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DiscussionsOnQuantifyingLightPollution

Taare Sadak Par

The following are 2008 discussions from the yahoogroup http://groups.yahoo.com/groups/Astronomy_Activities_2009

This archiving is to give an idea of the way the project has evolved and also keep all the useful discussions in one place.

Some thoughts that I shared with the Quest students during my talk at the Delhi University Science Centre, for involving just about everyone interested in the efforts towards quantifying light pollution from different regions have been uploaded here

http://nehruplanetarium.org/IYA2009/taare_sadak_par.htm

The writeup is rather hurried and so are the images. I will work towards refining all of these, but, most amateur astronomers would need no help towards carrying out naked eye observations and noting the magnitude of the faintest star visible.

We could also work towards a better help in this for the beginners - but, I do feel that just about everyone interested can contribute with data points for this quantification - which would have many local variations, of course, but, each of these data points would give us a beginning feel towards this quantification.

Is this a feasible excercise? Would it be possible to beg everyone to contribute data points to this Taare Sadak Par program?

Rathnasree, Sun Mar 23, 2008 11:43 am

I have a question about quantifying the light pollution: stars of different magnitudes will be found in different regions of the sky and very often the "light pollution" glow diminishes from the horizon to the zenith. How will we account for this effect? Simply saying that stars down to magnitude 4 were visible from a particular location at a certain time on a certain may not suffice, if we are spanning different regions of the local sky each time we make our measurement.

Tanmoy, Sun Mar 23, 2008 12:44 pm

hi,

This is an age old question. There were previous efforts like this one undertaken in the Manchester area of England. This is popular because it was done using a tool provided by the BBC and it basically, gave the group half-an-hour on its channels.

I think light pollution can be better prevented than detected and quantified. The above example worked on making street lights better and also reducing light emanating from stations and billboards. Since, many parts of India are now facing acute power shortage - Maharashtra, where the outer towns of Mumbai face upto 8 hours of powercuts, we might find it easier to communicate these light reducing techniques. Less light = Less power used?

I maybe wrong...but that's mostly because I'm an engineer and not a science student. I worry more about getting things done than worrying about how it is getting done :).

Pradeep Sun Mar 23, 2008 1:02 pm

Another thing struck me: The astronomical seeing or visibility of stars on a particular night depends also on the local weather quite strongly. On dusty days from the city, the day sky is a yellow-gray haze and the night sky is orange. On clear monsoon evenings, the air is crystal clear and one can see much fainter stars. The reports of visibility on any particular day from a given location will have strong systematic effects from the weather and dust as opposed to the effects of the true light pollution. How are we to account for this? Perhaps the maximum visibility from a given site be taken as the limiting effects of light pollution? If we are more determined, we can try to rope in local weather data, but that might make the exercise more complicated and I guess might make it lose some of its universality in application to sites all over the country. The analysis of the data will also be more involved!

Tanmoy Laskar Sun Mar 23, 2008 1:48 pm

Firstly, it would not make sense to combine data from different dates and seasons. The differences in sky conditions during different seasons, particularly for cities like Delhi, which have such drastic changes in weather conditions, are very large. So, one is looking for data points for given dates - a kind of an annual plot for a given location.

Even at that, a given date might give one limiting magnitude this year and a different one the next, for the same location - again depending on many local variations. However, these variations might perhaps be smaller than long term variations likely with either worsening light pollution or with any systematic improvements in case any action is really taken. We cannot be sure of that, but, we will not know until we make some measurements.

We might have just a few random measurements and no more than that, depending on the enthusiasm generated - but, I think, they would be useful thumb rule data points.

It would be good to have two measurements - one with a group of stars overhead and another with a group of stars towards the horizon, so that the differences can be noted. Again Horizon observatiosn might have many local and directional variations.

And finally, why quantify at all? I think, for one thing, to have a handle on long term changes - is anything making any positive difference, quantitativey? Or is something making a very bad difference -for instance the lighting that was installed at the Jantar Mantar would have made the limiting magnitude jump to negative values!

Another thing is that - such quantification will also help us better plan possible observations for people (from their homes and with no equipment) - what are the events that they can see and what would be missed by them. When people world over were writing in about naked eye views of comet Holmes - Delhi fared so badly over that - we did make some attempts to make people aware of this - but, numbers might make people realise this better - a comparison of numbers between Delhi and its outskirts.

