Scientists, amateur astronomers, and skywatchers had a field day May 9 seeing Mercury pass across the face of the sun – a spectacle that will not occur again before 2019.
According to NASA, the smallest planet in the solar system made its transit – a slow passage across the solar disk – at around 7:16 a.m. EDT. It started on the left side of the sun and traveled to the right on a downward flight, finally exiting the sun’s disk at around 2:38 p.m. EDT.
Images of the transit showed Mercury as very small, a mere black dot that leisurely moved at an angle across the imposing solar surface.
The event, which happens only a little more than once every decade, was seen from the entire North and South America, as well as in Europe, Africa, and a majority of Asia. From Earth's perspective, the planet finishes a transit of the sun around 13 times every given century, the last one in 2006.
The transit occurs only this frequently because although Mercury zooms around the sun every 88 days, the sun and the planet rarely align. Mercury orbits in a plane tilted from Earth’s orbit, so it typically moves above or below humans’ line of sight to the sun.
NASA and the European Space Agency (ESA) offered live views of Mercury’s transit, with the American space agency’s Solar Dynamics Observatory capturing a bunch of images of the small black spot journeying across the humongous, bright disk.
The Slooh Community Observatory likewise provided a live webcast with views from observatories positioned in multiple spots worldwide, such as Arizona, Nevada, Canary Islands, and France.
“Astronomers get excited when any two things come close to each other in the heavens. This is a big deal for us,” said NASA program manager Louis Mayo.
But more than serving as a visual feast, the transit of Mercury gives scientists a great opportunity to research the movement of planets and stars in outer space – information used to better understand the solar system and calibrate astronomical instruments.
A 1631 transit, for instance, allowed astronomers to measure the size of Mercury’s disk and estimate the distance between Earth and the sun. Back then, scientists were only performing visual observations on tiny telescopes based on today’s standards, explained Mayo.
A transit also allows modern instruments to study Mercury’s exosphere or very thin atmosphere, as well as to prove how small a transiting planet can become before it turns impossible to see how the object affects its star’s brightness.
North and South America will see the next transit of Mercury, but the ones in 2032 and 2039 won’t be visible in a huge part of the Western Hemisphere. The planet’s transit will be visible to this part of Earth again in 2049.
As the closest one to orbit the sun, Mercury is a rather difficult planet to study, with Mariner 10 – which had its flyby in 1974 and 1975 – as the first spacecraft to visit it. Not even half of Mercury was observed, making it an ongoing mystery for scientists.
Only fairly recently, too, did scientists propose why Mercury’s surface is so dark, pointing to carbon that has gradually accumulated from the impact of comets traversing the inner solar system.