Humans like light. We are creatures that rely heavily on what we can see to feel safe and secure. Not surprisingly, our eyes function at their best when plenty of light surrounds us.
Remove that light, however, and we are blind as the proverbial bat. Try it yourself at home tonight — turn all the lights off and you won’t be seeing an awful lot. But with patience, something almost magical happens, slowly your eyes start adjusting to the new darkened environment.
Understanding how this works is key to getting the most out of looking at the night sky.
Before light actually gets ‘detected’ in our eyes it travels through a layer called the cornea and passes through a tiny hole called the pupil, an aperture that gets adjusted by the iris.
The adjustment of the size of the iris is controlled by the amount of light available; if there’s a lot of light then the iris closes down letting less light in, but if light levels are low then it opens up or dilates to let more light enter.
This process takes just a few seconds and you can see it for yourself if you get close up and personal with someone. Get them to shut their eyes in the daytime for a few seconds then watch the pupil shrink as their iris closes up.
Once the light is in the eye it passes through the lens and into the gel filled portion of the eyeball called the vitreous body before hitting the rods and cones on the retina. The key to dark adaptation is found in the rods and cones themselves, which are the parts of the eye that detect photons of light.
During daylight vision when light is plentiful it is the cones that are doing most of the work but when light levels drop, the rods take over. While the eye is immersed in a world of darkness, the rods respond and change chemically to become even more efficient. Unlike the almost instant response from the iris in dilating, the chemical change in the rods can take up to an hour depending on the individual and the environment.
To get the most out of observing the faint objects in the night sky it is imperative that you give your eyes the chance to adjust to seeing in the dark. It is equally important to protect your new found night vision that you have just patiently waited for as exposure to bright light for even a short burst can instantly switch your eyes back to daytime mode.
There are occasions where astronomers do need a little illumination perhaps to read star charts or adjust equipment and for this a red torch is essential. Red light is the least energetic of the visible light spectrum and the one that doesn’t harm dark adaptation too much.
You will be amazed how much more you can see through a telescope or even just with your naked eyes if you give them a chance to adapt.
Your patience will be rewarded.