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IMO Starcounts

A customised User Guide

Nandivada Rathnasree, Nehru Planetarium, Nehru Memorial Museum and Library

So, that is what we do. When the whole world (!!!) sleeps, we count stars

Jab saari duniya soti hai, hum taare ginte rehte hain.

Well, the lyricist in the song above did not give much thought to the fact that the whole world does not have night at the same time and is not likely to be having a single time slot of sleeping.

Leaving such splitting of hairs aside, our task when we set out to do star counts is to do, well, star counts.

A userguide for star counts in January 2015, prepared by Arvind Paranpye, for amateur astronomers who have some experience of skywatching, is here

For those who have not had some amount of experience with skywatching, it might be useful to go through the contents on this page, first, before going to the link above.

During a sky observing session, one of the simplest quantitative activity which could be undertaken, is to make starcounts for light pollution measurements. In such an activity, one is measuring the number of stars visible in some specified location of the sky and from this number of stars which we have discerned as visible, a reasonably accurate estimate of the light pollution is given from the thumbrules arrived at, by the International Meteor Organisation.

The link above is our main resource in deciding which part of the sky we may wish to do our starcount observations from. If we scan through the pages of this site, we see the constellations abbreviated as here:

We need to decide which constellations will work for us on the day of our observations. We may wish to look at a skymap for our location and time and then decide that these constellations will be suitable for our observations on that date and time. Depending on the light pollution in our area we will see a sky anything like this, let us say for January evening skies;

We can decide which constellations will be useful to do star count measurements in, for our location and time, and then see the corresponding charts in the IMO page above.

A screen capture from the IMO page showing chart 4

For January star counts, it seems that regions 4 and 8 in these maps - Gemini and Taurus regions will be useful.

Print the corresponding images and keep with you during sky observations. What you need to do, is to count the number of stars within the geometrical regions (triangles, in this case) marked in these charts. The corner stars are also to be included in the star count.

Be careful about identifying the corner stars correctly. Also, be careful that the stars being counted are within the boundary and none outside the boundary are being included - you may mentally need to draw the boundary lines in the sky, while doing the star count activity.

Note down the location of observations, the constellation where star counts were done and the number of stars.

Once you count the stars and note it down, go back to the IMO page and scroll down where the tabulated Limiting Magnitude values are listed.

Limiting Magnitude is the magnitude of the faintest star visible from your observing location. Some beginner's discussions of the concept of star magnitudes is discussed here

A screen capture from the IMO page showing the Limiting Magitude values (Lm) corresponding to the number of stars (N) you have counted in ay constellation region (within specific boundaries marked in the charts)

Go ahead, count the stars and report your limiting magnitude on this page created by Arvind Paranjpye for the Light Pollution Watch which is a part of the co-ordinated monthly skywatch activities being co-ordinated by the Astronomical Society of India Outreach committee.

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Page last modified on January 17, 2015, at 06:40 AM EST