The night sky brightness in magnitudes per square arcseconds is loosely proportional to the logarithm of (distance-squared/population), the proportionality constant being close to two.
Vainu Bappu Observatory at Kavalur is 50 km distance from Vellore with a population of 2 lakhs. To get as dark sky near Chennai of population 50 lakhs, you will need to be 5 times farther or 250 km away! Kavalur is only 160 km away from Chennai which shows that there is significant contribution to Kavalur sky brightness by Chennai as well.
At 80 km from Chennai, the sky brightness would be about two magnitudes brighter than Kavalur. It would be 3-4 magnitudes fainter than inside the city. (I guess that straight line distance to the farm house from the centre of city may be less than 80 km)
Even at Kavalur, you can see the sky brightening towards north-east, the direction of Vellore. It is not surprising that people in the outskirts of Delhi could see the Delhi glow.
Most of the outdoor lights through as much light towards the sky as towards earth. Properly designed shades can give us darker skies even near the cities and save huge quantities of power.
With more of us looking at the sky, the humanity may learn to save power in addition to enjoying the grandeur of skies.
Tushar Prabhu IIA, Bangalore Tue Apr 8, 2008 7:03 pm
In response to
> Mrs. Ratnashree - Director > Nehru Planetarium > New Delhi > > Regarding the light pollution at our night sky watching place at > Chennai. we are glad to inform you that there is no light polllution > at all, as the place is about 80 km away from the Chennai, it is a big > farmhouse, near by no houses or human being but full of snakes and all > other type of insect, but excellent place for night sky watching. This > farmhouse is own by one of our TANASTRO member Mr.Shanmugasundaram, > who is extending a helping hand to all our members for the night sky > watch program. > > Thanks. > > T.S.Krishna moorthy > Tamil Nadu Astronomy Association > Chennai > >
Let me reply to this mail as I am the most frequent visitor to this farm house. The sky glow is quite high in south and and south east due to lights from Chennai, the northern sky is excellent as there is no town,picking up M51 with binos is quite easy, the best southern sky is at yelagiri where my friend amar saw LMC with ease on the scope
Suresh Mohan Wed Apr 9, 2008 4:24 am
I had estimated the sky brightness in Kavalur long ago perhaps 1980 - 81. I relish the lessons we had about this from none other than MKV Bappu himself. KK Scaria was with us. My notebook tells me that the B magnitude was 18.2 on a new mmon night, The exercise needs to be repeated; I am sure the light pollution has stepped in Kavalur too
Shylaja Wed Apr 9, 2008 2:54 am
Dear Shylaja Madam, When you had sent Akarsh and Me to Kavalur in June (to assist the college teachers in their project), 3 nights were all clouded up. The last and fourth night, skies amazingly cleared up! We were amazed to see the thickness of Sagi/Sco Milky-Way from there and density of stars, which we estimate to be equivalent to our observing site 70km North of Bangalore!
There was lots of light pollution (lp) at the horizons which "appeared" to be very dark. I feel the skies there will be 6.0 mag naked eyes now with some background lp. Maybe if we get a chance to see Kavaur skies once again, we can quantitatively estimate it once again. Thanking You.
Amar Wed Apr 9, 2008 10:47 am
Tushar Prabhu wrote: > The night sky brightness in magnitudes per square arcseconds > is loosely proportional to the logarithm of (distance-squared/population), > the proportionality constant being close to two. >
That was an interesting post. However, you didn't mention the additional constant in the relation: Magnitude \propto log (distance-squared / population) + c. In other words, what units should the distance and population be measured in? Assuming it to be only a scaling relation, works of course. Quantifying and verifying things is fun!
> Vainu Bappu Observatory at Kavalur is 50 km distance from Vellore > with a population of 2 lakhs. To get as dark sky near Chennai of > population 50 lakhs, you will need to be 5 times farther or 250 km away! > Kavalur is only 160 km away from Chennai which shows that there is > significant contribution to Kavalur sky brightness by Chennai as well.
I agree: what this shows is that there is a more significant contribution to light pollution at Kavalur from Chennai than from Vellore.
> At 80 km from Chennai, the sky brightness would be about two magnitudes > brighter than Kavalur.
Please correct me if I'm wrong, but I think it would be 2*k*log2 magnitudes brighter, where k is that proportionality constant in the first relation you mentioned. If k = 2, this gives a magnitude difference of 1.2.
