• Transit of Mercury 2016 taken by NASA (AAP)Source: AAP
Last night, Mercury crossed the face of our Sun, with awe-inspiring photographs captured by NASA. But why do astronomers get excited about this?
Shami Sivasubramanian

10 May 2016 - 4:05 PM  UPDATED 10 May 2016 - 4:05 PM

Last night at 9:12pm AEST, the planet Mercury made its transit across the Sun. Astronomers and amateur stargazers alike took to their specially filtered telescopes and safe transit-watching contraptions to watch the small dot-like silhouette travel across the large sun's backdrop.

Photographs of the transit were taken by NASA and have already been shared several thousand times across social media.

Transit of Mercury, as it is officially called, lasted about seven-and-a-half hours as the planet zipped in front of the sun at the rate of 48 kilometres per second.

Why is it so rare, especially when Mercury comes between the Earth and Sun nearly three times a year?

Since Mercury's orbit of the Sun is a mere 88 days to our 365, Mercury laps us every 116 days.

This means Mercury sits between the Earth and Sun about two to three times a year. And if that's the case, what makes this transit so special?

The problem, says Dr Alan Duffy, research fellow at Swinburne University's Centre for Astrophysics and Supercomputing, is that Mercury's orbit of the sun is at an angle to the rest of the solar system.

"The reason it's rare is you have to get the perfect alignment. Most of the planets in the Solar System sit around the sun like a vinyl record, on that plane. But Mercury is on a bit of an angle. It does lap us about three times a year, but because of that tilt, we never truly capture it in complete silhouette like that in front of the sun,"  he says.

"The transit of Mercury only really happens about a dozen times each century, which is why it is rather rare,' says Duffy. "It's because of its tilt; you can see that [in the photo above] in the slight diagonal line it tracks."

But I looked out my telescope and didn't see a thing. What gives?

Though many people across the globe did watch the transit of Mercury through their safe star-gazing apparatus, Australia was left quite literally in the dark, since the transit occurred during our night. 

"And I'm afraid we'll have to wait 'til 2032 transit," says Duffy, adding that the next transit of Mercury will be in 2019, but will once again only be visible to the northern hemisphere.

OK, so it's more about NASA's amazing photographs. Anything cool they discovered?

"Mercury still holds mysteries to learn about, so we definitely want to keep looking," says Alan Duffy.

NASA's state-of-the-art photo and video footage has yet to unearth anything new. However their images did capture a halo of the Mercurian atmosphere, which is something that can be further analysed.

"One valuable scientific fact is we can probe the atmosphere of Mercury as there is a little trickle of sunlight shining through Mercury's atmosphere," says Duffy. "That can give us a spectral and chemical fingerprint of the planet's atmosphere."

NASA has, however, already collected significant data on the state of Mercury's atmosphere through other research projects. Nonetheless, there's always room for more exploration.

In fact, Mercury is "shrinking" away, with parts of the planet being blasted off and trailing the planet as it moves through space, "almost like a comet's tail".

"Mercury has basically no atmosphere. It's actually being blasted off because of its low gravity. It has these cracks which occurs as the planet's surface heats up and cools down to extreme temperatures. " says Duffy, explaining how structurally vulnerable the planet is.

But ultimately, it's all about NASA's incredible photos and videos. You can view the transit of Mercury across the sun observed at different wavelengths of light here.

What's the significance of it all?

Alan Duffy says in spite of the transit not revealing anything new about Mercury, the transit, like most major astronomical phenomena, is a great reminder of how vast and dynamic the universe is.

"It's something were all look forward to in the scientific community." he says. "And it's a beautiful reminder that we live in a dynamic world."

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