Demonstrating the effects of Light Pollution

These following projects allows you to assess how light pollution effects you.

Star counting remains the most effective way of accessing how light polluted your observing site is. The brightness of stars is measured by the magnitude scale - the brightest star in the sky, Sirius, shines at magnitude -1.5, and the faintest stars detectable by the human eye shine at magnitude 6. A magnitude 1 star is exactly 100 times brighter than a magnitude 6 star. If this scale does not seem logical, it is because it isn't! It has randomly evolved over many thousands of years.
3000 stars are brighter than magnitude 5.55. The human eye has a field of view of about 2% of the entire sky, so you should be able to see 60 stars without moving your head, and if you look through a toilet roll tube you should, on average, see over 5 stars (although, of course, this depends on the direction you are looking). If you live under light polluted skies, you will be lucky to see 10% of these stars (a limiting magnitude of 3.6).

Here is a table of the total number of individual stars visible at different limiting magnitudes (strictly speaking, the number of resolvable stars; billions of stars are visible in the core of the Andromeda galaxy, which shines at magnitude 3.4, but you cannot resolve individual members). As you can see, the number very roughly doubles for every half a magnitude.

In addition:

• The Milky-way is just visible, with a limiting magnitude of 5.0
• At a limiting magnitude of 6.0, the Milky-way is obvious, Zodiacal light is just visible, and the Andromeda galaxy now looks like a galaxy
• If you can see stars fainter than 6.0, then you can view detail in the Milky-Way (i.e. dusty regions in our galaxy appear dark in front of the Milky-Way) which in this darkness also helps light up your path. Zodiacal light is clearly visible. Well known constellations become less obvious, due to the large number of faint stars now visible. Sporadic meteors are frequently visible.

And finally, to estimate the brightest stars you can see you will need to obsever in the best possible weather conditions. A full Moon in sight, or the presence of thin cloud, will all subtly limit the number of stars you can see, even if there is no light pollution present. Also, you could have no light pollution with the exception of one nearby light; make sure such lights are not in your field of view.
I recommend repeating these experiments a few times, until you truely do find the faintest star visible from your site!

How the human eye works: Pupil Dilation Projects

The main problem with light pollution is that the back-scattered light does not allow your eyes to become adjusted to the dark. Here are a couple of experiments that show how the pupils in your eyes work! Projects 1 to 4 look at how much time it takes the pupils in your eyes to adjust to darkness, and project 5 looks at how the pupil shrinks in daylight conditions.

I have given many public talks in the University of Leicester planetarium, which has a very faint Milky Way projector. It usually takes over 5 minutes for visitors' eyes to adjust to the levels of darkness enough for the Milky Way projection to become visible (this is much more rapid for younger visitors; often children have seen the Milky Way before I have!). This is because it takes time for your pupils to dilate to their maximum size. Your pupils dilate to their maximum size asymptotically - that is, as you look from a bright area to a dark area, your pupils expand, and continue to expand but at a continuously slowing rate. A sprinter provides a good analogy - the velocity of a sprinter increases asymptotically. As a race begins, the athlete accelerates rapidly over the first 5 metres, but their acceleration is not as rapid over the next 5 metres, and falls even further over the next 5 metres, until finally at 20 metres they are at maximum speed (no more acceleration) which they maintain to the finish line at 100 metres. Similarly, the diameter of the pupil in dark conditions continues to expand asymptotically for up to 30 minutes - it takes 30 minutes for the pupil to expand to its maximum! To take the analogy too far (?!), light pollution is a brick wall built across the running track at 3 to 5 metres from the start!

Here is a video of my eye (right, slowed down by a factor of 5), as done following project 5. I closed my left eye for about 30 seconds, covering it with my hand to make my eye get as dark adapted as possible. I then looked towards a window, looking into my video camera at the same time which was recording 15 images per second. Note! I was looking out through a North facing window, and I was looking away from the Sun. NEVER look directly at the Sun - such stupidity will blind you for life. It is also unnecessary; the key to this project is to get your eye as dark adapted as possible, and your pupil as wide as possible. Any natural light will make your pupil shrink. See also the warning below.