These are my thoughts - Pradeep might have a point - this might be a huge effort to have a few thumbrule data points - but, I am personally finding that collecting these data points is a lot of fun learning!

Rathnasree Sun Mar 23, 2008 4:53 pm

we could have a form where the person takes data for 1 year and then calculates the Average (mean) magnitude. This would eliminate the errors caused due to seasons.

We could ask people to provide other details such as date and location so even if one person isn't willing to or cannot make observations throughout the year, we can average the observations of several people from the same area.

Another problem is that if people don't wait for their eyes to adapt to the dark, they will see much fewer stars. Maybe we could have educators from organizations such as S.P.A.C.E. who travel a lot to do observations from their various dark sites or member schools etc..

Avnish Sun Mar 23, 2008 2:20 pm

I'm trying to develop a format for an observation -the "data point" - much like the observation tables we make for experiments in school. This is what I have so far: 1. Date 2. Time 3. Altitude (zenith / horizon (E/W/N/S)) 4. Limiting magnitude seen in that direction 5. Cloudiness (some scale might be proposed) 6. Humidity (another scale: dry / moderate / wet - perhaps can be more clearly defined in some way)

"...Even at that, a given date might give one limiting magnitude this year and a different one the next, for the same location - again depending on many local variations...."

I think it the limiting mag. during a particular time of year will be more or less same over years, apart from systematics like new constructions (for e.g.)

"...It would be good to have two measurements - one with a group of stars overhead and another with a group of stars towards the horizon, so that the differences can be noted. Again Horizon observatiosn might have many local and directional variations...."

So there will be more than one entry in the table per observation to have a measure of variation across the sky. As a separate point, perhaps we could have a database of stars we will use? Pleiades / Orion / Scorpius... ? Ideally, we should have an asterism with varying magnitude stars and that spans a small region of the sky so that effects of variations with altitude above horizon are unimportant for a given entry in the table (i.e. for the given asterism). Examples are Pleiades (not too good - all stars look very bright to the naked eye and are very close too!) and Hyades (proabably a better example). Also, Polaris!

Tanmoy Sun Mar 23, 2008 5:06 pm

More thoughts:

We can ask the observers to provide their latitude and longitude (easily available from Google Maps). Given the date and IST (which can be converted to LST), we can determine the altitude of the limiting magnitude star reported. This might enable a 3-D view of the light-pollution levels for each location. We could try and write a simple algorithms/programs to make these calculations, which would be very instructive (IST-LST conversions are easy, but non-trivial, same goes for RA/Dec <=> altaz conversions) and might provide a visual representation of the data.

Any takers?

Tanmoy Mon Mar 24, 2008 1:57 pm

Tanmoy, all of the thoughts that you have been posting, are useful. I think, we may need to have two kind of observing pages - a very simple one for beginners - just looking at very well known constellations and listing the stars they see.

Another, more advanced page for the amateur astronomers - noting ambient conditions, giving Altitude and Azimuth when they can (can be calculated, of course), limiting magnitudes and more.

I am just now reading the S&T article at

http://www.skyandtelescope.com/resources/darksky/3304011.html

Perhaps the Milky way Voyagers can rate the Nainital skies - the city, the observatory office location, and then the region of the 40 inch dome - using the thumbrules in this article.

B.T.W. as the Moon has started rising well after sunset now, we can all take our first data points and post in the group, or place them somewhere on individual websites so that they can all be linked together from different locations. Although the skies seemed to contain a few stars - considering that I am in the highly polluted Delhi University area, I was quite disappointed that I could not see Furud (mag 3.02). But, I think, that is my middle aged eyes, I will ask someone younger to confirm :-) (that is another factor - age or eyesight of the observer). I will place my observations on my website and post with a link.

Tanmoy, could you prepare a rough format for submitting observations and place it in the files section of the group?

Rathnasree Mon Mar 24, 2008 2:17 pm

Here is an article that is useful for measuring skyfog using a DSLR http://www.pbase.com/samirkharusi/image/37608572/original

Suresh Tue Mar 25, 2008 3:24 am

Dr. Rathnasree,

A very nice pointer from Suresh, Samir's website is right on topic. I should have a few results by the end of this week, including our nearby dark site - Nuh.

Ajay Talwar Wed Mar 26, 2008 5:26 am

Dear All,

This is a very interesting topic and everything I read here sounds remarkable and ambitious already. I shall stick to the 'simple for beginners' part of light pollution measurements.

