An uncommon cyclical phenomenon
Mercury is the closest planet to the Sun. Therefore, its orbit is much smaller than Earth's : while our planet circles the Sun in one year, Mercury completes its orbit in only 88 days.
Every 116 days on average, Mercury catches up to our planet and passes between the Earth and Sun. This is known as inferior conjunction. Normally though, Mercury’s orbit, which is tilted 7° with respect to that of Earth, carries it either well above or below the Sun’s disk.
But on rare occasions, when the inferior conjunction occurs within a short time window in early May or early November, Mercury’s alignment carries it directly in front of the Sun. This is called a transit. On average, there are only 13 or 14 transits of Mercury per century, and they are not evenly distributed over time.
Thus, the next two transits of Mercury will only take place on November 13, 2032 and November 7, 2039. However, they will not be visible from North America. The next transit that will be visible from beginning to end from Québec will only occur on May 7… 2049!
Significant events for astronomy
Transits of Mercury, and even more so those of Venus (much rarer), have greatly influenced the history of astronomy. In the past, careful observation of these events could be used to measure the actual distance between the planets in the solar system - and, from there, to estimate the distance to the stars. For all practical purposes, however, Mercury is too small to provide accurate measurements, and so 18th and 19th century astronomers focused their efforts on the transits of Venus.
Today, the distance to the planets is measured with great precision (with a plus or minus one meter accuracy) thanks in particular to radar echoes. The transits of Mercury and Venus are now of interest only because they are rare, and because of the history they evoke.
But the transits of Mercury and Venus in front of the Sun are not the only ones that we can observe. In recent years, we have measured the small dips in brightness caused by the transits of planets orbiting distant stars. It is thus possible not only to detect these exoplanets, but also to assess their size and density, and to calculate their orbital period.