Tuesday, November 11, 2008

evening sky and science myths

I love the change to standard time in the fall, because it means that I end up making my trip home from work or running errands right about sunset, and I get to observe the sky.

Right now we have two brilliant evening "stars" or actually planets, Venus and Jupiter, that are brilliant gems in the west/south/west in the early evening sky. These are not always visible from my home, deep in a holler with hills surrounding us, but at this time of year I can get wonderful glimpses of brilliant Venus and Jupiter on the trip home.

The past two days, I've been taking a (long) detour from work to home, over the mountain to Virginia to the Pound Veterinary Hospital where Nino the newly diagnosed diabetic cat has been staying. The one major plus of the trip is the spectacular views from both the Kentucky and the Virginia side of Pine Mountain, and at sunset the view is particularly stunning.

Yesterday, on my way to the vet, in the late afternoon light, the moon waxing towards full (but not quite there) hung low in the sky just above the edge of the mountains and seemed huge (I think the old saying is "as large as a dinner plate"). Later on the trip home after the sunset, the moon seemed so much smaller. I have often observed this phenomenon, and always assumed that it had something to do with the "magnifying" qualities of the atmosphere. But, lo I was wrong.

I googled "why moon looks bigger" and learned that the atmosphere has nothing to do with it. Moreover, I learned that if one were to use some objective measuring device, the visible surface of the moon is identical is size at the horizon and higher in the sky. It is only to the human eye and human brain that it appears larger. Our brain believes that things on the horizon are further away than things straight over head. For example, a cloud over our heads is nearer to us, than a cloud on the horizon (which could be as much as hundreds of miles further away). So our mind plays a trick on us, it says, hey, that moon low on the horizon just above the trees is further away than the moon straight over our head, and so our mind mentally adjusts the size of the image so that we think the moon is larger. When it is overhead we think it is nearer so our mind adjusts the image smaller. But of course, the moon is the same distance from us, whether it is above us or at the horizon.

This explains, finally, for me, why I will see the moon just above the tree tops and think "wow! how huge that is" and rush in to get the camera. But when I see the photograph, suddenly the moon seems so much smaller. That's because the lens of the camera is not fooled like our brain is.

Two neat websites that explain this with diagrams are:


Stacy (mama-om) said...

Wow! Thanks for that explanation. And thank you, too, for visiting my blog last week.


Billy said...

I almost majored in astronomy many years ago and remain fascinated with the discipline and am planning on a very large Celestron telescope in the near future in order to do stellar photography. I love when Jupiter and venus hang in the sky like jewels.

Qaro said...

Wow, really? I'll have to try getting the camera!