I don't know if you have heard of the 'Globe at Night' programme: http://www.globe.gov/GaN/

Every participants collect data on the number of stars visible in Orion using pre-drawn star charts with different numbers of stars, each corresponding to a magnitude limit. This is a very simple activity that is done by children, schools, families, etc. pretty much worldwide. They seem to compensate for local variations in visibility by having large numbers contribute.

This could be an interesting activity to involve children in with the added fun of being able to compare observations with what other children see in other parts of the world.

The annual global activity takes place in March - we've just missed it. But nothing stops you from taking the same star charts and running it across a region or the whole country on other dates.

There is also another 'Cornerstone Project' of the International Year of Astronomy who would be very interested in hearing about your discussions and the measurement techniques that are being so creatively discussed here: the Dark Skies Awareness programme:

http://astronomy2009.org/cornerstone-projects-mainmenu-80/dark-skies-awareness-mainmenu-87.html

I hope this helps.

All the best,

Carolina Wed Mar 26, 2008 10:39 am

Hello Everyone

This is to inform that I've uploaded some files in the group for help in the taare sadak par program under a folder by the same name. There is one Excel file containing a proposed observing scheme, which I have filled with some random sample entries to begin with. Also in the table I have included my own scale of cloudiness and light pollution which I have developed today. The scale is visible if you download the entire set of files in this folder onto your computer and open the "skycolour.html" file in your web browser.

This basically involves selecting a colour of the sky by moving a slider. There are two columns at the end of the table in the Excel file called "day time cloudiness" and "night sky glow". The day time cloudiness is a measure of the whiteness of the sky due to stratus clouds (those diffuse clouds that cover the whole sky). A value of zero is worst (grey) and 100 is best (deep blue). (For the curious this is constant hue and luminosity, but varying saturation.) An observation of this could be made during the day of the report, so that some measure of cloudiness can be made.

The night sky glow is less important and is simply a visual addition to your data point. This is the colour of the sky as you see it from your location at the same elevation in the sky as your report of limiting magnitude at the same time as you make your limiting magnitude estimate. (Again, for the curious this is constant hue and saturation, but varying luminosity.)

I spent the entire day learning html over the net, so I hope this is useful. Of course. I get to learn Javascript in the process, but it would be nice if the cloudiness scale can be put to use too :) If it is, one could think of hosting the webpage with the scale somewhere else and putting a link to it here, so that observers don't have to download the entire set of html files and scripts.

Looking forward to comments Tanmoy Wed Mar 26, 2008 12:59 pm

Tanmoy, I managed to view the sky colour html file. I think, this is a very useful tool. Can we upload it somewhere in such a way that people do not have to do all the downloading and so on and just use the sky colour bar?

Tanmoy, these are really very useful inputs, thanks.

Rathnasree Wed Mar 26, 2008 2:46 pm

Dear Rathna,

Sorry for holding it for a very long time.

One of the way to look at the light pollution estimation problem is to estimate ones limiting magnitude.

I suggest observers may follow the method suggested by International Meteor Organization.

http://www.imo.net/visual/major/observation/lm

IMO has identified 30 regions in the sky. What one need to do is to first identify a region (formed by 3 or 4 stars) and count number of stars visible in that region, Then using a table limiting magnitude can be estimated.

This is of course highly person dependent but 1. if a group of 10 or does this exercise then the lm would average out. 2. if a person does it consistently then personal equation can be found out and lm can be normalized. 3. We have developed a simple photometer that can be built for about Rs 500/- and can be used for this project.

I am in the process of standardizing the photometer and soon shall announce it.

Arvind Thu Mar 27, 2008 4:11 am

Dear Arvind, Thanks for these very useful inputs. The IMO method on the url you suggest, should definitely be a much more systematic and standardised method than any we could evolve ourselves. We will definitely use that method, use the digital camera method that was suggested a few days ago, the photometer method that you suggest - just about every method possible - and get a better feel for the observations that one needs to do, to contribute some useful data.

We have some very dedicated amateur astronomers from Delhi, in the group. Some amateur astronomers from Bangalore, Tamil Nadu, Gujrat and Kolkata are also present in the group and all of them are doing very good work - I am sure that we will have useful data collected from many regions. We should try and have more amateur astronomers from various regions also join the group. We also need more planetaria to join.

And finally, at a first glance, the IMO method will look a little intimidating to someone who has no acquaintance with the sky and has just joined here in this group to learn. We should have enough simple discussions in the group that would help remove that intimidation for beginners.

