What Is a Transit of Venus? The Event That Measured the Solar System
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Venus passes in front of the sun in a pattern that repeats over more than a century, and observing one from widely separated places on Earth was the method by which the distance to the sun was first measured properly. That made the transits among the most organised international scientific efforts ever mounted.
The pattern of recurrence
Venus passes between the Earth and the sun every nineteen months or so, and almost always misses the solar disc because its orbit is tilted relative to ours, so it passes above or below. A transit occurs only when the alignment happens near one of the two points where the orbital planes intersect, and the resulting pattern is strikingly irregular: transits come in pairs separated by eight years, and the pairs are separated by alternating gaps of about one hundred and five and a half and one hundred and twenty-one and a half years. The pairs in the modern era fell in 1631 and 1639, 1761 and 1769, 1874 and 1882, and 2004 and 2012, and the next will not occur until 2117. That spacing means most people who have ever lived never had the opportunity to see one, that each pair had to be prepared for by a generation that had never witnessed the previous, and that a failed expedition meant waiting either eight years or over a century.
How the distance was derived
The method rests on parallax and on a relationship that was already known:
- •Kepler's laws gave the relative distances of all the planets accurately, so the entire layout of the solar system was known in proportion but not in any absolute unit
- •Fixing any one distance in kilometres would therefore fix all of them, and the Earth to sun distance was the natural target
- •Observers at widely separated latitudes see Venus cross the sun along slightly different chords, because they view it from different angles
- •Different chords mean different transit durations, and timing the start and end precisely from two locations gives the angular difference
- •That angle, combined with the known distance between the two observing sites, yields the distance to Venus and therefore to the sun
- •Halley proposed the method in detail in 1716, knowing he would not live to see it used, and explicitly urged future astronomers to organise the observations
The expeditions
The 1761 and 1769 transits prompted an unprecedented international mobilisation, with well over a hundred observers dispatched by several nations to sites across the globe, cooperating across the boundaries of a continuing war. The results include some of the most quoted misfortunes in the history of science. Guillaume Le Gentil travelled to India for the 1761 transit, was prevented from landing by hostilities and observed from a moving ship where accurate timing was impossible, waited eight years in the region for the second, and was clouded out on the day, returning home after eleven years to find himself declared dead and his estate divided. Captain Cook's first voyage was mounted principally to observe the 1769 transit from Tahiti, with the exploration of the Pacific following on from that primary purpose. Observers across Siberia, Hudson Bay, Baja California and Scandinavia contributed data at considerable personal cost, and several died on the journeys.
The black drop and what came after
The timings proved harder to make than expected because of an optical effect in which the planet appears to remain attached to the edge of the sun by a dark ligament as it enters and leaves, blurring the exact moment of contact by many seconds. That black drop effect was attributed at the time to a Venusian atmosphere and is now understood to arise from a combination of atmospheric turbulence, telescope limitations and the way brightness falls towards the solar edge. It limited the accuracy of the eighteenth-century results, which nonetheless bracketed the solar distance within a few percent of the modern value, and the nineteenth-century transits with photography did better without solving it. The method was eventually superseded by radar ranging to Venus in the 1960s, which measures the distance directly with enormous precision, and the astronomical unit is now defined as a fixed number of metres rather than measured. The 2004 and 2012 transits were accordingly observed for public interest and for testing techniques used to study planets around other stars, where a transit remains the primary detection method.
The takeaway
Transits come in pairs eight years apart separated by gaps of over a century, so the next follows 2012 in 2117. Kepler's laws gave the solar system's proportions but no absolute scale, and timing a transit from widely separated latitudes supplied the one measurement that fixed everything else. Halley proposed the method knowing he would not live to see it. Radar ranging replaced it in the 1960s.