The tension was heavy in the air among the dozen people gathered expectantly at Greenwich Park gate. It was 0606BST and the gate still wasn't unlocked.
By Lucy Wilkins
BBC News Online
Jane Cartwright entertained with tales of astronomers past
Anxious that they were going to miss an event that no living person had witnessed, a couple scaled the 8ft ornate gate and ran for the hill.
Minutes later, a police car arrived with the keys for the padlock. The relief was visible. Several people raced over the well-kept lawns, heading for the Royal Observatory.
They were all there to see a Transit of Venus - last witnessed in the UK in 1882 - when Venus passes across the Sun, appearing as a black dot against the vivid orange circle.
The sky above London was utterly cloudless, blue and perfect for such an event.
Astronomer Jim O'Donnell, squinting in the June sunlight of a day forecast to be the hottest this year, said: "There was a lot of anticipation, so that made us a bit cautious in case it failed because it was too cloudy."
He was manning one of many telescopes set up in the observatory's courtyard for the public.
"People are really enthusiastic. Just like at other events, like the eclipse, a broad spectrum of the general public turns up - and they've always got lots of questions."
Astronomer Jim O'Donnell: Relieved the sky was cloudless
He said the crowd - a couple of hundred within 45 minutes of the start of the six-hour transit - was the biggest he had dealt with, but added "I enjoy going out and talking to the public".
Experts have warned the public about the dangers of looking directly at the Sun and urged observers to use a safe method such as pinhole projection.
Free solar glasses were handed out by observatory staff and many people had brought their own amateur viewing equipment.
Jenny Gristock, 32, and her ten-year-old son Finnian had left Brighton at 4am to get a prime position.
Initially disappointed at 0625BST - a mere five minutes since the phenomenon had begun - Ms Gristock was more excited by 0629.
Finnian and Jenny Gristock make a day of it
"Oh yeah, I can definitely see something on the left hand side,"
They were planning to stay for the duration and Finnian was looking forward to telling his classmates about his day out of school - "my teacher said 'just go for it'".
Others were also planning to spend the day at the southeast London park.
Julia Simpson, 26 years old and eight and a half months pregnant, said she had brought food supplies.
"I can think of nothing nicer to do on my maternity leave, than wandering round here. It's such a nice atmosphere," she said as her boyfriend fine-tuned a telescope.
"We saw an eclipse a couple of years ago in Australia - this isn't as spectacular, but it's quite good."
Pregnant Julia Simpson doesn't plan to call her child 'Venus'
She said she would definitely tell her baby about the event - but stopped short of calling her "Venus".
Actress Jane Cartwright, dressed in an 1890s costume, was entertaining the queues with tales of long-dead astronomers who had been unlucky in their attempts to see the transit.
Amateur astronomer Elizabeth Brown missed two transits - in 1874 and 1882 - because as a single woman she had had to look after her ailing father, Ms Cartwright said.
Out of character, Ms Cartwright was excited as her captive audience to be able to see the event.
"Having talked about it for so long, and knowing the lengths some men went to see, I am really excited."