It might also seem that there will be a little bit of problem determining the boundaries and corner stars for each region, even for seasoned amateurs, when there is severe light pollution - until we all practice with many rough cut observations. But, No, from my memory of observing over the last few evenings and estimating relative angular distances between stars to figure out which star I was observing, I do not think there will be much of a confusion, once we do these observations regularly.

Tanmoy, let us try out the IMO method over a few days and then we might want to make some changes in the excel data sheet accordingly.

Rathnasree, Nehru Planetarium, New Delhi Thu Mar 27, 2008 6:16 am

Dear Rathna,

> And finally, at a first glance, the IMO method will look a little > intimidating to someone who has no acquaintance with the sky and has > just joined here in this group to learn. We should have enough simple > discussions in the group that would help remove that intimidation for > beginners. > > It might also seem that there will be a little bit of problem > determining the boundaries and corner stars for each region, even for > seasoned amateurs - until we all practice with many rough cut > observations. But, No, from my memory of observing over the last few > evenings and estimating relative angular distances between stars to > figure out which star I was observing, I do not think there will be > much of a confusion, once we do these observations regularly.

In fact on the same ground we strongly recommend and urge amateurs to take up this activity as this strengthens up ones oneness with the stars. The corner stars are the bright ones and are easy to identify.

Some amateurs might find this method a bit difficut to start but no-pain-no-gain.

Arvind Thu Mar 27, 2008 8:42 am

All the AAAD members planning for today's observations at Nuh, please do conduct the star count observations from a few of the regions that are suitable for today, from the IMO website that was mentioned by Arvind a few days ago.

http://www.imo.net/visual/major/observation/lm

Anurag will be making several copies of some relevant printouts from these charts, which would be useful for today's star count and giving it to the group starting from the Planetarium.

Every observer can check their own observations for accuracy by matching their observations from different sky regions. And then, everyone's observations can be tallied together to check the kind of differences that are likely to arise when different observers conduct the same observations. Today's observations are an ideal oppurtunity for bench marking.

Tanmoy, I think, that the sky colour indicator that you have made would be something very useful to be used along with the star counts. Many people will be taking their laptops to the observing site, please do take your code and the relevant files along so that this indicator can also be bench marked.

Rathnasree, Nehru Planetarium, New Delhi Sat Mar 29, 2008 6:50 am

Despite the poor weather in Nuh last night with dust flying everywhere, we made some preliminary observations for Taare Sadak Par. I've uploaded an Excel spreadsheet in the files section. There were four observers. We looked at six IMO-regions in different parts of the sky. We spent ~ 10 minutes staring at each region.

Tanmoy Sun Mar 30, 2008 1:53 pm

Congratulations Tanmoy and his team.

I will check out the excel file later but in the meantime will you tell us about your experience with using the IMO maps? Please also tell us how many of you knew how to use star maps etc. Your input would be a good pointer.

"We spent ~ 10 minutes staring at each region."

This is important. While estimating the lm during the meteor shower observations we are not expected to stare in that direction of the sky but to take a quick look. This is because our aim here is to look for the meteor phenomenon that lasts for just about a second.

On the other hand to estimate the limiting magnitude for general sky observations it is better to take time to see ‘how faint’ one goes.

Arvind Paranjpye Mon Mar 31, 2008 4:53 am

Hello Tanmoy, Avnish, Ashmeet, Vishnu, Ajay, and everyone who participated in taking the limiting magnitude and other observations on saturday - these are the very first real observations for the taare sadak par program and this is as exciting as the 20" first light! Ajay was it also possible to make some DSLR attempts?

Anurag and I will be working on the wiki tomorrow, so that, everyone can now start adding data on their own to the wiki.

Tanmoy, as Arvind says, detailed feedback from all the observers would be very valuable for others attempting to make similar observations.

The really good thing emerging from this is the confidence that this is a feasible excercise for all of us to do. There is a reasonable consistency amongst the different observers.

Perhaps, very beginners may not be able to submit data with the kind of detail that you folks have provided - and then again, those wishing to do so would find all possible help given here in the group.

We can also have a separate format for simple beginner observations.

However, the kind of details that Tanmoy is putting in, are very useful. Tanmoy, giving Azimuth information would also be useful for mapping the light pollution in cardinal directions. For instance the last set of data show a lower limiting magnitude (although at a higher elevation than the previous observations) - and could that be related to relative Azimuth and city directions? Or, was it some local, temporal effect (lots of neighbourhood lights switched on late at night)?

