Taare Sadak Par - Simple methods for quantifying light pollution
This page is for summarising discussions under the topic mentioned above, at the yahoogroup
http://groups.yahoo.com/groups/Astronomy_Activities_2009/
1. Some thoughts that I shared with the Quest students during my talk
at the Delhi University U 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, Nehru Planetarium, New Delhi. Sun Mar 23, 2008
11:43 am
2. 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 Lashkar Sun Mar 23, 2008 12:44 pm
3. 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?
------ Pradeep Nair Sun Mar 23, 2008 1:02 pm
4. 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 Anand Sun Mar 23, 2008 2:20 pm
5. 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 observations 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, quantitatively?
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
6. "...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...."
Agreed. 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 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
7. 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