Albedo 0.1 Question

[deepblue]

Here's a question which I'm sure has a simple answer but thought it might be fun to ask.

The moon has an albedo of 0.1 meaning it reflects 10% of the light it receives from the sun.

In the sky, the moon and the sun occupy roughly the same area.

There are obviously lots of warnings about not viewing the sun through a telescope.

Why are there not similar warnings for viewing the moon? If it reflects 10% of the light is receives and occupies the same area in the sky as the sun, then surely viewing the moon at 100x magnification is equally as dangerous as viewing the sun at 10x.....???

[[1ponders] (Paul)]

The biggest difference DB is that the moon doesn't reflect the UV and IR radiation. It isn't the visible light of the sun that is the biggest problem (though it would certainly do damage in it's own right) it is the concentrated IR and UV radiation that would be the biggest danger.

[jjjnettie (Jeanette)]

Good question DeepBlue.
Thanks for the answer Paul.

[bojan]

There was another question within a question... And the answer is below :-).
Magnification does not add to the surface brightness of the extended objects, quite the opposite.
So, watching Moon with 100x is actually 100 times safer than watching the moon with 10X, because the apparent surface at 100x is 100 times larger (square law) than at 10x, and the amount of light is the same (provided the whole Moon can fit in the 100x eyepiece field of view.

[bojan]

Some more info on this topic can be found here:
http://www.asterism.org/tutorials/tut26-1.htm

[bojan]

Another point:
Someone may say OK, so I will take a neutral filter with 10% transparency (dark sunglasses) and the day should look like a night with full moon above hozon... and this is clearly not so.
Albedo of whatever (0.1 for Moon) means that the incoming light has been reflected from the surface with 0.1 attenuation in ALL directions, not only toward us. That means, the surface brightness of the Moon is NOT 10% of the surface brightness of the Sun, it is much, much lower. It will be ~20% of the surface brightness of the earth surface, roughly.. because Earth albedo is ~50%.
Take that into account and you will see that watching the Moon and Sun are two VERY different things in terms of amount of received light involved (or in terms of surface brightnesses involved)

[deepblue]

Many thanks to Bojan and 1ponders for your explanations. I must think up a harder question next time !!