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How Many Stars?


The collected star count data will be used for a comparative study of sky conditions across the country and possibly around the world.

In order to collect data that has the highest scientific value it is important that all observations be made under the best possible observing conditions and are also as similar as possible. This is so that any differences really reflect indiginous differences in the sky quality and are not due to transient differences in the observing conditons.

Part of this requirement means that observations should be conducted in a moonless sky after the end of astronomical twilight.

At the end of Civil Twilight (Sun 6 degrees below the horizon) it is no longer possible to read outdoors without artificial light.

At the end of Nautical Twilight, (Sun 12 degrees below the horizon) traditional navigation at sea is no longer possible (horizon is not visible).

At the end of Astronimical Twilight At the end of Astronimical Twilight (Sun 18 degrees below the horizon) the sky no longer contains scattered sunlight.

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Nights without Darkness

Many cities in Canada are without totally dark skys during the summer months. Some example locations are shown on the left side of the chart below.

Using the chart below, and a knowledge of your latitude, you can determine the dates during which there is either no sunset or no end of astronomical twilight.

Nights without Darkness

If possible, perform your star count after the end of
Astronomical Twilight and choose a Moonless Night


Prepared by YES I Can! Science Team at McMaster University,
for the Canadian Space Agency.