On May 9, Mercury passes in front of the Sun

When viewed silhouetted against the Sun, Mercury’s disk (seen at left on November 9, 2006) is clearly smaller than that of Venus (seen at right on June 5, 2012). Other dark markings are sunspots.
Credit: Montage Marc Jobin/Espace pour la vie; Mercure : Brocken Inaglory/Wikimedia Commons; Vénus : Marc Jobin
The transit of Mercury compared to the transit of Venus.
On May 9, Mercury passes in front of the Sun

On May 9, 2016, the tiny planet Mercury will pass directly in front of the Sun. For more than 7 hours, its miniscule round form will appear silhouetted against the surface of our star, producing a sort of “mini, partial eclipse” visible only though a telescope.

Though not visually spectacular, this event is still quite impressive given that our knowledge of celestial mechanics has allowed us to predict it to the nearest second! As well, observing a planetary transit allows us to experience an important chapter in the history of science.

A rare occurrence

Because Mercury and Venus orbit closer to the Sun than the Earth, they are the only planets that can pass in front of the solar disk. For example, Mercury takes 88 days to complete its orbit: Every 116 days on average, it catches up to the Earth and passes between the Sun and Earth, a circumstance known as inferior conjunction.

However, because the Earth and Mercury don’t orbit on exactly the same plane, an inferior conjunction doesn’t automatically produce a transit of the Sun’s disk. Favourable alignments only occur when inferior conjunctions take place around May 8 and November 10, which limits transits of Mercury to just 13 or 14 per century. Transits of Venus are even rarer!

Geography is another factor that limits the frequency of transits viewed from a given location on Earth. One has to be on the correct side of our planet, the side facing the Sun, in order to observe a transit when it occurs. And often, transits are already underway at dawn, requiring one to wait for sunrise in order to view them; likewise, observations can also be interrupted by sunset. This transit of Mercury on May 9, 2016, will be visible in its entirety from Quebec: The last time this occurred was November 7, 1960!

A date with history

In the 17th century, Kepler’s laws allowed astronomers to calculate the distance of each planet in our solar system, but these calculations were in terms of the Earth-Sun distance as a basic unit of measure: Known as an astronomical unit, it is still used today. But what is the actual value of this distance in kilometers?

Observing transits from sufficiently separated locations on Earth, and knowing our planet’s diameter, would allow terrestrial distances to be transposed into real distances in space. It was a simple matter of trigonometry, but one that required very precise measurements.

Transits of Mercury turned out to be impractical, because of the planet’s minuscule size. So attention turned to the much rarer transits of Venus, which is closer to Earth and considerably larger than Mercury. Great expeditions were sent to the four corners of the globe to observe transits of Venus in 1761, 1769, 1874 and 1882.

These efforts established the Earth-Sun distance to be about 150 million kilometres. The foundation had been laid for measuring the actual size of the Sun and Planets, and eventually developing a scale of distances in the universe, starting with the closest stars.

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