- A Beginner's Guide To Stargazing
- Dark Eye Adaption - How We See In the Dark
- Light Pollution
- Using Star Charts and Measuring Distance
- Top Tips for Binocular Astronomy
- Moon Watching - How to Observe the Moon
- Buying Your First Telescope
- Your First Night With Your First Telescope
- Sky Orientation through a Telescope
- Polar Alignment of an Equatorial Telescope Mount
- All About Telescope Eyepieces
- Useful Astronomy Filters for Astrophotography
- How to Photograph Constellations
Before observing, you must give your eyes time to adjust to the darkness. This is called dar eye adaption. You may have noticed that your pupils are larger in dark conditions and smaller on a bright sunny day. This is the way the eye controls how much light to let in - exactly the same way as the aperture works on a camera lens. In dark conditions the iris opens and the pupil becomes larger to let in more light, while in bright conditions the iris closes and the pupil becomes smaller to limit the light coming in and preventing you from becoming dazzled. This is actually only a small part of what your eyes are up to, and the process of getting used to the dark is called Dark Eye Adaptation.
Your eyes adapt to whatever the prevailing lighting conditions are. Let's take an example - a room at night with the lights on. It all looks fine because your eyes have set themselves to work in whatever light there is around. Now turn the lights off and the first thing you'll notice is that the room appears almost black for a short time. Your eyes, sensing the lack of light, have gone into dark adapting mode - your pupils grow to let in more light and then the all-important chemical changes begin to switch on the low-light-intensity 'rods' which fill the backs of the eyes. This process is Dark Eye Adaptation and it actually takes around an hour, but a good proportion is complete within 10 minutes or so.
In order to see the best of the faint night sky, shield your eyes from all bright lights for a good few minutes before you start stargazing. You should also make sure you cannot see any bright lights, such as street lighting, while you are observing. If you need to use a torch whilst observing you can get special red light torches that will help preserve your night vision. Remember - Dark Eye Adaption can be ruined by just one glance at a bright light.
How Does Dark Eye Adaption Work?
Your eyes gather light by means of rod and cone receptors on the back surface of the eyeball. The cones are responsible for seeing fine detail and colour vision in good light, while the rods allow us to see more effectively in dim lighting conditions. Cone sensitivity adjustment takes around 10 minutes which is why we are able to see reasonably well quickly in the process, but with less detail. You may notice your night vision being "fuzzy". The rods take longer to increase their sensitivity, say 20 to 30 minutes or so, and once they have adapted you should be able to see details and your night vision will be less fuzzy.
The 30-minute time course for adaptation is about how long it takes for the Earth to shift from twilight to the darkness of the night.