The UK countryside charity CPRE is asking for help with its Orion Star Count 2022 project. You can take part this weekend by counting how many stars you can see in Orion and letting them know the results. That’s it. This activity brings people together and allows us to appreciate nature and experience the sky above. It illustrates how heritage is linked to the sky and what we’re losing through light pollution. After the count, you might look at your own house – and its lights – and consider what you can do to help keep our skies dark.
The star count will map the current status of light pollution by showing where the darkest skies are in the UK. It can offer local councils information to take action and limit light pollution. Some culprits include badly installed lighting at monuments, sports grounds or excessive and unsuitable street lighting.
Rather than trying to measure how bright the sky is – which is pretty tricky – we can use stars. Stars differ in brightness levels, and usually the unaided human eye under good conditions can spot stars as faint as 6-7 magnitudes.
A magnitude is a measure of a star’s brightness. Roughly speaking, the faintest visible star is approximately 6 magnitudes and the brightest around 0 magnitudes. This system is a legacy of the Greek astronomer Hipparchos – and was later improved by one of Nottingham’s own astronomers, Norman R. Pogson.
As the sky gets brighter – due to bad lighting on Earth illuminating the sky and sending light back to us – we can’t spot the fainter stars anymore. As a result, if we pick out a certain area in the sky, the amount of stars we can see in it will give us a pretty good idea how bright the sky is.
How to take part
If you want to join, the star count is happening until March 6. Make sure you pick a clear night with no clouds or haze in the sky, as that will make the sky brighter and the viewable stars even fainter. Wait until 7pm at least, so the sun has set substantially and skies are as dark as they can get.
Find a spot that is dark, where you feel comfortable and safe to watch the southern sky. Take your time to wait at least 20 minutes for your eyes to adjust to the darkness. Your eyes are marvellous and very sensitive, but need a bit of time to fully switch to night time vision. You can use that time to explore the stars in the sky – and find the constellation of Orion.
At this time of year, look south after sunset and Orion (see image below for guidance) will be a handspan above the horizon. Once you have found it, you will need to count the number of stars you can see inside the rectangle formed by the four corner stars in the Orion constellation. Include “Orion’s belt” but not the four corner stars. Then report the number back using the CPRE website’s online form.
Orion is a beautiful winter constellation. There are 88 constellations that have been recognised by the International Astronomical Union. Orion is one of the easiest to spot, having seven bright stars resembling a human. Especially striking are the three bright stars forming the line of Orion’s Belt, as well as the infamous red star Betelgeuse at the top left shoulder and the white star Rigel at his bottom right foot.
Stellar heritage
While gazing at Orion, you might not think it much resembles a mythical hunter as described in the Greek myths, but something entirely different. But when you look at these stars, you see what the builders of Stonehenge saw, the Pharaohs of Egypt gazed upon, and what was used to navigate between the islands of Polynesia. It forms a window into our past heritage that light pollution is now closing on us.