10 May 2016

NZers miss out on Mercury crossing the sun

10:19 am on 10 May 2016

Skywatchers across the globe have watched Mercury transit the Sun, but New Zealand is in a small band of countries that missed out.

The planet Mercury is seen from NASA Headquarters in Washington DC, in silhouette, lower left, as it transits across the face of the sun.

The planet Mercury is seen from NASA Headquarters in Washington DC, in silhouette, lower left, as it transits across the face of the sun. Photo: AFP

Mercury's sojourn between Earth and the Sun lasted from 11.12am GMT (11.12pm NZ time) until 6.42pm.

It was the third such pass of 14 this century; Mercury will not make another transit until 2019 and then 2032.

The event is impossible - and dangerous - to view with the naked eye or binoculars, but astronomy groups worldwide are offering the chance view it through filtered telescopes.

Live views from space and ground telescopes were also available online.

They showed Mercury as a tiny black circle, smaller but darker than many sunspots, slowly traversing the Sun's giant yellow disc.

Mercury spins around the Sun every 88 days, but its orbit is tilted relative to the Earth's. It is that discrepancy which makes it relatively rare for the three bodies to line up in space.

From western Europe, north-western Africa and much of the Americas, Mercury's glide across the Sun was visible in its entirety.

A further swath of the planet was able to catch part of the transit, depending on local sunrise and sunset times.

The only land masses to miss out completely were New Zealand and Australia, far eastern Asia including Japan, Korea, the eastern coast of China, parts of South East Asia, and Antarctica.

Because Mercury is so small - just one-third as big as Earth and, from our perspective, 1/150th of the Sun's diameter - its transit could only be glimpsed under serious magnification.

The "eclipse glasses" used by thousands of people to view last year's solar eclipse were useless, and to avoid permanent eye damage, telescopes had to be fitted with a solar filter before being trained on the Sun.

The British Astronomical Association explained on its website how amateur stargazers could enjoy the spectacle safely.

Open University's David Rothery said the celestial event did not present any novel scientific opportunities - but was special nonetheless.

"From this transit, we're unlikely to learn anything we don't already know," he said. "But what a wonderful event for showing people Mercury. It's a hard planet to see.

"Historically, transits were of immense importance."

In the 1700s, for example, it was observations of Mercury and Venus slipping across the Sun that allowed astronomers, led by Edmund Halley, to pin down the dimensions of the known Solar System.

Professor Rothery is a Mercury expert and a leading scientist on the European Space Agency's BepiColombo mission to the diminutive planet, which will launch in 2017 or 2018.

Mercury has already been visited by two Nasa probes: Mariner 10 flew past in 1974 and 1975 and Messenger spent four years in orbit until its planned crash landing in 2015.

"[Messenger] told us an awful lot. It really told us we don't understand Mercury - because there's a lot of things which just don't stack up," Prof Rothery said.

"It's an airless body, with lots of craters ... But there's been a long history of volcanic activity, fault activity - and the composition, that began to be revealed by Messenger, is weird.

"There's very little iron at the surface but it must have a ginormous iron core, because it generates a magnetic field - which Venus, Mars and the Moon don't."

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