Transcript

INSTRUCTOR:

By now, I hope you've had some clear weather, and been able to get outside and start identifying some constellations. Now if you're in a built up area, you've probably found that you can pick out the main stars of whatever constellation you're observing, but it's quite difficult to make out more details.

And to do better than that, you need to get away from the street lights and any surrounding bright lights and find yourself a dark site that's safe to observe. Perhaps out in the countryside where you're away from the city lights. And then immediately, you'll be able to see more details.

If you've spent any time outside in the dark, you'll also probably have noticed that initially, you can see just the brighter stars. But after a short period of time, you'll find that you can start to see more and more. And what's happening is that your eyes are becoming more sensitive to the light. They're becoming dark-adapted. And there are two changes that take place.

First of all, the pupil in your eye reacts to light. It expands and contracts. And if you're spending time in a dark location, the pupil in your eye will expand to admit more light. That happens quite rapidly. And then over the next 10 or 15 minutes, your eyes will gradually become more sensitive to the light as the retinas in the back of your eye develop more sensitivity. We call it dark adaptation.

For this reason, once you begin to understand how your own visual system works, then you'll be able to see more and more. And as astronomers, we're careful to preserve our night vision, so we'll always use a dim red torch to look at our star charts. We won't use any bright lights, because then we'd have to spend more time redeveloping that dark adaptation.