STAR SAVERS

Imagine for a minute you are driving in a car with a very dusty and dirty windscreen. It may not obstruct your view and you can see just fine for a while, but then you turn west and are heading into the setting sun. Suddenly you can’t see much outside and it’s almost impossible to make out what’s ahead. It is a very dangerous situation. So why did the sun shining directly onto the windscreen make such a difference? Well, that fine layer of dust and bugs scatters the sunlight from the surface of the windscreen in all directions, so suddenly the windscreen has now become a bright light source immediately in front of you. The windscreen outshines some of the dimmer objects on the road ahead making visibility very poor. Exactly the same thing happens to sunlight in Earth’s atmosphere. The nitrogen molecules in our atmosphere act like the dust on the windscreen and scatter the shorter blue wavelengths of light very well. This is the reason that the sky appears blue (it is called Rayleigh Scattering). The amount of blue light scattering from the air above us exceeds the amount of light coming from faint stars so we cannot see stars during the daytime. You will notice however that it is possible to see the Moon in the daytime just before and after a full Moon. This is because the Moon is very bright compared to the stars, so we can still distinguish it above the glow of the blue sky. Because our atmosphere scatters blue light so well, it acts like a filter, allowing the longer red and orange wavelengths to pass through most easily, hence the sun appears very red at sunset and sunrise. This is most noticeable from a flat, open location with a very low horizon such as in Australia’s interior and around Winton of course. When the sun is just above the horizon it’s light travels through the more length of atmosphere and the blue light gets filtered out very well, leaving the long red and orange wavelengths to dominate and thus the spectacular colours of sunsets and rises. So what does this tell us about light pollution you ask? It tells us that bright blue lights (or cool-white light bulbs) emit light that scatters off the air above us, while dimmer warm-white lights do not scatter as significantly. In our houses, buildings, streetlights and cities it has become the trend to try to mimic natural bright sunlight as much as possible. However, this means when we leave those lights on at night, they literally act like sunlight and their blue components mimic the sun in our atmosphere. This causes the air above cities to glow glow, thus reducing the number of faint nebula and stars that can be seen. This is light pollution. The sky glow above large cities can be so significant that night constantly appears like a full Moon is shining down and very few stars are visible. To reduce the sky glow from artificial lights, organisations like the Australasian Dark Sky Alliance (ADSA), and DarkSky International (formerly the International Dark-Sky Association) raise awareness about light pollution and measures to protect and preserve night skies. Some of the best ways that you can help are : •visit an International Dark-Sky Place such as our Museum Jump-Up which was accredited as the first International Dark-Sky Sanctuary in Australia (also, Warrumbungle National Park in NSW, the River Murray Dark-Sky Reserve in SA and the newest Dark-Sky Sanctuary in Arkaroola, SA) •Use warm-white lights outdoors in carports or front doors with shielding to ensure the light is directed downwards (bulbs with a 3000K temperature rating are best as they emit very little blue light) •Use small pathway lights at ankle height where required rather than large and bright overhead lights •Visit https://www.australasiandarkskyalliance.org/ and https://darksky.org/ to learn more As you make your outdoor lighting more night-sky friendly you will quickly discover how little light you require to see and move safely in the dark. Clear and dark skies to all. GRANT SALMOND Gondwana Observatory (first image) One of the sights you can see in a truly dark sky is the Zodiacal Light. The band of light on the left of the frame is the Carina Nebula and our Milky Way galaxy. On the right side of the frame you can see a distinct spike or cone of light glowing upward into the sky. This is the Zodiacal light – sunlight is scattering off interplanetary dust particles outside Earth’s atmosphere in the plane of our solar system. (second image) The sign at the base of The Jump-Up announcing our accredited status as Australia’s First International Dark-Sky Sanctuary with star trails in the background. The dense and bright cluster of trails to the right of the sign is from the rich field of bright stars in the Carina Nebula region, Southern Cross (Crux) and Centaurus constellations.