How dark can we go?

planetarium_Geoff Wyatt

This coming Friday and Saturday evenings (July 21 & 22) Sydney Observatory is holding its second annual Festival of the Stars. The Sydney Harbour Bridge floodlights will be turned off for the events and a number of City and North Sydney buildings will be turning off their exterior signage. Maybe, maybe they they will be joined by a major Sydney landmark.

For the first time Sydney Observatory will be measuring the brightness of the night sky during the event while the lights are turned off. This is with a special Sky Quality Meter borrowed from the Sydney City Skywatchers. Last night (Monday 17 July at 9:00 pm) the reading was 17.82 magnitudes per square arcseconds. How dark can we go? Any guesses welcome – post them as comments. Note that the higher the reading the darker the sky. A really dark sky in the country is 23 mag/arcsec2, but that is not feasible in Sydney.

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