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Five days before the start of the 100 Hours of Astronomy there is an important event. It is not the major school reunion that I sadly will be unable to attend. It is not my birthday, although it does happen on that day, as that is something I prefer to forget. The important event on Saturday 28 March is Earth Hour when between 8:30 pm and 9:30 pm local time people all over the globe are asked to switch off their lights.
Switching off lights even just for an hour is beneficial for astronomy. It provides a great opportunity to organise star parties and look at the night sky even from cities. The organisers are well aware of the effect of light pollution on astronomy, or at least were quickly made aware after the first event in 2007, so that the date chosen is close to new Moon with its darker sky. In the centre of a large city like Sydney the difference in sky brightness as building decorative lights and lit advertising signs are switched off is barely noticeable. Yet at sky viewings here at Sydney Observatory during the previous two events, the effect of seeing the floodlights switched off on the Harbour Bridge and the office lights and advertising signs dim on the office buildings on the opposite side of the Harbour was dramatic The lights may be off for only one hour, but a star party on 28 March gives a perfect platform to talk about light pollution. It is important to stress that astronomers do not expect dark cities as cities need light. At night lighting is needed to illuminate road surfaces and footpaths or sidewalks, to give people a sense of security and to make advertising signs and decorative signs visible. However, the assumption made by most people that the more light you throw at a task or problem the better is wrong. To improve night time visibility we need to improve the quality of the lighting not its quantity. With good lighting, that is light directed where it is needed and not spilling into the sky we can see better, save energy and make the sky darker. A win-win for all!
The International Dark-Sky Association has a large amount of information on good lighting on its website. Basically, light fittings should be full-cut off, that is should not radiate light above the horizontal. Lights should always be aimed downwards, not angled or pointed upwards. There should be no glare from lights into people’s eyes as that makes it harder to see, especially for older people. And security lights are much more effective if they are on sensors so that they only switch on when people approach.
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Earth Hour started in Sydney, Australia just two years ago. This time over 1400 cities in at least 80 countries are taking part and will be dimming their lights. The idea is to make a symbolic gesture to emphasise the urgent need to reduce energy consumption and thereby reduce greenhouse gas emissions. The organisers expect over a billion people to take some partin Earth Hour and regard each of them as voting for the future of planet Earth.
