When you look up into the night sky, you are looking into the past.Gazing at the stars, you are seeing them as they looked years ago, not as they are now. But for the first time in three years, Hampstead Observatory is now looking to the future once again.
The observatory, perched atop the Kidderpore Reservoir, has been closed to the public since 2016.
On Sunday it will reopen its doors and roof to coincide with the 50th anniversary of Apollo 11's return launch, which saw the first people ever to set foot on the moon.
It will be three years since it closed as work was being done to repair the membrane that the observatory sits on top of.
This led to the discovery of dry rot, meaning a lengthy repair process had to take place.
Simon Lang, co-secretary of the Hampstead Scientific Society along with Doug Daniels, has been involved with the facility since 1984.
He's hoping it will give youngsters and locals the chance to get involved with the observatory.
As well as the public being able to have a look around, there will be an exhibition on about the historic mission 50 years ago. This will allow Simon to carry on one of his favourite things about being involved with the society: passing on his enthusiasm to others.
"I love all things astronomical," he said. "I enjoy engaging with the public and teaching them how it all works.
"I love the build-up when they don't know about something new and it then exciting them. If you show them Saturn through the telescope they can't believe what they're looking at."
His own interest was piqued as a child. He remembers as a nine-year-old, being woken up by his parents at 3am to see Neil Armstrong and Buzz Aldrin set foot on the moon.
"We watched it on an appalling black-and-white TV," he said. "It was what was happening that was amazing. I remember thinking: 'This is something else.'"
He then got involved at the observatory years later around the time of the return of Halley's Comet in 1986.
The visit was witnessed by more than 1,000 people at the observatory.
He explained: "I got very keen. There was a lot of evening classes and I was desperately keen to learn about astronomy, but it wasn't very sexy. It turned out that it was being given by Terry Pearce, who was involved at the observatory."
The observatory has been on the top of the Hampstead reservoir since 1910.
Another had existed previously on Hampstead Heath, according to Simon, but users had difficulty finding it in the dark without lights.
The location is almost obvious, he points out: it's the highest place at ground level in London. The society was formed in 1899.
There had been plans to reopen the observatory last year, after Thames Water had finished work on the reservoir underneath.
But the discovery of dry rot, along with "life getting in the way", set those plans back.
Sunday's opening will be a one-off to celebrate the anniversary, before opening more regularly in September.
It houses two telescopes. The larger, six-inch Cooke refracting telescope dates from early in the last century.
Another four-inch one, Wildey, is able to be moved outside. The portable telescope is named after one of the society's former astronomical secretaries, Henry Wildey.
Despite being the only observatory open to the general public, it receives no formal funding and is a registered charity.
People can visit free, which is a key part of the observatory's purpose, according to Simon.
"We've had one substantial donation before," he said, "but otherwise we rely on people putting money into the donations tin. We need that to stay open.
"We're open for educational purposes, and not charging people means anybody can come along and learn.
"We just say to everybody that if they can give us a donation then we will be very grateful.
"They are often more generous than they used to be."
On Sunday, the observatory in Hampstead Grove will open from midday to 10pm. Visitors will be able to use telescopes to view the sun's surface, and enjoy an exhibition about the Apollo moon landing, featuring models, fact sheets and "pop bottle rockets".
Perhaps, much like a young schoolboy sat in front of his parents' TV 50 years ago, this weekend's open day could be the start of a lifetime of astronomical interest for children in Camden.
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