Mercury in transit: everything you need to know about this rare celestial event

The journey of the planet across the face of the sun marks its first appearance in almost a decade.

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At this very moment, a rare celestial event is in motion. The innermost planet of our solar system, Mercury, is currently visible and in transit across the surface of the sun. The transit marks the first sighting of the planet in front of our star in almost a decade.

2016 Mercury transit path: 

Mercury is a peculiar planet: its days exceed its years in length (a day on the planet is twice the length of its year, which lasts 88 days), its orbit is notably eccentric and it is punched with craters that were formed by meteors penetrating its thin atmosphere. Skyward surveys on Mercury would display a sun three times larger than the sun in our skies, while recordings of the planet’s temperature have found fluctuations between highs of 427°C and bitterly cold lows of -179°C. And, despite being the planet closest to solar heat, it isn’t quite the hottest planet in the solar system (that crown goes to Venus).

To visualise Mercury is to visualise a world entirely different from our own. It passes between the Earth and sun approximately 13 times per century, and was last observed 8 November 2006. It won’t be seen again until 11 November 2019, which means between lunch and 7:45pm today, scientists, stargazers, and astronomy-enthusiasts alike have been watching the little black dot of Mercury in transit. It is currently visible from Europe and the Americas, with large parts of Asia and Africa also positioned to see the planet’s passing.

But the journey cannot be seen with the unaided eye, and use of binoculars through eclipse glasses will not help. Speaking to broadcaster Adam Rutherford on BBC’s Inside Science, David Rothery, Professor of Planetary Geosciences at the Open University, warned not to employ the flawed combination of binoculars and eclipse glasses as “heat will be concentrated onto glasses”.

Looking directly at the sun can seriously damage eyesight, making it essential that all appropriate safety measures are taken if forms of magnification are to be used to see the transit. Professor Rothery recommends that people, “project the image of Mercury with binoculars or a telescope onto white card or have a telescope with a bespoke solar filter in front of it to cut out all the light and heat before you start magnifying it”. Nasa has noted that solar filters should be, “made of specially-coated glass or Mylar”.

Mercury is a planet that still holds pockets of mysteries. Nasa’s MESSENGER spacecraft, sent to orbit the planet in 2011, ended its mission in 2015 with questions about the planet’s atmosphere and other details still unanswered. Though the observations of today’s transit won’t shed light on any of Mercury’s remaining mysteries, the transit will serve as an apt reminder of the critical importance transits have had in the past.

First observed in 1631, the discovery of Mercury’s transit across the sun paved the way for astronomer Edmond Halley to make his own observations of the travel. It allowed him to work out the distances between Earth, Mercury and the sun, reshaping our conception of the solar system’s scale in the process.

Future plans have been devised to unveil the secrets of the swift planet, with both the European Space Agency and Japanese Space Agency working together to launch an investigative mission into the magnetic field and particle environment of Mercury’s surface and atmosphere.

Just hours remain until the planet falls out of sight again until 2019; so to everyone with a penchant for all things cosmic – now’s the time to catch a glimpse of Mercury.

(Note: for those unable to access binoculars and telescopes equipped with the correct solar filters, live streams of the transit have been made available by Nasa and the European Space Agency.)

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