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Message: <blockquote data-quote="Binastro" data-source="post: 3175812" data-attributes="member: 111403">. Hi Mark,I don't know if you mean today or March 20. Either way there is not much difference.To all intents and purposes, one cannot take low elevation of the Sun into account.In some countries such as Switzerland, at about 10,000 feet or 3000 m in the mountains I've experienced exceptionally strong sunlight even in the winter. This also applies to some countries at sea level that have very clean air.The water vapour in the atmosphere also plays a part.. But I have seen the sun at 5° elevation, which is absolutely blinding. This doesn't normally happen in southern England.In England, with a lot of pollution and water in the atmosphere, the Sun doesn't seem quite so bright.I normally use a welders glass 13 tilted as necessary to get the correct density. But I think the difference in densities is around 2.7 times for each shade. The scale is something ridiculous like the seventh root of 1000, I will look it up again.. It is normally considered that a welders glass 14 should be used. However, welders glasses were not designed for solar viewing and some folks prefer not to use them, thinking that special solar filters are better.I'm not so sure of this. Also I don't know whether there is a shelflife on a welders glass, or if it is affected by continual radiation either from arc welding or by the Sun. In essence, I think that a low Sun might be around half the intensity of a high Sun.Normally I think that high ultraviolet readings occur with the sun's elevation above about 45°.Human beings have some protection against the Sun. Our eyebrows and recessed eyes protect us from a really high Sun.Blinking is normally fast enough to prevent eye damage.Averting our vision is another.The pupil response is far too slow and anyway not very effective.With the Sun at 20° elevation even a welders glass 13 untilted is often not dense enough even in polluted England. I often have to tilt it slightly, which is why I use a larger size welders glass 13, which is sufficiently high vertically to allow tilting, without risking direct exposure to the sun.The problems come when we deliberately stare at the Sun, as we are tempted to do with all the hype of solar eclipses. If there was no advertisement at all, few people would notice a partial Eclipse. But with modern communications really large numbers of people will be aware of such events.We have to consider that normally the full moon is considered the brightest thing we should look at with direct vision.The Sun is about 450,000 times brighter than this. If at a really low elevation it is 200,000 times brighter than the full moon, it is inevitable what will happen if somebody stares at the Sun.The solar eclipse glasses, which meet British and European standards reduce the sunlight by about 100,000 times and this is considered good enough for three minutes continual exposure and intermittent exposure for the few hours of an eclipse. But they should not be used by persons who have had eye surgery or have eye disease.I think they are considered to have a shelflife of five years if they're kept in their original wrapper in the dark.But low elevation of the Sun is something that should not even be considered as providing any protection whatsoever.</blockquote>

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