The Darkest Skies in the GTA – The Results
by Paul Markov, February 2001

In the October 2000 issue of Scope I asked for your participation in an experiment to determine the faintest star visible with the unaided eye from your observing location within the GTA. In this article I will summarize the results of the experiment and attempt to draw some conclusions based on the observations received.

The experiment was launched in mid-October and we were very lucky to have several clear and moonless nights during the last two weeks of October. The moonless periods during both November and December were mostly cloudy, thus only a few observations were received during those times. The experiment came to a close on December 31.

In total we only had 15 members participate in the experiment, which is disappointing for a group the size of the Toronto Centre. Of these 15 people, only 6 submitted at least 3 sets of observations, which was the requirement for eligibility in the Toronto Centre golf shirt draw.

The range for the faintest star visible to the naked eye was quite wide; magnitude 3.5 to 5.5, with an average of 4.8. In the experiment I defined the "faintest star" as a star that was "barely visible with averted vision - the star ‘pops’ in and out of view." It’s not surprising, however, that observations at the low end of the scale came from individuals who deemed themselves to be "beginner" or "intermediate" observers, while individuals who described themselves as "advanced" observers generally reported seeing fainter stars.

As expected, observers in suburban areas of the GTA reported some of the faintest magnitudes, such as 5.3 (Ajax and Mississauga) and 5.5 (Pickering), but much to my surprise, two of the faintest reports came from observers in the heart of the city! Patrick McDonald observed magnitude 5.4 and 5.5 on two separate nights from Woodbine and Gerrard, while Irena Wilk observed magnitude 5.5 from Dundas and High Park. Are the skies really less light-polluted at these locations, or do these two individuals have keener eyesight? I cannot tell you for sure, but I suspect the latter. I could only draw a more definitive conclusion if I had received observations form others at those same locations.

I was planning to present the results in a graphical format, with all the observed magnitudes superimposed on a map of the GTA by location, however I do not think there were sufficient observations to make the map meaningful. In fact, it may have been more misleading than useful – for example, an observer in Brampton could have been lead to believe he should go observing near Dundas and High Park in Toronto!

After doing a simple analysis of the data, I feel comfortable in stating that an average observer with good eyesight who is located within Metropolitan Toronto can expect the faintest star observed (as defined above) to be between magnitude 4.3 and 5.0 on a clear moonless night. Observers in Metro Toronto’s suburbs can expect this limit to increase to magnitude 5.5. Below is the actual data for your review:

Location Age Bracket Experience

Category 1

Category 2

Category 3

           
Warden & Sheppard 26 - 35 advanced

4.8

4.9

5.1

Westney & Tauton (Ajax) 46 - 55 intermediate

4.8

5.1

5.3

Torbram & Hwy 7 (Brampton) 36 - 45 advanced

4

4.2

4.3

York Mills & Leslie over 55 beginner

3

3.3

3.8

Bathurst & St. Claire 46 - 55 advanced

4

4.5

4.8

Bathurst & St. Claire 46 - 55 advanced

3.8

4

4.3

Warden & Sheppard 26 - 35 advanced

4.1

4.3

4.6

Torbram & Hwy 7 (Brampton) 36 - 45 advanced

4.2

4.4

4.6

Kingston & Warden 36 - 45 intermediate

4.3

4.6

4.8

Torbram & Hwy 7 (Brampton) 36 - 45 advanced

4.2

4.3

4.4

Warden & Sheppard 26 - 35 advanced

4.8

4.9

5.1

Woodbine & Gerrard 46 - 55 advanced

4.6

5.2

5.4

Lorne Park & Lakeshore (Mississauga) 46 - 55 advanced

4.9

5

5.3

Eglinton & Leslie over 55 intermediate

3.6

4

-

York Mills & Leslie over 55 beginner

3.3

3.5

4.2

Harwood & Bayley (Ajax) 46 - 55 beginner

4.6

4.8

4.9

Woodbine & Gerrard 46 - 55 advanced

4.6

5.2

5.5

Eglinton & Leslie over 55 intermediate

4

4.1

4.3

Dundas & High Park 36 - 45 intermediate

5.1

5.2

5.5

Avenue & Eglinton over 55 advanced

3.3

3.8

4.1

Warden & Sheppard 26 - 35 advanced

4.1

4.6

4.9

Kingston & Warden 36 - 45 intermediate

4.1

4.3

4.6

Warden & Sheppard 26 - 35 advanced

4.6

4.9

5

Eglinton & Leslie over 55 intermediate

4

4.3

4.7

Park & Gibb (Oshawa) 36 - 45 advanced

4.9

5.1

5.3

Hwy 7 & Westney (Pickering) 36 - 45 advanced

5.3

5.4

5.5

Dundas & High Park 36 - 45 intermediate

4.8

4.9

5.1

Dundas & High Park 36 - 45 intermediate

-

-

4.8

Kingston & Warden 36 - 45 intermediate

3.8

4.3

4.6

York Mills & Leslie over 55 beginner

3

3.5

4.3

Keele & Bloor 36 - 45 advanced

3

3.3

3.5

Hwy 7 & Westney (Pickering) 36 - 45 advanced

5.3

5.3

5.5

           
AVERAGE    

4.2

4.5

4.8

Definitions:

Category 1: a star that is just visible with direct vision.

Category 2: a star that is only visible with averted vision.

Category 3: a star that is barely visible with averted vision, the star "pops" in and out of view.

Note: Observations from repeat locations were made by the same individual, but on different nights. We did not have different individuals observing from the same location.

Part of the experiment was to try to make a correlation between age and faintest magnitude visible. Unfortunately I only received observations from three individuals who are over the age of 55, but interestingly all three observers scored below the expected range noted above. Observations from these individuals ranged from magnitude 3.8 to 4.3. There definitely is not enough data to draw a conclusion, but these results seem to support the notion that the eye’s sensitivity to faint light decreases with age.

Despite the low participation, I think the experiment yielded enough data to formulate an approximate magnitude limit within the GTA. What was not achieved was the identification of a "dark spot" that offers the darkest skies within the GTA, but perhaps we can try the experiment again next Fall and find that "magic" place. Thanks to all of you who participated!

 

Copyright (C) 2001 by Paul Markov


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