> It would be 3-4 magnitudes fainter than inside > the city. (I guess that straight line distance to the farm house from the > centre of city may be less than 80 km)
Yep, I concur. Assuming the radius of the city to be 8 km (R = sqrt(area / pi)); area = 181 km: wiki). Assuming Delhi to be a city of radius 22 km with a population of 17 million, the sky brightness here should be 3 magnitudes brighter than in Chennai. The sky at Kavalur should be five magnitudes fainter than Chennai (assuming that the population of Kavalur is much less than that of Chennai). So all in all, the sky brightness in Delhi should be 8 magnitudes brighter than at Kavalur. Goodness! What have we done to our skies?
Also, this would be an underestimate, wherever the city has a higher population density than the average, such as in the "heart". Source of all data is the Wikipedia articles on Delhi and Chennai.
>Even at Kavalur, you can see the sky brightening towards north-east, > the direction of Vellore. It is not surprising that people in the > outskirts of Delhi could see the Delhi glow.
Hmm, I've never been to Kavalur, but from this, shouldn't Chennai be having a larger effect?
> Most of the outdoor lights through as much light towards the sky as > towards earth. Properly designed shades can give us darker skies even > near the cities and save huge quantities of power. > With more of us looking at the sky, the humanity may learn to save > power in addition to enjoying the grandeur of skies. > > Tushar Prabhu > IIA, Bangalore
I was just remarking this to my friend during the skywatch we had on Sunday - the Delhi metro has set up these huge poles and put dazzling halogen lamps on top. They shine directly into our eyes from over 750m away. We could read by their lights - while counting stars for Taare Sadak Par. It's crazy.
Tanmoy Lashkar Wed Apr 9, 2008 4:17 am
Assuming that each city emits certain amount L of light per person, and a fixed fraction of this gets scattered from the sky, and using inverse square reduction of light, one obtains sky brightness in magnitude per square second of arc (mu) equal to -2.5log(L*P/D**2 + B) where P is the population, and D the distance and B the brightness of the night sky from naturals sources. When B is much smaller than manmade light, we get the sky brightness mu = const+2.5log(D**2/P)
There is considerable spread in this relation (+/- 2 magnitudes) due to the differences in lightning, aerosol content and altitude of the location with respect to the city, etc.
Late Dr Syuzo Isobe, who was Chairman of IAU Commission for Preservation of Astronomical Sites, had determined the zero point to be around 16.5 magnitudes per square second of arc when population is counted in the units of 10000 persons and distance in km. Note that the sky can be this bright even for a small town if it consumes same amount of power per person as bigger cities, and if we continue to send half the street light up into the sky.
Kavalur has been slightly on the brighter side because of its lower altitude and higher aerosols due to its tropical location. Yet, it is indeed the best site for astronomy south of Bangalore. Prof. Vainu Bappu who located it had told me that he had indeed considered the Yelagiri hills first. However, the lighting at Jolarpettai Junction and developing towns of Vaniyambadi and Tirupattur on either side made him go a bit deeper in Javadi hills between Alangayam and Jamnamattur. Yelagiri hills are now also a tourist destination, a phenomenon which is making many of Indian hills unsuitable for astronomy. One may contrast it with some Japanese public telescope sites which are tourist destinations that preserve qualities for astronomy.
Glad to see that you tried to get more quantitative than me. Use k=2.5 instead of 2 and you will be closer to reality. It will be interesting to check the actual values within our cities to feel appalled with evidence. There are various dark sky associations and you can consult their webpages to see the recommended shades for outdoor lights which can make enormous difference. It will be great if the city of Anil Agarwal and Sunita Narain becomes the first to adopt this.
Paul Murdin had set some standards for IAU in terms of ALCOR (Astronomical Lighting COntrol? Regions for optical observations) according to which our cities may be unsuitable for even casual viewing of bright objects. In terms of difference in magnitudes between normal lighting and curfew (or major power failure for us?), and in brief, the classification is: Class dm astronomy 0 4 casual sky viewing of bright objects 1 2-3 casual sky viewing and amateurs with up to 30cm telescope 2 0.7-1 amateurs and students, 0.5-1m telescope 3 0.4 academic work with 1-m class telescopes 4 0.2 observatories of international standing 5 0.1 world-class sites
--- Tushar Thu Apr 10, 2008 5:46 am