Transit of Mercury: Stargazers watch planet move in front of the Sun and see just how small we are

Transits have historically been used as a way of figuring out our place in the solar system – and are now used as a way of mapping out the huge scale of the universe

A jet airliner leaves a vapor trail as the planet Mercury is seen in the bottom left transiting across the face of the sun (Reuters/David Becker)

Some lucky stargazers have managed to see one of the sky’s most stunning sights – the transit of Mercury, as it moves across the front of the Sun. But at the same time they were watching one of the most historically important events that we can see from Earth.

The event happens about 14 times each century and is one of the few times that we can see Mercury directly. It appears as just a tiny black spot on the face of the Sun, since despite Mercury’s huge size – it is 3,000 miles in diameter – it is dwarfed by our star.

And it’s that image of the planet that allowed the event to become so important. Through history, astronomers have used them as a way to calculate the true scale of the solar system and our place within it – transits like that of Mercury provided us with one of the first ways of knowing just how small we are.

But they’ve not lost that importance. Nowadays, scientists can use the events to find solar systems far from our own – and, just as astronomers once used the planets to understand our place in the solar system and its size, they can now do the same with the universe.

For all the grandeur, many people wouldn’t – or shouldn’t – have been able to see it at all. People were advised not to look straight at the Sun to see it, and only watch events with the right equipment.

Since seeing a transit means looking at the Sun, it can be incredibly dangerous and can lead to people losing their eyesight. And since Mercury is so small in front of the star, it requires a telescope – making it even riskier than normal.

Instead, stargazers were encouraged to head out to public events where people could make use of telescopes with special solar filters that keep out the dangerous brightness of the Sun.

Luckily for those unable to make it, Nasa and the European Space Agency held events to let people watch through their telescopes over the internet. Some of those images even came from space – helping avoid any potential problems caused by clouds.