transitus

Latin transitus derives from transire: trans ('across, through, beyond') + ire ('to go'). The verb meant to pass over, to cross, to go through. Transitus was the noun for such a crossing. Roman writers used it for a passage of time, a crossing of territory, and the movement of a body through a space. The astronomical application — a smaller body crossing the visible disk of a larger one — is precise and geometrically defined: unlike an occultation (where the larger body hides the smaller) a transit occurs when the smaller body is seen as a dot crossing the face of the larger. Mercury and Venus can both transit the Sun, appearing as tiny black circles moving slowly across its disk. These events became among the most scientifically consequential observations in the history of astronomy.

Edmond Halley recognized in 1716 that transits of Venus, observed from multiple widely separated geographical locations on Earth, could be used to determine the distance from Earth to the Sun — the astronomical unit, the fundamental yardstick of the solar system — with unprecedented precision. The method was geometric: observers at different latitudes would see Venus trace slightly different paths across the Sun's disk, and the difference in these paths, combined with the known geometry and precise timing, would yield the solar parallax and hence the Earth-Sun distance. The mathematical insight was brilliant; the practical execution was brutally difficult, requiring coordination across entire hemispheres and observations from locations not yet known or charted.

The transit of Venus occurs in pairs separated by eight years, with each pair separated by gaps of over a century (the pattern runs 121.5, 8, 105.5, 8 years, repeating). The pair of transits in 1761 and 1769 triggered the most ambitious scientific coordinating effort in history to that date: observations from over 150 stations across the globe, including Captain James Cook's voyage to Tahiti for the 1769 transit (the ostensible scientific purpose of the voyage that would lead to his charting of New Zealand and Australia). The 1874 and 1882 pair generated comparable international effort. The next pair fell in 2004 and 2012; the next will not occur until 2117 and 2125. Nobody currently alive will see another Venus transit.

In contemporary astronomy, transit has been repurposed for exoplanet detection. When a planet passes across the face of its host star from our viewing angle, it blocks a tiny fraction of the star's light — typically less than 1 percent — producing a detectable dip in brightness. NASA's Kepler space telescope, launched in 2009, discovered over 2,600 confirmed exoplanets almost entirely through this transit method, monitoring the brightness of 150,000 stars simultaneously for the telltale periodic dips of transiting worlds. The Latin word for crossing over became the primary tool for discovering planets orbiting other suns. The same geometry that Halley used to measure the solar system was scaled up to survey the galaxy.