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In 2006, the Solar and Heliospheric Observatory, a US-European satellite, captured this time lapse sequence of Mercury transiting in front of the sun. The dark splotches at upper left and upper right are sunspots, regions of magnetic disturbance on the sun’s surface that are considerably larger than the passing planet.NASA/ESA

It was just a tiny black dot, but when Pierre Gassendi saw it projected on a screen in his darkened Paris apartment, he knew he was making history.

The dot was the planet Mercury and on Nov. 7, 1631, Gassendi became the first person to describe its five-hour passage across the face of the sun – an interplanetary spectacle that astronomers refer to as a transit.

This Monday morning, Mercury will be at it again. The solar system’s innermost planet is set to pass in front of the sun starting at about 7:35 a.m. ET. The phenomenon is rare – the whirling geometry of the heavens only makes it possible about 13 times per century. Venus, the only other planet that can pass between Earth and the sun, does it even less often. (The next time is 2117). Since Mercury won’t be in transit again until 2032, astronomy enthusiasts are eager to seize the celestial moment while they can.

“It really is the choreography of the universe in action,” said Paul Delaney, a professor of astronomy at York University in Toronto. “When these opportunities come up, you don’t want them to pass you by.”

Weather permitting, York’s on-campus observatory will be open to the public from 8:30 a.m. until after 1:05 p.m. ET, when Mercury concludes its slow motion traverse across the sun’s disk.

Similar viewing opportunities are planned in other Canadian locations and various observatories around the world are set to live stream the event. (A partial listing of local events can be found here.) Because of the risk of staring at the sun directly, this is the kind of event best watched alongside those who have the know-how and the equipment to do so safely.

To observers, Mercury will appear just as it did for Gassendi nearly four centuries ago – in silhouette, as a tiny mote swimming in front of a blazing solar backdrop.

Gassendi’s success marked a watershed moment for astronomy. At 39, he was the scientific revolution’s version of today’s digital natives. Like his contemporary, René Descartes, he was part of the first generation to come of age with a new technology and a new way of thinking about the world.

In Gassendi’s case, the new technology consisted of the telescope he used to magnify and project the sun’s image. The new way of thinking was embodied in the mathematical tables that Johannes Kepler had devised to explain and predict planetary motion. Kepler even predicted the transit of 1631, but he died one year too soon to witness it.

Kepler had alerted astronomers to start watching a couple of days early in case his numbers were off. The sky above Paris was cloudy on both Nov. 5 and 6. When Nov. 7 dawned, things did not look much better. But then the clouds parted around 9 a.m. and Gassendi saw a spot on the sun that hadn’t been there before. The transit was under way.

In an era before accurate clocks, Gassendi had to stomp on the floor to make sure his assistant in the room above was simultaneously measuring the height of the sun above the horizon in order to time the transit’s progress. (One wonders what the people in the room below thought of this.)

Referencing Mercury’s mythological namesake, Gassendi later wrote: “I have been more fortunate than those hunters after Mercury who have sought the cunning god in the sun. I found him out, and saw him, where no one else had hitherto seen him.”

His successful observations validated Kepler’s prediction and also provided the first reliable estimate for the size of another planet. And while there are no scientific breakthroughs expected from Monday’s transit, the same technique offers astronomers their best chance at finding Earth-like planets in other solar systems.

Whenever such planets pass in front of the stars they orbit, the star exhibits a momentary dip in brightness. Since the size of the dip is directly related to the size of the planet, and the timing to its orbital distance, the method can be used to find planets that are similar in size and temperature range to Earth.

Eve Lee, a theoretical astrophysicist at McGill University in Montreal, said that, in some cases, the method can be used to reveal whether a planet has an atmosphere.

“Then you can go deeper and actually say something about the profile of the planet," she added.

As technology improves, astronomers are looking to a day when a transit may allow them to identify a planet whose atmospheric characteristics suggest it may be biologically active, due to the presence of gasses such as oxygen, methane or water vapour.

If so, it will be a scientific triumph of a kind that Gassendi could hardly have anticipated but that Prof. Delaney said makes Monday’s event all the more worth talking about.

“It’s like a birthday,” he added. “You don’t have to celebrate it, but it’s more fun if you do.”

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