A rare astronomical event will take place on November 11, 2019: Mercury will pass directly between the Earth and Sun. For several hours, the planet will appear silhouetted against our daytime star.
What’s a transit?
Mercury is the closest planet to the Sun. Its orbit is therefore 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 degrees with respect to that of Earth, carries it either above or below the Sun’s disk. But on rare occasions, when the inferior conjunction occurs in early May or early November, Mercury’s alignment carries it directly in front of the Sun. This is called a transit. There are only 13 or 14 transits of Mercury per century, and not all of them are visible from any given place.
Transits of Mercury, and even more so those of Venus, have greatly influenced the history of astronomy. Indeed, astronomers James Gregory (1638–1675) and Edmund Halley (1656–1742) described a way to use these events to measure the actual distance between the planets, and from there, 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 very rare transits of Venus. See our special report on the June 2012 transit of Venus for more information.
Today, the distance to the planets is measured with great precision thanks to radar. The transits of Mercury and Venus are now of interest mostly because they are rare, and because of the history they evoke. More recently, however, transits of planets around other stars have been observed: the resulting small dips in brightness of these stars tell us much information about the size and orbital period of the exoplanets that revolve around them.