On May 9, astronomers will be able to watch one of the rarest celestial events as Mercury passes between the Earth and the sun.
Known as a planetary transit, Mercury will appear as a dot as it passes in front of the sun’s face. The event is expected to last for seven and a half hours – from 7:12 a.m. until 2:12 p.m.
Although the planet closest to the sun completes one revolution around the sun in 88 days – nearly a quarter of one Earth year – the Earth, sun, and Mercury align very rarely. What further lowers the chances of an alignment is Mercury’s tilted orbit space with respect to Earth’s, making the astronomical event occur only 13 times in a century.
According to Space.com, the last transit occurred in 2006, while the next one is scheduled to occur in 2019. The planet is too tiny to be seen with the naked eye, and one may require a telescope or binoculars equipped with a solar filter to make it safe to look at the sun directly.
“Astronomers get excited when any two things come close to each other in the heavens,” said Louis Mayo, program manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This is a big deal for us.”
According to Discovery News, the transit will occur in the following phases, as seen by the Solar Dynamics Observatory:
- Corona contact: Witnessed only from space, Mercury first enters the sun’s corona, the superheated plasma atmosphere of the sun. The planet will come in contact with the corona an hour before the first contact.
- First contact: The contact of the edge of Mercury with the edge of the sun.
- Second contact: Consequently, the planet starts moving onto the sun’s disc.
- Transit midpoint: This occurs when Mercury is halfway across the sun’s disc.
- Third contact: The contact of the edge of Mercury with the edge of the sun on its way out.
- Fourth contact: This phase occurs when the planet leaves the sun’s disc completely.
- Last corona contact: Witnessed only from space, when the planet moves out of the sun’s corona.
The first Mercury transit was observed by astronomers in 1631, which helped them measure the size of the planet and estimate the distance between the sun and the Earth.
“Back in 1631, astronomers were only doing visual observations on very small telescopes by today’s standards,” said Mayo, according to NASA.