Darkness is disappearing and that’s bad news for astronomy

Dec 23, 2015 Off Comments in News & Stuff by

Darkness is disappearing and that’s bad news for astronomy

Astronomers have much to celebrate in the International Year of Light and Light-Based Technologies (IYL). Until the 1930s, every scrap of information about the universe came to us in the form of light.

Admittedly, once radio telescopes began to make the first inroads into the invisible regions of the electromagnetic spectrum, the game changed. Today, there’s no portion of that universal hum of radiation that is off-limits to ground- or space-based telescopes. But optical astronomy – the old-fashioned kind, using visible light – still reigns supreme.

Today’s optical astronomers are able to glean the most astonishing information from starlight. For example, with exotic calibration tools like iodine cells and laser combs, they can measure a star’s speed with a precision better than one metre per second – a slow walking pace.

Over time, this miniscule Doppler shift can reveal the existence of orbiting exoplanets by the wobble they induce on their parent stars. More exciting still are the possibilities offered by the coming generation of Extremely Large Telescopes, which will boast mirrors larger than 20 metres in diameter.

Within the next ten years, astronomers will have the capability not only to see the distant exoplanets directly, but also to detect signatures of life in their atmospheres. The discovery of any such biomarkers would profoundly alter the way we see ourselves, and our place in space.

With optical astronomy on the brink of a new golden age, it’s no idle boast that the sky is, indeed, the limit.

The threat to the night sky

But that’s the problem. In optical astronomy, the sky really is the limit. When astronomers observe celestial objects, they see them superimposed on the natural luminous background of the night sky.

The Earth’s rarefied upper atmosphere contributes to this, as its air molecules relax after a hard day in the sun. There’s also light from sunlit dust in the solar system, together with a faint background of light from myriad distant stars and galaxies. Pushing to observe ever-fainter celestial bodies, astronomers are sometimes measuring objects whose brightness is only one percent greater than the natural night-time skyglow.

So you can easily imagine what happens if the night sky is polluted by artificial light from towns, cities and industrial complexes. The faint objects simply disappear. For this reason, astronomers site their giant telescopes well away from centres of population.

Australia’s national observatory, for example – a A$100 million infrastructure investment – is located at Siding Spring Mountain in the Warrumbungle Range, 350km from Sydney. But due to the scattering of light by the Earth’s atmosphere, remoteness is no guarantee of darkness, and from Siding Spring, the glow of Sydney can clearly be seen on the horizon.

That light-scattering process turns out to be much more efficient for the blue component of light than for its red component. That’s why the sky is blue; sunlight’s blue constituent is very effectively scattered in all directions. But the same is true for artificial light. Light with a high blue content (think of those intense white LED headlights now seen everywhere on our roads) makes a bigger contribution to light pollution than warmer, cream-coloured light.

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Shared from the full article at https://theconversation.com/darkness-is-disappearing-and-thats-bad-news-for-astronomy-51989