Stargazers across the world are watching as Mercury moves across the face of sun - a rare event that happens just 13 or 14 times every 100 years.
The planet is almost 5,000km (3,032 miles) in diameter and appears as a tiny black dot on its journey, which lasts from 11.12 until 18:42 GMT.
Mercury will not make another transit until 2019 and then in 2032.
The event is dangerous to view with binoculars, but astronomy groups around the world are giving people the chance to view the event through filtered telescopes.
Public Astronomer Dr Marek Kukula said: "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."
Dina Sadek, a stargazer watching the event from Abu Dhabi, said: "I feel like this is something we should cherish.
"I went back and looked at the telescope twice because I couldn't register this is happening and it's not going to happen for a while, so I had to look at it twice."
Mercury's transit is entirely visible from western Europe, north-western Africa and throughout much of the Americas.
The only regions to miss out completely are Australasia, Antarctica and far eastern Asia.