NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Basics of computing sunrise/sunset
From: Frank Reed
Date: 2009 Jun 18, 20:48 -0700
From: Frank Reed
Date: 2009 Jun 18, 20:48 -0700
You wrote: "which is what telescopes or binocs do: they gather more light radiation and concentrate it into the fovea. " That's not really what telescopes do. If you look at the Moon in the night sky for example, without optical aid, a small image of the Moon is formed on your retina, and naturally if you're looking right at it, that image is on the fovea. When you look through a telescope at the Moon, that image on your retina has the SAME brightness per square millimeter, but the image is larger in proportion to the magnification, and only a portion of it falls on the fovea --you have to "scan around" to see the different parts of the Moon. But the key principle here is that there is no more energy per square millimeter than when you look at it without optical aid. The brightness of an astronomical object per unit solid angle is not increased by purely optical systems. The image is enlarged. This increases the TOTAL energy striking the retina since the area is larger, but the brightness per unit area of the retina is the same. So there's no mistaking, I don't wish to imply by any of the above that looking through a telescope at the Sun won't hurt your eyes. It will! The point is merely that the mechanism cannot be by concentrating more light on the fovea simply because telescopes don't do that (and can't do that). I can't recall the mechanism right now but clearly if the whole retina is getting intense light instead of just one tiny patch receiving light of that same intensity, there would be a much more rapid rise in temperature. By the way, it's also true that the brightness per unit solid angle cannot be increased by getting closer to an object. If you're standing on the planet Mercury, the Sun's light is just as intense as if you're standing on one of the moons of Neptune. The difference, of course is that the total angular size is smaller out by Neptune in proportion to the distance squared. If you hold up a sheet or metal foil with a tiny pinhole in it and look at the Sun from either vantage point, it would look just the same. The apparent "surface brightness" doesn't change although the total energy naturally does increase. Similarly, if you've ever looked at the Orion Nebula through a telescope and wondered what it would look like from "close up", you needn't wonder; you've already seen it. It would be larger in apparent size in the sky if you were closer to it, but that faint gossamer light would have the same apparent surface brightness (brightness per unit solid angle) at any distance from the source. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---