See how the pupil of my eye shrunk considerably within just 10 frames (about 0.7 seconds)! The images to the top left show the dramatic difference.

Light pollution forces the pupils in our eyes to contract (as in frame 14), to stop the light from blinding us. To see the beauty of the night sky in all its splendour, we need our pupils to be wide open, as in frame 4.

• Project 1

  You have probably done this experiment many times and not realised it significance. For this project you have to go to bed, in a dark room, and without street lights shining into your room!
  Turn off the light, and you stumble to bed, not being able to see where you are going! However, as you allow your eyes to get accustomed to the dark, you begin to see objects in your room. Five minutes later, when your eyes have adjusted to the dark, the shape of wardrobes, desks and chairs have all become clearly visible. And, after half an hour when your pupils have dilated to their maximum size, you can see everything quite clearly.
  Time how long it takes it takes for you to see certain objects in the room, and compare your time to other people's.
  
  

• Project 2

  Similar to the previous project, this is to be done at night, in a room with a light. With the light on, close or cover one eye such that it is in the dark, for about 30 seconds, leaving your other eye open.
  Now turn the light off, and rapidly switch between using your left eye only, and then your right eye.
  In the dark, you will be able to see far more detail with the eye that was closed for half a minute (i.e. your dark adapted eye), and much less detail with your other eye (the eye that was kept open in the light).
  There is a striking difference between what you can see with your two eyes, which lasts for about 2 seconds until both eyes become adapted to the dark.
  
  

• Project 3

  You need two people, a dark room and paper or card for this experiment!
  Cut out several different shapes out of pieces of paper or card (say, a circle, square, star and a triangle - make them about 6 inches in size). One person places the shapes in the dark room in a random order. Now, in total darkness, allow the other person into the room and see how long it takes their eyes to become accustomed to the dark enough to identify the order of the shapes!
  
  

• Project 4

  Count stars! Go outside, look straight up, and quickly count the number of bright stars you can see! Wait 5 minutes (while remaining in dark conditions), and then repeat the experiment. Unless you live under very light polluted skies, on the second attempt you should be able to see many more stars. Again, this is because your pupils are still dilating, and your eyes are adjusting to the dark.
  
  

• Project 5

  Close or cover one eye such that it is in the dark, for about a minute. Set up a video camera (webcams are the cheap and cheerful version) so that it is focused on your eye. Start videoing your eye, and open it. Notice how long it takes the pupil of the eye to re-adjust from the dark to normal light levels! The exact timing of the pupil dilation time (which occurs rapidly) can be done by counting the number of video/webcam frames that have elapsed before the pupil stops expanding. And plotting the diameter of the pupil against time shows how this occurs asymptotically.
  
  

  WARNING

  Normal daytime light levels will be sufficient to see your pupil dilate. There is no need to aim bright lights at your eyes - this will not make any dramatic difference to your pupil size and could blind you. For a more dramatic effect, make sure the darkness is darker, rather than the brightness brighter. Never aim bright lights into your eye. A few centuries ago, a once great astronomer went blind due to this very reason - he was stupid enough to look directly at the Sun (I think it was Galileo). Blindness is the worst handicap to an astronomer!
  
  

For more information about how the human eye works, take a look at the following links:

Focus on the cause & cure: lighting

These projects look at lighting - both the cause and the cure for light pollution

• Project 1

  Design your own lighting! This could be a good project for school children. Start by getting them to design some lighting. Then teach them about the need for well designed lighting using the tips and ideas contained in these pages. Then get the children to look for faults in their own designs, and encourage them to redesign their original ideas if necessarily. This not only teaches children about the need for dark skies, but will also teach them about the process of design and redesign - a process that occurs regularly throughout life.
  
  

• Project 2

  A simple one this one, requiring a map and a compass: what is the most distant light that is visible from your home? From many areas of Sheffield, for instance, street lights are visible right across the valley in which the city is built. Street lights can be seen for many tens of miles. And this highlights the absurdity of poorly directed lights - why is a street light many miles away trying to highlight my face? It is simply unnecessary!