Rare Occurrence: The Sun, Mercury and Earth Lining Up In Space

The planet Mercury will pass between Earth and the sun today.  Out of the 8 planets in our Solar System, Mercury is the closest  to the sun, Earth is third from the sun.  

Mercury's journey between the Earth and the sun will be invisible to the naked eye or binoculars.  In fact, it is dangerous to attempt to view it.  However, astronomy groups across the world are offering the chance to view the rare spectacle through filtered telescopes.  

Even with the help of special viewing devices, certain areas of the world will not be able to see it at all, such as far eastern Asia and Antarctica.  Much of the United States and western Europe will have a good view for more than seven hours.

While Earth orbits the sun every 365 days, Mercury orbits the sun every 88 days.  However, Mercury's orbit is tilted, which means it will only pass directly between the Earth and the sun 14 times in the next 100 years.  Aside from today, the next time will be in 2019 and after that, 2032.

Mercury is a relatively tiny planet.  From our perspective it is only 1/150th the size of the sun.  That means the lining up of the three bodies must be viewed with significant magnification.  The "eclipse glasses" used by thousands of people to view last year's solar eclipse will be useless. And to avoid permanent eye damage, any telescope must be fitted with a solar filter before being pointed at the Sun. 

Open University's Prof David Rothery said scientists are not likely to learn anything about Mercury from the transit that they don't already know, but that it's still an interesting phenomenon.

"It's a hard planet to see," he said.

"Historically, transits were of immense importance," he continued. 

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

Mercury has already been studied extensively, most recently by the Nasa probe, Messenger, an expedition Rothery said yielded unanswered questions.

"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," he said. 

"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," he continued.