Rathna Mon Mar 31, 2008 6:46 am

Here is how we conducted the observation:

We chose the regions that we would work with. I had previously identified 3,4 & 9 (Leo, Gem and UMa?) for use, but since we had time, we went ahead with more. We looked at the IMO maps with the region boundaries drawn on them and then glanced at the sky to confirm that all of us knew what the corner stars of the chosen region were. We then proceeded to switch off our torches and lie on our backs (Ashmeet had cleverly brought a chitai with him). We then counted the stars in the chosen region, each his/her own. Often we voiced aloud whether others saw the same stars within the region that one of us thought he was seeing. Several of us often confirmed, but sometimes we disagreed.

When everyone was sure of their own star count, we would switch on the torch, note down the time and check in the IMO and other maps whether the stars we had counted actually existed! We were hindered by the fact that we didn't know the limiting magnitude in the star charts we had, so we didn't know if we could actually be seeing stars not on the map. Occasions on which the stars seen were not on the map were rare, however and we were reasonably confident that we were not imagining the stars we thought we saw. The counts were written down and another region chosen and the above step repeated.

The exercise was held from ca. midnight to 1:30 AM. During this period, we were interrupted several times by bright lights shining from nearby houses and cars from the road shining their headlights onto the site. When the sumo/jeep with tea arrived, we were blinded for several seconds and lost our night vision for a while. Measurements are therefore somewhat suspect at places where it is possible that the observers did not have full night vision. Since the interruptions were intermittent and went on throughout the night, the effect cannot be documented.

My first impression with using the IMO maps was that the regions that they have prescribed seem to encompass too few stars for the statistics to be of any use. We saw ca. 3-4 stars in any one region clearly and the rest had to be stared at and often averted vision used to tell that there was a star there. Faint groups of 2-3 stars within our regions also confused the hell out of us a few times - when the observers did not know what they were seeing and often reported the asterism as a single star. The reason is probably that the site and weather were not too good and the regions tend to enclose very few bright stars and not too many faint ones (below about mag 4) either. So the numbers of stars inside a given region as we have reported them are subject to large Poisson (counting) errors (Poisson errors are proportional to the square root of the number of observed stars, so the relative Poisson errors scale as one over the number of observed stars).

Three of the four observers eat star maps for breakfast. It was the first such outdoor observing expedition for the fourth one amongst us - but I'm sure it was a good experience! We helped each other find the correct regions anyway. Overall it was a fun thing to do - as I am sure the others would corroborate. I hope the observation under the given limitations of sky conditions, ambient light and intermittent beaming interruptions can still be put to some use.

Tanmoy Mon Mar 31, 2008 6:49 am

Some quick thoughts on mapping the data:

One easy way we could map the data would be to make a simultaneous use of wikimapia and google earth.

If we go to wikimapia.org and type Nuh, in the search box we find some locations already marked. Using these and working around the map, the exact location of the observing site can be determined on wikimapia.

Create a wikimapia account and place a marker on this location. Write a little about the location of general interest and also mention the taarewiki link as a location that will give information about the light pollution data for this location (after we have placed the data on the taarewiki).

When we create a box around this location, clicking on this box will give the latitude and longitude for that location (we can check it against the GPS data that AAAD members have, maybe also from detailed ASI maps etc. to know the relative accuracies involved).

Anyway, the wikimapia can be used by observers who do not have their latitude and longitude determined. Once, the latitude and longitude are determined through wikimapia, one can type those in google earth - I have generally been finding a small offset between wikimapia and google earth - but, not much. Anyway, one can create a google earth placemark for the observing site and upload the placemark to the wiki.

The spirit of wikimapia is "Lets describe the whole world!" We can add the dimension of light pollution, to that description, by adding our inputs in to wikimapia :-)

Rathnasree Mon Mar 31, 2008 7:24 am

Tanmoy, Avnish, Ashmeet - would you care to repeat your observations from your own locations today - take an evening segment of the sky - and then, many people could join from their own locations.

Shall we all join in the observations today - each of us from our own locations, time to time compare feedback online at the group, and then go back for observations and so on?

Beginners could use easy to use software like skyglobe (just type skyglobe and download in google and download version 3.6) to locate Leo, Taurus, Gemini and Virgo - which will be useful parts of the sky for evening observations today. There are many ways of getting aquainted with the sky - Karunakar posted with a link to wikisky which would also be useful for this exercise.

