Winter is the best time to see the rich beauty of the sky when we look straight into the centre of the Milky Way.
And Sunday night is the longest night of the year in the Southern Hemisphere, which makes it perfect for counting the stars in the Southern Cross.
If you do, you'll contribute to a world record attempt to map light pollution across Australia.
Whether or not you get to see full beauty of the Milky Way — or even the Southern Cross — depends upon where you live, says astronomer Fred Watson.
If you've struggled to find the Southern Cross from your backyard during COVID you're not alone.
In areas that have high levels of light pollution you can see only four — or even three — of the constellation's main stars.
While satellites can detect raw points of light across the globe, there is very little data about how Australians are affected by light at ground level.
But if you count how many stars you can see in the Southern Cross this Sunday night, during the winter solstice you could help fill in some of the gaps.
The information collected contributes to the Globe At Night international citizen science program, which measures light pollution around the globe.
There were only six readings from Australia until April, when the Australasian Dark Sky Alliance, ran its first star count during lockdown. Now there are 770.
The idea is to do the star count again on a much larger scale to get a baseline across Australia and New Zealand, says Marnie Ogg, who heads the Alliance.
"It's a way of working out where we sit in the world. How much of our continent is dark? Where are the light patches or the patches people are preserving? she says.
"And it becomes a way councils can gauge what their residents are interested in and give the councils some statistics about how bright it is there as well."
Why can't I see my night sky?
Light pollution is caused by three different factors.
Firstly, light shining upward can bounce off clouds and molecules in the atmosphere.
"In urban lighting typically 30 per cent goes up into the sky," Professor Watson explains.
"The more light you've got going above the horizontal plane, the more it scatters from the molecules in the atmosphere."
This produces a phenomenon called sky glow that can be seen from hundreds of kilometres away from cities.
If you are under it, a lot of that scatter light comes back down to your eye.
"So you look up and you can't see the stars."
The second factor is glare.
If a bright light is directly in your field of vision, your iris closes down to protect your eye, so again, you can't see the stars.
The third factor is reflection.
A proportion of the sky glow of a city comes not from just directly the lights themselves, but the reflection of whatever it is the light is trying to illuminate.
"So there's some upward spill from a sports ground for example because — even if the lights are perfect and they're only pointing downwards and illuminating the sports ground itself — you've still got the reflection from whatever that playing surface is," Professor Watson says.
Dark surfaces such as asphalt reflect less light, but concrete is a knock-out. Not only does it scatter light, it reflects it upwards.
The amount of light pollution you experience can vary from night to night — it will be greater if there is cloud in the sky — from suburb to suburb, even within your backyard.
Which is why, if you did the star count back in April, it would be good to do it again, says Ms Ogg.
"In April, you may have had cloudy sky, or you may have had beautiful weather, but with science you don't just rely on one set of data, you go out and do it again and again."
How is light pollution measured?
Different groups use different measurements of light pollution.
Professional astronomers, who study the stars from dark-sky areas, do it by measuring the brightness of a given star — known as its magnitude.
The higher the magnitude, the fainter the star.
The naked eye can generally see objects up to a magnitude of 6 in dark sky areas.
But the fifth star in the Southern Cross ε (Epsilon) Crucis, which is magnitude 3.59, may be harder to spot in light polluted areas as it fades into the brighter sky.
Amateur astronomers describe the level of light pollution in the sky by what elements you can see in the sky, depending upon their magnitude.
This leads to something called the Bortle Scale, which is divided into nine light conditions.
Number 9 is the most polluted night sky condition, aka inner-city sky, and it means that nothing higher than magnitude 4 is visible. At this level, you're lucky if you can see the fifth star in the Southern Cross; the Milky Way is not visible at all.
"Certainly I haven't been able to see [Epsilon] from the steps of the Opera House," Professor Watson says.
A newer way of measuring the sky is to use a sky quality measure — a photometer that measures the light directly above your head on a cloudless night.
"If you want to put together an application for a dark sky park you've got to use the SQM," Professor Watson says.
But that is not entirely straightforward in the Southern Hemisphere, as Professor Watson discovered when putting in the proposal for the Warrumbungle Dark Sky Park.
The centre of the Milky Way doesn't pass overhead during the middle of the year in the Northern Hemisphere, where much of the work has been done to develop this tool.
"The centre of the Milky Way actually brightens the sky so it makes it look as though you've got light pollution where you don't — you've just got the natural light!"
How to count stars in the Southern Cross
Counting stars in the Southern Cross for this weekend's world record attempt is much easier.
All you need to do is count how many stars you can see using just your eyes (not binoculars or a telescope) and match it to the maps on the website that best represents what you can see.
These maps provide an approximation for the star's magnitude, and the Bortle Scale.
You can also note cloud conditions and nearby light sources.
And don't let any cloud cover — even rain — put you off.
"Even if your sky is cloudy ... it doesn't negate the entry," Professor Watson says.
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