Our night sky

As the sun sets in Cambridge, Chris Smith steps outside the studio with Paul Fellows to examine the state of our night sky...

Paul - Well, sadly, we can't really see very much and it's getting worse here in Cambridge and elsewhere in the country. The amount of light scattered up from all these artificial lights, the street lights, the office lights, cars, everything, is just hitting the sky and bouncing back to us.

Chris - Can we actually see any stars?

Paul - At this precise moment, I've only just come outside, but no I can't. Even if I look where I know they ought to be...

Chris - There's one look, straight above your head.

Paul - Oh yes, you've spotted one before I did. That's Deneb in the constellation of Cygnus - well done!

Chris - Have you ever seen the Milky Way?

Paul - Oh yes, definitely! And we do see it on a good night here in Cambridge. Just right above your head at this time of year - near that star actually, it goes through the constellation of Cygnus.

Chris - But how old were you when you first saw it?

Paul - Oh I should think I was probably in my early teens; thirteen something like that when I first saw it. And I lived down in Portsmouth, which is a fairly big city and you could see it then because the skies were so much darker.

Chris - But not today?

Paul - Not today, and very rarely here in Cambridge these day, unfortunately.

Chris - I must admit, I probably had the same sort of epiphany that you did when you first saw it in your teens, but it took me until my late twenties. It wasn't until I went to Australia and was knocking around in the outback and staring up at this amazing sky and I suddenly realised what I'd been missing for almost the first three decades of my life.

Paul - Yes, I had the same idea. I went up to the Isle of Arran in Scotland, off the coast, away from all the lights and I was lost. I mean I know my way around the stars but there were so many and the Milky Way went right from one horizon above my head over down to the other side, and I was speechless.  

Chris - Now explain this to me. We're surrounded by street lights here and we can look at the impact in terms of objective measurement of that in a minute, but why should a bit of light here on the ground make a difference for our ability to see stars up there in the black void?

Paul - Well there are several reasons. Firstly, the light leaks from where it's supposed to go. It either goes directly straight up into the sky or it hits the ground and then bounces back up and it's scattered all over the place, and then it hits the dirt, and the dust, and the particles in the air and is bounced back to us again. So we see the sky as bright, it's not actually black.

And the second reason is to do with our eyes. Our eyes accommodate to the darkness by the pupil opening up and by the chemistry in the eye adapting to be more sensitive, and it takes about twenty minutes in full darkness for that to happen. But, if you don't have full darkness it'll never happen, your eyes, the pupils will stay small and you won't see any faint objects at all.

Chris - Well I've brought a newspaper out to see if I could read the newspaper. I thought I'd bring the Sun because that sounds suitably astronomical. So I can clearly here, apparently on this dark night, read every word on that page, certainly the headlines and even the smaller stuff. But I mean being able to read a newspaper under plain sight, that says quite a lot of light pollution doesn't it?

Paul - It does really and, of course, we're standing here and I can see you in colour and that means the colour cells in my eyes are perfectly happy and the rods that do the really sensitive work are shut down. So we've got no chance of seeing anything faint like the Milky Way in the amount of light we've got here.

Chris - How do we actually quantify the degree of the light pollution?

Paul - Okay well, there's a very useful scale called the John Bortle scale invented in 2001 and it runs from nine to one.  One is the best perfect conditions which we just don't ever get in England and nine is the centre of London or some big city like that where you can really only see the brightest objects like the planets, the moon and maybe one or two of the brightest stars.  Here in Cambridge we get about six which means we can occasionally see the Milky Way directly above our head and many hundreds of stars on a good night. If you go somewhere like the outback of Australia you might get down to a three or two and start to see other phenomenon, such as the zodiacal light which is actually dirt and dust in the plane solar system being illuminated by the Sun, and you see that just at the horizons coming up in the plane of where all the planets are.

Chris - Do you think in the grand scheme of things Paul this really matters?  As in we're not going not do astronomy we can just go somewhere like you're saying where it is dark and do the astronomy.  Does it really matter that we're polluting the night sky?

Paul - I think are a whole series of reasons, both astronomical and financial really. We're wasting energy by ruining the night sky with all this stray light if somebody's paying the electrical bill to do that.  But from an astronomical point of a view, it's a great way that kids get into science.  I run the cambridge young astronomers programme and we have kids from seven to eleven every Saturday once a month.  Some of them have gone onto get their PhDs now. It's a great of way of getting kids into science.

Chris - Is that what happened to you?

Paul - Absolutely, my first experience of looking at the night sky was with a small telescope my dad bought home and we pointed it up at the sky and said "what's that object there?" and "wow"  it was saturn with its rings.  So the next thing I did was build my only telescope.  Slightly bigger, actually bigger than I was at the time and that lead to a school prize and a trip to see Patrick Moore.  Tea and telescopes at his house with the great man himself.  

Chris - Patrick Moore invited you over?

Paul - Yes, I went round and he came bounding down the driveway in an old t-shirt and welcomed me and I spent hours there looking around all his telescopes and observatories that he had in his garden in Selsey down on the south coast and listening to him talk.  It was fantastic, I was completely hooked.

Chris - And I suppose in those days there wasn't too much light pollution was there?

Paul -   No, even there, he had a pretty good view.