Mercury moves across sun's face in rare phenomenon

Krystal Johnson and agencies

For the first time in 10 years, Mercury has passed directly between the Earth and sun, resembling a black dot against the massive star's vast, glowing face.

Stargazers turned to the internet as NASA provided almost real-time images of the black dot inching over the moon in seven-and-a-half hour trek, courtesy of the Solar Dynamics Observatory.

Mercury is visible as a tiny black dot crossing the face of the sun in the rare event known as the

Mercury is the smallest recognised planet in the Solar System and completes an orbit every 88 days. It usually darts above or below the Earth and Sun every 116 days.

This rare astronomical phenomenon only happens 13 times a century and the next transit won’t occur until 2019.

"It's something rare, because it requires the Sun, Mercury and Earth to be in almost perfect alignment," said Pascal Descamps of the Paris Observatory.

The cosmic show began at 7:12 am EDT (9.12pm Monday AEST) and was visible from the eastern US and Canada, as well as Western Europe, western Africa and most of South America.

Those places were privy to the entire event but Australia, New Zealand and New Guinea missed out altogether.

This of the transit of Mercury was seen from St.Petersburg, Russia on Monday. Photo: AP

NASA warned amateur stargazers to use high-powered binoculars or telescopes equipped with special filters to protect their eyes from the glaring sun, as it could have caused permanent eye damage

Eclipse glasses were useless for spotting 4828 kilometre-wide Mercury as it crossed the 1,390,000 kilometre diameter of the sun.

“What happens during a transit is really all about perspective,” said Jim Green, director of NASA’s planetary science division who viewed his first transit of Mercury 46 years ago.

Hot and cold

The closest planet to the Sun and a third the size of Earth, Mercury is one of the Solar System's curiosities.

It is one of the four rocky planets of the inner Solar System but has no atmosphere and its metallic body is scarred by collisions from space rocks.

Daytime on Mercury is six times hotter than the hottest place on Earth, and nighttime can be more than twice as cold as the coldest place on our planet.

It rotates so slowly - three times for every two orbits - that, bizarrely, Mercury's day is twice as long as its year.

The transit of Mercury was first recorded by French astronomer Pierre Gassendi.

Daytime on Mercury is six times hotter than the hottest place on Earth, and nighttime can be more than twice as cold as the coldest place on our planet. Photo: NASA

He observed it through a telescope in 1631, two decades after the instrument was invented.

German astronomer Johannes Kepler had correctly predicted that transit, but died in 1630 before he could witness the event.

"It is always exciting to see rare astronomical phenomena such as this transit of Mercury," said RAS President Martin Barstow.

"They show that astronomy is a science that is accessible to everyone."