Astronomers should have their telescopes at the ready, because next month they’ll be able to observe a rare celestial event.
On May 9, people in Europe and America will be able to witness the transit of Mercury in front of the Sun, an event that happens about 13 or 14 times every century.
As Mercury and Venus orbit closer to the Sun than Earth, they can pass in front of the solar disk, appearing as small dots on the Sun’s surface. The transit doesn't happen very often because their orbits are tilted at different angles with respect to Earth’s.
The transit can happen either in May or November, with the May ones being half as frequent. Due to slight changes in the orbits of the planets, though, the transits are gradually moving later in the year. Before 1585, they occurred in April and October.
The last transit of Mercury happened in 2006 and the next one will be in 2019. This year’s transit will start at about 7 a.m. EDT (noon BST) and finish at about 3 p.m. EDT (8 p.m. BST). The precise time depends on your exact location on Earth, and it can be found using a planetary app. A list of the transit time covering 100 U.S. and Canadian cities is available from Eclipse Wise.
Global Visibility of the Transit of Mercury on May 09, 2016. F. Espenak, Eclipse Wise
The entire transit will be visible in Western Europe, western parts of North Africa, north-eastern North America, and most of South America, with a partial transit visible in the rest of Europe, Africa, America, and Asia. The transit won’t be visible at all in Australasia and Eastern Asia.
The observation of the transit of Mercury cannot be done with just any instruments, and it should not be attempted at all with the naked eye. Mercury will cover about 0.004 percent of the Sun’s disk, so to observe it, you’d need a telescope with the correct solar filter.