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Solar eclipse viewing

Posted: Mon Mar 09, 2015 5:33 pm
by Mr. JohnB
My school is considering purchasing glasses for pupils to observe the solar eclipse later this month. Is it safe for pupils to look through glasses from those sold at a website If the product is known to be reputable how long would you advise it to be safe for young children to look at the sun through them?

Thanks for any answer to this question and any other advice you may think pertinent.

John B

Re: Solar eclipse viewing

Posted: Tue Mar 10, 2015 2:39 pm
by David Frydman
. Dear John,
I cannot advise how reliable the supplier you mention is.
There is very good advice in the recent popular astronomy and also I think on this website perhaps by Robin.

The current sky at night magazine has excellent advice and a free Eclipse glasses.
The coverage in astronomy now magazine is in my opinion not good enough.

With the free Eclipse glasses given with the sky at night magazine it says that the maximum time for viewing is three minutes and also intermittently for shorter periods over the few hours of the eclipse.
I would advise perhaps one or two minutes maximum.

One should hold the glasses with one's fingers, especially if using them in front of prescription glasses, as they can slip or fall off.

It would be a good idea for the school to get eclipse glasses from a reputable source.
Maybe the Royal astronomical Society gives a list of reliable suppliers, I've seen a list of suppliers recently somewhere.

For a school, another good way is as Robin says to project an image of the Sun in a small hand mirror with a triangular hole about 5 mm with the rest covered. This will give an image of the Sun about 5 cm across at 5 m projection distance. Or 10 cm across at 10 m projection distance. At this distance perhaps a 7 to 10 mm hole would be better.
The classroom should be fairly dark and it is better to have windows on one side so that the projection screen or white card is in the dark part of the room. I have found that projecting into a dark area gives a much brighter image.

Never look at the Sun with unprotected unaided eyes.
Never look at the Sun with an optical instrument as instant permanent eye damage is probable.

There is a present a sunspot about 40 arc seconds penumbral size that I just saw with protected unaided eyes. It is about one third of the distance from the limb to the centre disc.
One might need keen eyesight to see this with eclipse glasses.

Best wishes, David

Re: Solar eclipse viewing

Posted: Tue Mar 10, 2015 2:48 pm
by David Frydman
. Using mirror projection as described above I suppose that a whole classroom could see the Crescent Sun. The resolution is not that great but one can easily see the Crescent Sun, but not sunspots.

How many children are there in the school that you wish to partake in this event?

If there are many children you may need more than one classroom or perhaps relays of children.

There should be responsible adults, such as science teachers supervising the whole event.

How many eclipse glasses are you considering and for how many pupils?

Regards, David

Re: Solar eclipse viewing

Posted: Tue Mar 10, 2015 5:14 pm
by brian livesey
The problem with eclipse specs David is that they are not very durable, especially with kids.
I knew a lady who had some eclipse specs in the back of a drawer and they were in a terrible state. She was inexperienced and thought that the specs could still be used.
A No.14 welder's glass will last a lifetime and, at today's prices, costs a mere £1.20.

Re: Solar eclipse viewing

Posted: Tue Mar 10, 2015 7:05 pm
by David Frydman
I agree, Brian.
The eclipse specs are really one use only, although they supposedly last five years in the dark with the original rapper (I've left dragons spelling in).

Brian, are the welders glasses shade 14 generally available nowadays? Can you buy single ones, or do have to buy say five?

My chemist friend says that organic compounds used in various filters do break down, whereas inorganic compounds last longer.
I am still not sure how welders glasses are made, and whether they do have a shelflife or not.

Nice day today.

Regards, David

Re: Solar eclipse viewing

Posted: Wed Mar 11, 2015 2:37 pm
by brian livesey
I had ordered three No.14 welders' glasses from a local engineers' suppliers David. This particular supplier only had No.12s onboard ( more suitable for brazing than arc welding ).
As you probably know, the stained glass windows in ancient churches have natural mineral colours, rather than vegetable colouring, which is why the colours haven't faded down the centuries.
I suppose we should ask a welder about the permanence or otherwise of the dark- glass tinting. It's my bet that the tint is permanent and won't bleach.
If gradual bleaching out of the tint occured, a novice or casual welder might not be aware of bleaching until it was too late to avoid retinal damage.