And then use the charts on the site

http://www.imo.net/visual/major/observation/lm

to determine the limiting magnitude.

If more help is required - a single tentative post in the group asking a question will be flooded with helpful replies :-)

Rathnasree Mon Mar 31, 2008 10:42 am

Ok. Can we also benchmark the sky colour indicator? It seems to run well only in firefox, though I am not sure. Both daytime colour (much before sunset if poosible) and nighttime colour (in the specific IMO region) would be needed. The daytime colour is more important!

Let us also work on the same regions, 3 (Leo), 4 (Gem) & 9 (UMa?), say to begin with. May I also suggest we observe at similar times? Say 7:30 PM, 8:30 PM, 9:30PM and 10:30 PM - or does anyone have any other suggestions...

Tanmoy Mon Mar 31, 2008 11:07 am

Roadmap for collaborative observations

Here are some thoughts I have, as a roadmap for collaborative observations, starting today. Collaborative observations, where all the beginners and college students who are in the group and who do not have any familiarity with the sky, can also participate.

Starting today, over the next few days, let all of us (experts included) concentrate mainly on doing collaborative observations of one overhead region of the sky.

This will be the Leo region for April - moving roughly overhead at around 10 PM these days, at 9:00 PM middle of the month, and at 8:00 PM towards the end of the month. (Small variations will be there, location to location, but, we need not worry too much about that).

We plan to count the number of stars visible in the boundary region marked by the stars - Regulus, Algeiba, Zosma and Denebola of Leo. Saturn being close to Regulus, might confuse beginners but, might also be a useful way of finding Leo as this is the brightest object in the overhead regions at the times mentioned. Saturn is not to be included in the star count!

Help for beginners to locate these stars is being provided at

http://nehruplanetarium.org/taarewiki/pmwiki.php?n=Site.BeginnersHelpForAprilObservations

The pages are still being written, hopefully, I can finish them soon and hope that they would be of sufficient help so that that many beginners will join for today's observations.

All we have to do is count the number of stars in this boundary region and post here in the group, for now, until table matters on the wiki are sorted. We just post with the number of stars observed and the observing location. Those who cannot determine the latitude and longitude of their location can post with the address of their location and take help from the group to search this location in wikimapia, to obtain their latitude and longitude.

So, will this be a good idea - all of us participating in just the beginner observations for a few days? Those interested can also contribute with other expert observations - while participating in the beginner segment, also.

After observing, before, or while taking a break, we could post here in the group, with feedback.

And now, volunteers for today's observations?

Rathnasree, Nehru Planetarium, New Delhi Wed Apr 2, 2008 11:34 am

Will this endeavor be restricted to beginners and college students alone? and for members situated in india alone?

i'm not quite a beginner, and college days were 'long long ago so long ago", and i'm not in india! :) i guess i can add something, being situated at 93.01 W and 45.08 N.

cheers, sridhar Wed Apr 2, 2008 4:33 pm

Sridhar,

Observations from everyone, from anywhere in the world, are welcome. Those who wish to give a more refined estimate of the limiting magnitude, using any method they like, are also welcome.

It would be good to bench mark the observations through using many possible methods.

And, we do want observations from outside India - just to compare.

I was reading in the Sky and Telescope article - a statement about the worst kind of inner city skies where they talk of a limiting magnitude of 4.0.

But, if I am to take my own observations seriously - the Delhi University regions in the last two weeks have been showing a limiting magnitude of about 3.3! (I would, of course wish for someone younger to also measure from these regions and confirm that). If such an alarming magnitude of light pollution is existing in Delhi - it has not been quantified so far and it needs to be done, compared with inner city values world wide and bought to the notice of the authorities.

I think, Delhi has an additional problem of squall like conditions, haze and dust. Perhaps studies need to be correlated with other environmental studies like aerosol content and so on.

Rathna Wed Apr 2, 2008 4:49 pm

:) sounds good... thanks!

last i heard, limiting magnitude here near home should be around 4. i'm a member of the Minnesota Astronomical Society, and they have a few dark sky locations where lim.mag. should be closer to 6.5.

i think the method you suggested - comparing apparent magnitudes - will work as well as counting the visible number of stars... i'll try to do my best tonight and revert.

cheers, sridhar Wed Apr 2, 2008 6:02 pm

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