error with the index arm set to zero
degrees after viewing the horizon, without swinging down the horizon mirror
shades.  Fortunately I sensed the very bright light just starting to flare
in the periphery of my field of vision before I got a retinal blast, and
quickly turned away.  I did not have any immediate or subsequent symptoms.
I have had laser surgery a couple of times: it really flared my vision green
for several minutes after the procedure, and was far more stimulating than
that brief peripheral solar exposure.

Being a physician you stimulated my professional curiosity, so I did a
brief, limited literature search on Pubmed to learn more about solar eye
injury.  I posted the literature search results in a separate email.  I
found not a single reference to navigational sextants in Pubmed, for any
health reason whatsoever.  Interestingly the world's medical literature
appears to be completely silent on navigatonal sextants!  No news is good
news, I guess.

I get the sense that humans are well adapted to moderate sun exposure.
Although sun-gazing can definitely cause damage, much of it appears to be
reversible, probably because sufferers are quick enough to turn away soon
enough.  I am by no means expert on this issue, but I get the sense from my
reading so far that for significant damage to occur, the eye has to be
exposed to direct sunlight for relatively lengthy periods, or has to suffer
repeated exposures over long periods of time.  My conclusion is that human
eyes are well adapted to deal with limited exposures to direct sunlight, but
that we do not want to push the edge of that envelope.

As Kieran pointed out there are risk groups, such as the intoxicated.  But
hunters, sailors and other people who occupationally stare at bright skies
are also at risk of acute and chronic eye damage.

Sunlight can acutely damage all the tissues of the eye in different ways:
- Lids and conjunctiva: sunburn (UV).
- Cornea: sunburn (UV damage, same as welders' flash).
- Lens (UV, heat).
- Vitreous.
- Retina (heat, UV, and components of light).
There is also a link between UV exposure and ocular melanoma.

I think it is clear that, as with all things in health, minimizing exposure
to noxious stimulus is ideal, so scrupulous eye care is wise when using a
sextant, or when out in bright sunlight for any other reason.  I recommend
sunglasses between sights, as Kieran suggested, lots of sunscreen, minimal
observation time, careful attention to the condition of the shades, deep
caution when swinging up to view the sun, and very careful attention to use
of the shades.

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
Outgoing mail scanned by Norton Antivirus
-----------------------------------------

   

Reply