Astronomers and space enthusiasts had the rare opportunity to watch as Mercury made a transit of the sun.

The smallest planet in the solar system could be seen as a tiny black disc moving across the glowing orb, starting in the UK at just after 12:35 PM on Monday.

(NASA/SDO/PA)

Members of the public were urged to join amateur astronomical societies and public observatories across the UK to witness the occasion safely.

And although poor weather conditions left a few people disappointed, we now have pictures of the occurrence.

The last time Mercury passed the sun this way was in 2016, which is not too long ago. But the next passing is not due until 2032.

NASA revealed some of the first images of the transit, taken from its satellite monitoring the sun on Twitter.

The entire event was visible from the eastern United States and Canada, the south-western tip of Greenland, most of the Caribbean, Central America, the whole of South America and some of west Africa.

(Nasa/SDO/PA)

Sadly, in Europe, the Middle East and most of Africa, the sun set before the transit ended, so the latter part of the event was not visible.

Every 88 years Mercury completes each orbit around the sun, and it passes between the Earth and sun every 116 days. Because the planet's orbit around the sun is tilted, it normally appears to pass above or below our nearest star.

(PA Graphics)

A transit can only take place when the Earth, Mercury and the sun are exactly in line in three dimensions. Which makes capturing the moment incredibly hard. So, it's cool that many took a picture of the event.