Star Count: measuring light pollution

Eliza Hermann

When did you last see the Milky Way, that pale band which appears to arch across the night sky, formed from many billions of stars which make up our own galaxy? Our view of the stars is a source of infinite amazement for scientists and casual observers alike.

What is Star Count?

Each year in February CPRE asks people from all across the country to become citizen scientists and join in with its Star Count. Choosing a clear night, people are asked to look up at the constellation of Orion and let CPRE know how many stars can be spotted.

Stargazing is a great activity for children, which you can do from your doorstep or garden or even through a window. There are lesson plans available for Key Stages 1 and 2 children on the national CPRE website, developed by CPRE with the Royal Astronomical Society. Don’t forget to look at the moon as well: you can notice its different phases and, with the help of binoculars, pick out its surface features.

During February, without any special equipment, these are two constellations you can easily find on a clear night:

  • The Plough or Great Bear looks like a giant saucepan. You can use it to find north. Draw an imaginary line through the side of the saucepan away from the handle: follow its line upwards until you see a very bright star. That’s the Pole Star and if you stand to face it you are facing north.
  • Orion (the hunter) with his central belt of three stars, is a very distinctive feature in the night sky, especially from January to March. The constellation includes two of the brightest stars in the sky, Rigel and Betelgeuse. These, the belt stars and two others form an hourglass shape.

Light pollution

Sadly there are places in the country where ‘pollution’ by night-time light from many sources spilling into the sky obscures the best view of the stars. Light pollution at night also disrupts wildlife’s natural patterns. A really dark sky helps to distinguish rural areas from urban. With light pollution steadily encroaching further into the countryside, CPRE is a leading voice among organisations trying to stop, and even reverse, this trend.

CPRE nationally has produced an interactive map which show how much light pollution there is in your local area. In Hertfordshire the darkest skies can be experienced in the north eastern areas of the county. The national CPRE website includes information on how to campaign to reduce light pollution in your locality. You could for example lobby your local council if a local light source is a problem, and ask the council to ensure that new developments have well-designed lighting schemes or are refused permission if they would cause light pollution in existing dark places.

Have a think about your own house too, as domestic security lighting is a major source of light pollution, especially when it is left on all night. You can make sure that your exterior house lights only come on when needed and point downwards, just illuminating the area which needs to be lit.

Star Count results

In 2020 CPRE’s Star Count found that 61% of participants lived in areas with severe light pollution – based on the number of Orion’s stars they could see. This was an increase over the figure for 2019. On the plus side there was a small increase in the number of people who could see more than 30 of Orion’s stars, an indication that they have truly dark skies.

In 2021 a record 7,800 people took part in Star Count across the UK, during the period of lockdown. The good news was that the percentage of people nationally able to see 10 stars or fewer in the constellation of Orion fell from 61% in 2020 to 51% in 2021. This means that light pollution appears to have diminished, for 2021 at least, possibly as a result of the drop in human activity in urban areas. Those able to see more than 30 stars in Orion increased from 3% in 2020 to 5% in 2021: these people are fortunate in being able to enjoy truly dark skies.

In Hertfordshire a slightly higher percentage of people – just over 53% – saw 10 or fewer stars in Orion in 2021, and are living in areas with severe light pollution. This is perhaps not surprising in one of the most densely populated counties in the UK. Nonetheless nearly 6% reported being able to count more than 30 stars in Orion, and are experiencing really dark skies. Huge thanks to the 158 people who submitted their sightings from Hertfordshire.

CPRE is now calling on councils and government to take action on light pollution so that more people can enjoy the remarkable tranquillity of a dark starry sky, a distinctive feature of the countryside.

We’ll post more information early next year about how to join in for 2022.