Our astronomer, Tom Kerss, discusses the Transit of Mercury and how you can see it here in Greenwich. 

On Monday 9th May, the innermost planet Mercury will cross the face of the Sun as seen from the UK and many parts of the world. The entire event, which lasts about seven and a half hours, will be visible from London, and we’re celebrating by inviting you to join us for an afternoon of special observing. Using safely filtered solar telescopes, we’ll show you the night side of this small, curious planet, as it silently slips across the solar disk at a distance of under 52 million miles, appearing like a crisp, inky black spot on the face of our star.
 
We also have special ticketed viewings taking place through the Observatory’s flagship Great Equatorial Telescope later in the afternoon. Can’t make it to Greenwich? We’ve got you covered. We’re planning to host a live stream of transit direct from our Victorian telescopes, so you can tune in online wherever you are.
 
A specially designed solar filter for our 28-inch aperture refractor. Credit: Tom Kerss.
A specially designed solar filter for our 28-inch aperture refractor. Credit: Tom Kerss.
 
Transits of Mercury occur roughly 13 times per century, and can happen in May or November, with May transits being slightly more favourable, as Mercury’s disk reaches 12 arcseconds in diameter. Although Mercury overtakes us several times per year on its relatively quick journey around the Sun, we don’t see transits every time, because Mercury’s orbit is quite highly inclined relative to that of the Earth. Fortunately, transits of Mercury are considerably more common than transits of Venus. The next one won’t occur until 2117!
 
Mercury’s orbital tilt relative to the Earth. Credit: ESO.
Mercury’s orbital tilt relative to the Earth. Credit: ESO.
 
Transits of Mercury are historically important. German astronomer Johannes Kepler predicted one would occur (along with a transit of Venus) in 1631. He recognised the value in measuring transits, and published a notice to the astronomical community in 1629. Sadly, he died the following year, but the French astronomer Pierre Gassendi, among several others, succeeded in becoming the first to witness a transit of Mercury.
 
Kepler’s warning to astronomers of ‘rare marvels’ in the year 1631, Venus and Mercury in conjunction with the Sun.
Kepler’s warning to astronomers of ‘rare marvels’ in the year 1631, Venus and Mercury in conjunction with the Sun.
 
Gassendi’s observation secured support for the Keplerian model of the Solar System, and generated controversy over the size of Mercury, which was poorly established at the time. Today, we know a great deal about Mercury thanks to precise ground-based measurements, and more importantly, interplanetary spacecraft that have visited the little planet. Mariner 10 (in 1974) and Mercury MESSENGER (in 2008) made close flybys of Mercury, mapping its surface and measuring its properties. They discovered its surprisingly strong magnetic field, its geological history and its tenuous exosphere - an ultra-thin gas envelope surrounding the planet. A once strange mysterious world has become much more familiar thanks to these missions.
 
The surface of Mercury as revealed by MESSENGER. Credit: NASA/JHUAPL.
The surface of Mercury as revealed by MESSENGER. Credit: NASA/JHUAPL.
 
Many questions about Mercury remain, and probing its history is imperative to understanding the formation of our Solar System. The European Space Agency (ESA) in cooperation with the Japanese Aerospace Exploration Agency (JAXA) is currently building the next interplanetary mission to Mercury – BepiColombo, which is due to launch in January or February next year. It comprises two orbiters, which will make unprecedented observations of Mercury’s magnetic environment, exosphere and surface for at least four mercurian years.
 
Artist’s impression of BepiColombo orbiters. Credit: ESA.
Artist’s impression of BepiColombo orbiters. Credit: ESA.
 
As we wait for BepiColombo’s launch, we can enjoy the spectacle of the Mercury transit, and appreciate the work performed by previous generations of astronomers before such space technology was available. In 1661, Dutch astronomer Christiaan Huygens (best known for his study of Saturn’s rings) was in London, and observed a transit of Mercury on 3rd May, the day of King Charles II’s coronation. A few years later, the king, who had a keen interest in astronomy, established the Royal Observatory in Greenwich. In 1677, Edmund Halley, who would later become Astronomer Royal in Greenwich, watched the Mercury transit of November 7th from St Helena. He realised that transit observations would be instrumental in measuring the distance between the Earth and the Sun. Nearly a century later, during the 1769 transit on November 9th, English astronomer Charles Green accompanied James Cook at an excellent observing site now known as Mercury Bay in New Zealand. Green noticed that Mercury’s disk appeared very sharp in comparison to that of Venus, determining that it had little or no atmosphere. 
 
Photographs of Mercury in transit taken at the Royal Observatory. Credit: RAS/NASA
Photographs of Mercury in transit taken at the Royal Observatory. Credit: RAS/NASA
 
In 1914, and again in 1927, astronomers observed transits of Mercury from Greenwich with a variety of instruments. These observations were used to make corrections to our understanding of Mercury’s orbit, and a careful study was carried out in 1914 to determine whether or not Mercury had any moons. None were found, and indeed none are known of today.
 
In the spirit of the astronomers who worked here a century ago, we’ll once again turn these instruments towards the Sun for this rare event, and we hope you’ll join us to see it for yourself
 
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