Stargazers have been watching intently as Mercury moves across the face of the sun.
The planet - which is almost 5,000km (3,032 miles) in diameter - appeared as a tiny black dot during the Transit of Mercury - an event that happens around 14 times a century.
Even though it looked as though the planet crawled across the surface, Mercury actually moves at a speed of 50km (31 miles) per second.
Organised observation events have been taking place around the world, including at the Royal Observatory, Greenwich. The event has also been live-streamed online by the European Space Agency.
The Observatory pointed its Great Equatorial Telescope - one of the largest lenses in the UK at 28 inches - towards the sun for the first time since 1927 and used a special solar filter for the viewing during an eight-hour window from midday.
Public Astronomer Dr Marek Kukula said: "The last one was in 2006. These events don't happen every day, so it's a lovely chance to see it.
"Events like this are important for two reasons - historically they have helped astronomers work out how big the universe was, and now they are used to detect solar systems outside our own and help us understand the scope of the universe."
The viewing is dependent on weather conditions and some cloud cover obscured the view for a time in Greenwich. Those gathered to see it were warned not to attempt the viewing unless at a specially organised event with precautions in place.
Daniel Hardiman-McCartney, clinical adviser to the College of Optometrists, said: "This is a rare opportunity to see the Transit of Mercury across the sun, and people will be very excited to make the most of it.
"You may be tempted to look directly at the sun - but it is very important that you don't do so, as this will put your eyesight at risk."