Ever stepped outside into the dark from a brightly lit room and realised that you are blind as a bat? Ever noticed that after a while being in that dark environment that actually, you really can see where you are going and don’t have a need for a torch? This is your eyes getting adapted to seeing in the dark.
Its like this you see, there are two things that happen when you plunge into darkness; first, the iris in your eye opens up as wide as possible to let as much light in as it can. This change takes no more than a minute. Along with this, there is a chemical change that goes on in the eye that makes it more efficient at seeing in the dark. This change takes a lot longer and can take up to 40 minutes to complete. Once it has, you are fully dark adapted and on all but the darkest of nights, you will easily be able to see what you are doing.
For astronomers, this is of crucial importance. Most of the things we look at are faint, sometimes even on the limit of what the eye can actually detect. To give us the best chance of seeing these faint objects we need to be fully dark adapted, otherwise they will stay hidden from us. You can see this simply by looking at the night sky. Look at it after going outside and you will see only the brightest stars, give it 40 minutes and you will be surprised how many stars there are! So to get the most out of astronomy, make sure you get dark adapted and stay dark adapted.
Unfortunately, exposure to bright lights, for even the shortest period of time can instantly destroy your dark adaption leaving you with another 40 minutes to get it back. If you do need some light and invariably you will, then red lights have much less impact to dark adapted eyes than any other colour so a red torch is a must. The rear red lights of bicycles are ok but still a bit bright, bright red lights cause as much damage as white. A much better solution is to buy a specialist red torch from an astronomy supplier