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How Many Stars?

Estimating Sky Conditions at Midday

The number of stars visible in the nighttime sky depends upon both the transparency of the sky and the brightness of the sky caused by upward shining artifical sources of light such as unsheilded street lights.

Prior to making your star count it is helpful to determine the transparency of the sky in your neighbourhood.

The 6 point scale below is a guide to help you determine the sky's transparency. Make this determination the same day that you will be performing your star count.

Air Quality: Good

Clear and very dry (low humidity), no clouds overhead

Air Quality: Good

Clear dark blue horizon in the direction opposite to the sun.

Low relative humidity. Distant objects appear closer than usual and distant colours are sharp and vibrant.

Shadows are sharp and black.

Air Quality: Moderate

Clear and humid, no clouds overhead

Air Quality: Moderate

The sky at the horizon is very pale white or grayish.

The Colours of distant objects have a soft grayish-white tint.

Air Quality: Unhealthy to sensitive groups

Thin haze and humid

Air Quality: Unhealthy to sensitive groups

The sky at the horizon is very white and the horizon is difficult to see clearly.

Distant objects lack colour.

Air Quality: Unhealthy

Sky grayish-white

Air Quality: Unhealthy

Sky at the horizon is grayish and slighlty obscured.

Distant object seen faint and slighlty blurred.

Shadows slightly gray.

Air Quality: Very unhealthy

Haze and smog

Air Quality: Very unhealthy

The sky at the horizon has a slightly yellowish tint.

Distant objects (on the horizon) are obscured.

Winds are generally light winds.

Shadows are faint.

Air Quality: Hazardous

Very hazy/dense smog

Air Quality: Hazardous

Horizon obscured. The sky near the horizon has a distinct brownish colour. The distant horizon is obscured.

The air has a high relative humidity. Winds are generally light.

Shadows are faint or non-existant.


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