I’m not sure if this was asked in the plethora of eclipse threads, so I apologize if this is a repeat:
From our perspective on earth, the sun is just slightly larger in the sky than the moon, which presumably creates the famous and dangerous ring of light during a solar eclipse. What would happen if the moon a was slightly larger or closer to earth (or if the sun was slightly smaller)? The moon would then completely cover the sun. Would we still have the potential of figuratively burning our eyes out if we stared directly at it? Any other differences on earth with other size differtials between the 2 celestial bodies?
Okay… usually, from our perspective, the moon is slightly larger than the actual body of the sun. This is what allows a total eclipse. As I understand it, the moon is never larger than the apparent size of the sun with its radiative atmosphere, (the corona,) which you can see during a total eclipse. If that were possible, then you could stare directly at the moon without any danger of eye damage because there would be nothing brighter than stars or planets in that part of the sky.
Sometimes the moon is further away from earth and visibly smaller than the sun in the sky. If there is an eclipse at this time, then it will never be a total eclipse, and may be an “annular” eclipse, in which you can see part of the body of the sun on every side of the sun - a very dangerous ring of light, as it’s as bright as in any similar partial eclipse. Monday’s eclipse was not an annular eclipse.
During a total solar eclipse, the Moon is larger than the Sun (by angular measure) and thus completely covers the surface of the Sun … during this time it is safe to look directly at the eclipse …
During and annular solar eclipse, the Moon is smaller than the Sun and thus the surface of the Sun is visible and can cause severe eye damage to look at directly …
During a total solar eclipse, the Moon is larger than the Sun (by angular measure) and thus completely covers the surface of the Sun … during this time it is safe to look directly at the eclipse …
During an annular solar eclipse, the Moon is smaller than the Sun and thus the surface of the Sun is visible and can cause severe eye damage to look at directly …
And everything under the sun is in tune
But the sun is eclipsed by the moon.
Only the partial eclipse and annular eclipse are dangerous. During total eclipse it’s perfectly safe to stare at the corona surrounding the sun without any filters. An annular eclipse is similar to a total eclipse, but happens when the moon is just a bit further away from earth, so it doesn’t cover the whole sun.
If the Moon were larger, a total eclipse would be more common, and the total eclipse path will be wider when it happens. It will still be safe to stare at it. But it will be somewhat less impressive, because the prominences and inner corona will be hidden by the Moon, except at the very beginning and end.
Googling suggests that when viewed from the Earth’s surface, the angular size of the Sun ranges from 1891 to 1953 arc-seconds, whereas the Moon’s range is 1760 to 2046 arc-seconds.
The Moon’s distance from earth increases at a rate of around 1.5" per year, which means that eventually there will be no more total eclipses - all will be annular. Moral: don’t miss any opportunities to see a total eclipse.
Thanks. I assume this nuance between total and annular solar eclipses must be a rarity in the universe, only made possible by the coincidence that the moon and sun are so close in size in our sky.
Would the harmful effects on the eyes from an annular eclipse be reduced if the moon was 1/2 its diameter. When would they go away completely? Would I damage my eyes in a similar manner if I simply held up a quarter or silver dollar so that only the corona of the sun was visible?
Not an expert, but
I think the harmful effects would be greater if the moon were smaller, since more sunlight would slip around the edges.
Yes, holding up a quarter to block out the sun is most certainly a bad idea
If you just hold up a quarter, you’ll protect your eyes (provided you can hold the quarter steady enough), but you won’t be able to see the corona, because the scattered sunlight (what makes the sky blue) will be enough to wash it out.
A partial eclipse of any sort will not decrease the severity of damage to your retina, but it will decrease the area that the damage covers.
Definitely not - smaller moon blocks less sunlight, so more reaches your eye.
When the moon’s apparent size is large enough to fully cover the sun - as was the case along the path of totality during Monday’s eclipse.
If what you hold up entirely covers the sun such that no direct sunlight reaches either eye, you are safe. That’s easy to do by holding up your outstretched hand; not so easy with a coin.
But (as Chronos notes) you wouldn’t then see the solar corona: it’s washed out by sunlight scattered by the Earth’s atmosphere. If you were at a place with no atmosphere between you and the sun (e.g. in orbit), this would work.
And to extend that, there are satellites in orbit that do observe the corona, and they do it by, more or less, “holding up a quarter” (though in practice, the actual obstructions they use are larger than a coin).
The eye moves quickly about any scene you are inspecting, to bring the best part of the retina (the fovea) to bear on every interesting part of the image. If the gaze remains too still for too long, the receptors and ganglia begin to see “no change” and send fewer signals to the brain. The brain stitches the image together into what you perceive. This is true of even a steady gaze, but normally it is too rapid to detect.
I’m pretty sure that you’ll fry your fovea if you look long enough, no matter how partial the eclipse.
"Mama always told me never to look into the sights of the sun. ‘But Mama, that’s where the fun is!’ "
How big was the moon relative to the sun when Alley Oop saw eclipses (say 65-100 mya). Was the difference perceptible? How far back do you have to go to have a moon twice as big as the sun?
Someone said that this rarity is the basis for a sci-fi story that aliens travel to earth to view such eclipses. Looking around at the crowd during totality, that might have been true.
Does anyone know the name of such a sci-fi story about alien travel to a viewing? It sounds like something Dr. Who would do an episode about.
There is in the core of this question also this idea of the “harmful ring”. The ring of sun seen is only harmful because if it is thin enough you won’t trigger the eye’s reflex to look away from the sun, and thus you can look at the sun without discomfort, although it will still burn the retina - as the ring itself has just as great surface brightness as before the eclipse.
So any eclipse - even partials, annular eclipses, or total eclipses during the partial phases, are dangerous because there are times when you can look comfortably at the sun and your eye may be damaged. This is really all there is to it.
There are apparently a couple of moons of Saturn where the geometry works out well enough: Pandora and Epimetheus. But Pandora is elliptical enough that it doesn’t cover the whole solar disk, and both of them are moving so fast across the sky that the eclipse totality would only be a fraction of a second. (Don’t forget that the Sun appears a lot smaller from Saturn as well.)
And, of course, it’d be hard to find a place to stand to view these eclipses. Saturn is a gas giant and its “surface” is really just the tops of its clouds.
Yeah, as soon as the totality ended and the “diamond ring” appeared I slipped my glasses back down, then decided to glance at it naked-eye anyway, and it was still blindingly bright even at something like 99.95 percent coverage.
I was at DFW (Dallas’ main airport) during the later stages of the eclipse. Which peaked at ~80% there. This was awhile after the peak. A bunch of people were looking at the sun through a skylight wearing dark glasses. The general scene outside was a normal cloudless sunny day; there was no evidence of reduced total sunlight.
Using the same technique I’d use to look at a full sun I whipped my head and gaze quickly across the sun. Which was, of course, blindingly bright and slightly painful just as it would be any other day. But I had the distinct impression of having seen the yellow disk with 1/3rd to 1/4th of it missing. And no harm to my vision. Just like any other day.
A few minutes later I got to go outside and borrow somebody’s eclipse glasses. I looked up at the sun and saw the same shape I thought I’d seen.
