Where are the Darkest Skies in the GTA?
by Paul Markov, October 2000

In this month’s installment I will stray a little from the usual deep sky discussion, however the results of the experiment I am proposing below will eventually tie back into the general theme of deep sky observing. The purpose of this month’s article is to ask you to participate in an experiment to determine the limiting visual stellar magnitude in the Greater Toronto Area (GTA), and if you accept the challenge, there may be a very nice prize in it for you. The cool late Fall temperatures typically bring skies free of haze and moisture, which make this a great time of the year to obtain reasonable magnitude estimates that are unaffected by atmospheric conditions. If you live or observe from these urban areas you are encouraged to participate: Oakville, Mississauga, Brampton, Metro Toronto, Vaughan, Richmond Hill, Markham, Pickering, Ajax, Whitby, and Oshawa. Please feel free to participate if you are located in adjacent townships to the ones listed above, as long as you are in an urban or suburban location.

The experiment is very simple, will only take about 20 minutes of your time on each night you wish to participate, and the only "equipment" required is your eyes, the sky chart below, a red flashlight, and pen and paper. The experiment can begin as soon as you read this article, which I hope will be in late October, and will end on December 31, 2000. I think that just over two months will provide enough clear nights free of moonlight interference to allow for several observations from you and other members. The reason I am looking for several observations from several different nights is because atmospheric conditions are never the same from night to night, so that by averaging the results from many different nights should yield a reasonable visible limiting magnitude expectation. The target area of the sky is parts of Pegasus and Andromeda, which ride nearly overhead in November and December in the evening hours. You will not be able to observe during the period of about November 3 to 11 and December 3 to 11 (first quarter Moon to full Moon) as moonlight interference will be unavoidable. Make sure you observe when the target stars are high in the sky, and not later in the night when they are lower in the north-west. Observation beyond December 31 will not be as useful because Pegasus and Andromeda will begin their descent towards the horizon.

There are only a few pre-requisites before making an observation. The sky must be clear and free from visible haze or high thin clouds, the Moon must not be in the sky, and your eyes must be dark adapted. On that last requirement, it is recommended that before making your observations, you let you eyes dark adapt for about 15 to 20 minutes, and that while observing, no sources of light are shining directly in your eyes. If you are near sighted, even just a little, make sure you wear your prescription glasses, otherwise your observations will not be accurate. Because the target stars are nearly overhead, I recommend you get comfortable in a lawn chair otherwise this experiment will literally become a "pain in the neck".

Click here to view the required sky chart

Use the sky chart included in this article for estimating the faintest visible star from your backyard, balcony or favourite observing spot within the GTA. Do not use any optical equipment, such as binoculars, spotting scopes, or telescopes – this is strictly an unaided eye experiment. Start by spotting easily visible stars, such as the two top stars in the Great Square of Pegasus, then progressively move to fainter magnitudes until you reach the limit. Please report the three faintest magnitudes you see for each observing session, such as, for example, 4.8, 4.9 and 5.1 (don’t report three different stars of the same magnitude, such as 5.2, 5.2, and 5.2). From my experience, as you approach the limit, the three faintest stars will fall in these categories: i) just visible with direct vision, ii) only visible with averted vision, iii) barely visible with averted vision, the star "pops" in and out of view. If you have a hard time distinguishing between these three categories while observing, just report your three faintest observations, with any personal comments you might have as to the visibility of these three stars.

The sky chart included here shows parts of Pegasus and Andromeda at their highest point in the sky, which happens to be November 15 at 21:00 EST in this printout. Although the chart may appear intimidating at first, all the information shown is very straight forward. Only stars that begin with an alpha character are to be used in this experiment. The other stars are just shown for reference. The number following the alpha character represents the star’s magnitude, but with the decimal point omitted. The use of an alpha character in front of the magnitude is just the method I adopted for differentiating multiple stars of equal brightness from one another. For example, star A49 is the first 4.9 magnitude star I selected on the chart. Similarly, star C45 is the third 4.5 magnitude star I selected. There are a total of 64 target stars shown in the chart ranging from magnitude 3.0 to magnitude 5.5. I chose several stars of the same magnitude, where available, to allow for a greater choice when observing.

When reporting your observations, let me know the following:

The reason I would like to know your age bracket is because it is generally accepted that as you age your eyes’ sensitivity to faint light diminishes. I would also like to know your experience level in observational astronomy because advanced observers typically have "trained" their eyes to see fainter stars. Here’s a sample report from one of my trial observations in September: Paul Markov, Sept. 26, 11:30 pm, Warden and Sheppard in Scarborough, 26 to 35, advanced, (i) B43, (ii) D46, (iii) A48. Please follow this format when reporting your observations.

I would prefer to receive your reports after each observing session rather than receiving them all at once at the end of the experiment period. The best way to send me your observations is via email at pmarkov@ica.net.

The only way this experiment will be successful is if many of you participate. The more data points, the more meaningful the results. In order to encourage as many of you to participate, the Observational Activities Committee of the Toronto Centre has donated a beautiful RASC Toronto Centre golf shirt (a $30 value!) to be raffled randomly (only RASC Toronto Centre members are eligible for the raffle). All you have to do to become eligible for the prize is to submit at least three observing reports from different nights. The draw for the golf shirt will take place at a Members’ Night Meeting in the New Year (you don’t have to be present to be able to win, but you must be a member in good standing). Once the experiment comes to an end, I will summarize the results in an upcoming article. I hope to determine where one can find the darkest sky within Metropolitan Toronto, and within the GTA. I will also try to correlate differences in age and experience in respect to seeing faint stars. I look forward to many reports!


Copyright (C) 2000 by Paul Markov

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