The public will have an "exciting" opportunity later to see the planet Mercury pass in front of the sun.
The Transit of Mercury will see what appears to be a tiny black dot moving across the sun during an eight-hour window from midday.
People will be able to watch the event safely at a number of locations around the world, including the Royal Observatory Greenwich. It will also be live-streamed online by the European Space Agency.
Those gathered at the Royal Observatory will be able to view the event through the 28in wide Great Equatorial Telescope - the largest lensed telescope in the UK - during two special half-hour viewing sessions.
The telescope has not been turned towards the Sun since 1927 and will be using a new solar filter to make the viewing possible.
Others will be able to take advantage of free telescope viewings next to the Astronomy Centre between midday and 7.45pm.
Public Astronomer Dr Marek Kukula, who noted that it is not ever safe to look directly at the sun, encouraged people to take the opportunity to watch the event at one of the many organised viewings.
He 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."
Asked what those gazing through the lens might see, Dr Kukula said: "It appears as a tiny black dot moving across the sun. You can't see it with the naked eye.
"When you are watching this tiny black dot crawling across the face of the sun you are actually looking at something about the size of Africa 100 million kms away moving at a speed of 50km per second. It is quite exciting."
The viewings will depend on weather conditions, but Dr Kukula said conditions in London are looking good for later.