'They're going to pop up everywhere. Everybody's going to want one,' says Mario Di Maggio, his arms waving about in excitement, as if defying gravity. 'People are fascinated with astronomy. It's a very sexy science. It's in the news all the time.'

Di Maggio is the planetarium manager and head astronomer at Thinktank, Birmingham's science centre at Millennium Point - earthly home to the first purpose-built digital planetarium in the UK. Thinktank is just the latest addition to Britain's growing galaxy of more than 20 planetaria, many of which are attached to museums or science centres.

And next spring, London's National Maritime Museum will launch the greatest of all out-of-this-world experiences when it opens a planetarium housed under a mighty bronze cathedral-like cone at the Royal Observatory, Greenwich. This is part of the museum's Time and Space project, a £15m redevelopment of the observatory site.

The Royal Observatory's planetarium, which is still under construction, sits on the world's Prime Meridian, longitude 0 degrees, with the cone inclined at 51.5 degrees North - the latitude of Greenwich - so it points to the North Star. A glass roof reflects the clouds by day and moon by night. 'A building that reflects its place on the earth,' says Robert Massey, the Royal Observatory and National Maritime Museum's public astronomer.

But is this leap for the stars nothing more than pandering to populist interest in the planets? And is a planetarium really just an entertaining sideshow on a curved ceiling that gives you a crick in the neck?

Di Maggio springs to the defence of his new hi-tech toy. 'Astronomy and astrology - people don't know the difference,' he says. 'This is a fantastic opportunity to clear that up. I'm just interested in communicating science. That's the only long-term solution
for humanity.'

A planetarium undoubtedly draws visitors into a museum's orbit. In the first three months since its opening, Thinktank attracted 27,000 stargazers. The Royal Observatory welcomes more than 800,000 visitors each year. Massey sees a big market for its new 120-seater since the recent closure of the commercial London Planetarium, which
Madame Tussauds converted into a film-based attraction called the Stardome.

'If you look at the number of people studying astronomy, from an amateur level to the number of telescopes being bought, it's huge. People are spending money on astronomy. We'll pick up the market that Tussauds dropped. We'll be London's planetarium.'

Melanie Quin, the executive director of Ecsite-UK, the UK network of science centres and museums, agrees that a dome above and reclined seats below, coupled with celestial calm, are at the top of many visitors' wish lists.

'If you were to start with a greenfield site, a sketch book and a limitless pot of gold, you would inevitably include a planetarium in your scheme,' Quin says. 'It's one of the facilities and attractions of an institution. Quite simply, it's a change of pace. There comes a point in the afternoon when people just want to take the weight off their feet and sit down. And a new show in the planetarium can be marketed as a reason to come back. The National Space Centre in Leicester, for example, is always advertising new shows.'

But in the thrust to fill the planet-arium's darkened rows, does science have to take a back seat? Will the new planetarium in Greenwich be science or entertainment? Massey is determined Greenwich will respect its illustrious scientific roots. 'It's a mixture of both,' he says. 'But you don't start from the values of the entertainment industry. It's not Disneyland or Hollywood. It's about making big concepts exciting. That's not easy - but it is possible.'

Massey admits the marriage of popularity and scholarship was not made in heaven. 'Let's be clear - if you've got a PhD in astronomy and you come here, you won't learn anything new. But if you've got a PhD in art history, you don't make discoveries in the National Gallery either. But an informed lay person should be excited by what's here.'

The four modern astronomy galleries adjacent to the Royal Observatory's planetarium are still under construction and details of the design and interpretation are yet to be finalised, but Massey has a clear idea of his priorities. 'It's not going to be geared towards the lowest common denominator,' he insists. 'We want to retain an adult audience. We don't want a place full of coloured plastic. It's about the scientific concepts.'

Nick Winterbotham, the chief executive of Thinktank, also sees no conflict between educating and entertaining: 'If someone's enjoying something, they're also learning from it. It's fun, it's interesting. And it's exactly what they asked for.'

Di Maggio puts scientific education at the heart of his space mission. 'School kids don't know how beautiful the night sky is. Most have never seen the Milky Way. If you're living in a big city, you might be able to spot about 200 stars. In here, you can see more than 2,500. So it's good for education.' Thinktank's planetarium is a 70-seater, designed to hold two school groups.

Quin says the educational value of a planetarium has always been clear: 'Some of the oldest planetaria in Europe are attached to famous museums. One of the first was at the Deutsches Museum in Munich - they recognised right from the start that this was the only effective way to teach space science. Planetaria deliver a portion of the curriculum that couldn't possibly be delivered in class. It's an area of science that is out of the reach of teachers. And with digital technology now, you can do a lot more in terms of space navigation.'

And it's people, not the planets, that are at the core of a planetarium's success, Quin believes. 'The most successful, from the visitor satisfaction point of view, is where there's live interpretation - that's what visitors like.'Greenwich relies entirely on 'human explainers'.

Roy Clare, the director of the National Maritime Museum, is dismissive of starstruck institutions such as the Hayden Planetarium at the Rose Center for Earth and Space in New York, where shows are narrated by Robert Redford.

'We don't do voiceovers; a voiceover could be in a cinema or in front of your PC,' says Clare. 'We have human beings, and what we offer is the expertise and authority of the staff here. Someone says this show is for you, and we'll tailor it to the kind of group we think you are. The pace of the show goes at the dynamic of the audience. The advantage of a human presenter is flexibility. That is the key difference.'

Clare fondly remembers a former human explainer who worked in the old Greenwich planetarium. 'We didn't have Harrison Ford, we had Harry Ford. A real man. He got an MBE when he retired. People loved him.'

Thinktank also uses live presenters - 'real astronomers on the staff who speak to the audience' - who give the opportunity to question and contribute. 'Important questions come up,' says Di Maggio. 'How did the universe begin? Do you believe in God? The Big Bang? Creationism? It's a forum for debate.'

Winterbotham believes science museums, and the planetarium within them, don't only stimulate debate on important issues, but they could also change attitudes and help to explain issues such as climate change. But there's one question the audience won't ask the human explainer - are planetariums as much about generating money as generating debate?

Greenwich is going to run popular Dinner with the Stars evenings, seating 110 people in the new cone. 'There's a market for that,' says Clare. 'But museums are places people trust. We can't squander that. We're not just selling something because we're
making money on it.'

The Birmingham Chamber of Commerce held its annual general meeting in Thinktank's planetarium, showing a film about Van Gogh. Di Maggio has plans to introduce club nights, where you can safely get spaced out, to attract teenagers back into the museum. Massey is more modest: 'I don't think we'd go down the nightclub route. Classical concerts - maybe. But clubbing - I'm not sure.'Now planetaria - or immersive cinemas, as Di Maggio prefers to call them - are going digital, they can be put to almost any use. 'They're so much more than just planets and stars,' he says.

Di Maggio wants to soar still higher. 'We need a big one,' he says, looking around his humble 70-seater. 'This one is way too small. This is just to prove that it's valuable. Every museum should have one. And they will.' mj

Dea Birkett is the founder of Kids in Museums

www.kidsinmuseums.org.uk