On the top of a hill, above Northumberland’s beautiful Kielder Water lake, a group of people are waiting in a car park next to a strange wooden building. They are here because of the darkness and this is Kielder Observatory, the centre of Britain’s latest industry – astrotourism. The people who are waiting outside are lucky. Many more people apply for a night of stargazing at the observatory but not everyone can come because numbers are strictly limited. 
Inside, the observatory’s founder and lead astronomer, Gary Fildes, speaks to his colleagues and volunteers. The team discusses that they might see the northern lights but Fildes doesn’t think they will. Instead, they decide to use their powerful telescopes to look at Jupiter and Venus and, later, to find stars such as Capella and Betelgeuse. An extra attraction is the appearance of the International Space Station. 
Fildes is a leading figure in the UK’s growing astrotourism industry. The key moment for Northumberland came in 2013 when the entire national park, about 1,500 square kilometres in area, got Dark Sky Park status. It is the only one in England. Dark Sky Parks are rare. Research in 2013 showed that only 5% of the UK population can see more than 31 stars on a clear night. 
The International Dark-Sky Association (IDA) gives the status of Dark Sky Park only to places that take big steps to prevent light pollution. The areas must also prove their night skies are very dark. In Northumberland Dark Sky Park, it is so dark that Venus casts a shadow on the Earth. 
Duncan Wise, visitor development officer for the Northumberland National Park, helped to lead the campaign for dark-sky status. “We usually think that 'landscape' is everything up to the horizon,” Wise said. “But what about what’s above the horizon?” Wise and others spent years preparing their application to the IDA – they collected thousands of light readings. 
Because of their hard work, many of the 1.5 million tourists who visit Northumberland each year are now aware of its Dark Sky status. “A lot of people come here to see the sky now,” says a man who works for a local car-hire company. “They come in autumn and winter, when it’s darkest. It’s good for the local hotels because tourists come all year round now.” 
Wise agrees that Northumberland needs to do more to take advantage of its dark skies, which are very rare. He believes the region needs more observatories to make sure that visitors will see what they came for. A new £14-million national landscape discovery centre will have an observatory when it is completed in a couple of years. 
Fildes has big ambitions. He is planning Britain’s first “astrovillage” – it would have the largest public observatory in the world, a 100-seat auditorium, a 100-seat planetarium, and radiomagnetic and solar telescopes. The multimillion-pound project would also have a hotel and attract 100,000 visitors a year – that is four times the number that are currently able to use the observatory. 
However, Northumberland has competition. Galloway Forest Park in Scotland also has Dark Sky Park status. Exmoor in south-west England became Europe’s first International Dark Sky Reserve – one level below Dark Sky Park – in 2011. A number of local businesses there now offer stargazing holidays. The UK is not as good as northern Chile, which has more than ten tourist observatories and some of the clearest skies in the world. 
So, why do people want to look at the night sky? The media have helped. TV programmes about astronomy have attracted a new generation of stargazers. Technology has also made astronomy more popular. Apps such as Stellarium now turn smartphones into pocket-size planetariums. But Fildes believes that, most importantly, people are starting to appreciate the sky. “If you had to build a visitor attraction from the beginning, what could be better than the universe?”