Though my soul may set in darkness,
It will rise in perfect light,
I have loved the stars too fondly
To be fearful of the night. –Sarah Williams
Everyone knows how you see things during the day: sunlight makes it possible. Delivering huge amounts of visible light to the entire “day” side of the planet, everything becomes illuminated to human eyes.
But things change rapidly once the Sun goes down. Even with a full Moon in the sky (like last night), the amount of light reaching the Earth at night is over 100 trillion times less than during the day.
Still, we have a great pair of eyes, and instead of narrow pupils with our cones (which see color) forward in our eyes, our pupils dilate, letting more light in, our cones retract, and our rods (which see monochrome in low light) become more prominent. As a result, we can see at night, just not as well as during the day.
But what about when the Moon isn’t out? Somehow, even in the absence of the Moon and Sun, when the amount of light reaching us from the sky is over a billion times dimmer than when the full Moon is out, humans can still see (at least a little bit), and many animals can actually see very well!
So how is this possible? How can we see anything at all, given that the amount of light out there is so mind-bogglingly small?
There’s a two part answer. The first part you’ll guess, and the second one you won’t. First, your eyes simply adapt. Something like grass in starlight, even though you would be completely blind to that signal in weak daylight, becomes prominent to your eyes at night. Over a trillion times dimmer than the brightest things you can see, it still appears clearly to the dark-adapted human eye.
Well, this is great for humans, who see right in the middle of the visible spectrum at night. Guess what? Not all animals do. In fact, many night-adapted animals can see infrared light. The greatest amount of infrared light reaching the ground on the night side of the Earth — at least far away from cities — comes from the Sun, not from the stars!
Could this really be true? Could the Sun really be affecting us at the darkest part of the night, impeding our ability to see the stars but allowing us to see the land around us? Here’s how: imagine floating in space on the far side of the Earth, away from the Sun. What would you see as the Earth appeared to move between you and the Sun? (Pay close attention to the atmosphere, courtesy of NASA.)
You see, our atmosphere reflects, refracts, and scatters light. The blue light scatters more easily, which means that the reddest light (and infrared light) makes it the farthest. In addition, the Earth absorbs visible light during the day, but re-radiates it as infrared light. This means that any animal that can see these longer wavelengths of light is better adapted to being a nocturnal or crepuscular creature.
In fact, those “night vision” goggles you can get are extra-sensitive to this infrared light, and can produce images like this, thanks in part to thermal heat signals and red light left over from the Sun!
And that’s not only why we can see as well as we can in incredibly low-light conditions, it’s why many animals can do it better than we can! Blame the stars, the Earth, and the Sun for the light that us and the other animals see at night, but the one that deserves the most blame depends on what your eyes can see!