Stargazers have had a rare opportunity to witness Mercury fly directly across the face of the Sun, a sight that unfolds about once every 10 years, as Earth and its smaller neighbouring planet came into perfect alignment.
Key points:
- Transit best seen from eastern Americas and western Europe and Africa
- Could be seen with high-powered binoculars as a precisely round spot
- Mercury is slightly larger than Earth's moon
Mercury was seen through telescopes as a black dot inching over the face of our star, providing a celestial spectacle lasting seven-and-a-half hours.
"It's something rare, because it requires the Sun, Mercury and Earth to be in almost perfect alignment," Pascal Descamps of the Paris Observatory said.
The smallest recognised planet in the Solar System, Mercury completes an orbit every 88 days and passes between the Earth and the Sun every 116 days.
But its orbit is tilted in relation to Earth's, which means it usually appears — from our perspective — to pass above or below the Sun.
The best vantage points to observe the celestial event, known to astronomers as a transit, were eastern North America, South America, western Europe and Africa.
In those regions, the entire transit occurred during daylight hours, according to Sky and Telescope magazine.
But Mercury is too small to see without high-powered binoculars or a telescope, and looking directly at the Sun, even with sunglasses, could cause permanent eye damage.
NASA and astronomy organisations provided virtual ringside seats for the show by live-streaming images of the transit in its entirety and providing expert commentary.
The tiny planet, slightly larger than Earth's moon, started off as a small black dot on the edge of the Sun at 9:12pm AEST (1112 GMT).
It travelled 48 kilometres a second across the face of the Sun, which is about 1.39 million kilometres in diameter, or about 109 times larger than Earth.
"Unlike sunspots, which have irregular shapes and grayish borders, Mercury's silhouette will be black and precisely round," Sky and Telescope said in a press release.
The event came into view in the western United States after dawn and ended at sunset in parts of Europe, Africa and most of Asia.
The transit of Mercury was first recorded by French astronomer Pierre Gassendi in 1631.
German astronomer Johannes Kepler had correctly predicted that transit, but died in 1630 before he could witness the event.
The last Mercury line-up was 10 years ago, and the next will be in 2019, followed by 2032 and 2049.
"It is always exciting to see rare astronomical phenomena such as this transit of Mercury," Royal Astronomical Society president Martin Barstow said.
"They show that astronomy is a science that is accessible to everyone."
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