In March 1716 Edmond Halley became one of thousands of northern Europeans to witness a powerful display of the Northern Lights. The aurora manifested in England as streams of reddish-yellow light, perpendicular to the ground but spreading at the zenith to form a “corona.” Halley, later famed for predicting the periodicity of a certain comet*, published his observations of aurora borealis in the British Royal Society’s principal periodical. His article proved remarkable for two reasons. First, Halley attributed the aurora not to the glory of God or Earth’s venting of luminiferous vapors, but to an effusion of “magnetical effluvia” that the Earth’s magnetism drew toward the North Pole. Halley was only partially correct - today we know that the Northern Lights are caused by charged solar particles impacting the Earth’s magnetic field - but he did at least connect the phenomenon with terrestrial magnetism.
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| Photo by AstroAnthony via Wikimedia |
Second, Halley observed that the Northern Lights had become quite rare in Europe. Apart from a few low-powered manifestations over the winter of 1707-08, he knew of no recorded appearances of the aurora since 1621. The Lights had in fact appeared a few other times in the previous century: in 1661-62 over England and Germany, and a couple of times in Romania in the early 1700s. That aurora borealis had not been seen in Halley’s homeland in over fifty years currently seems indisputable, and worthy of explanation.
In The Global Crisis (2013, p. 13), Geoffrey Parker pointed out the likely cause of the aurora’s long absence. As the Sun is the active agent in producing auroras, during periods of reduced solar radiation they tend to weaken or disappear altogether. Halley’s Hiatus (to coin a phrase) coincided with a period of drastically reduced sunspot activity - strong evidence of weakened solar output - that modern Earth scientists call the Maunder Minimum (1640-1715). This same period saw, not coincidentally, intermittent spells of drought and foul winter weather throughout the world, culminating in the devastating winter of 1708-09. The return of the auroras to northern Europe coincided with an increase in sunspot activity and solar output, and the subsequent decades saw a steady increase in the world’s population and food supply and the stabilization of some of its more powerful empires (e.g. Russia and China). Foul weather and failed harvests did not become commonplace again until the late 1780s. What Halley and his contemporaries were witnessing, then, was not an ill omen but a predictor of better times to come - the climatological end of the seventeenth century.
* A disappointing one, for those of us who remember the dismal display Halley’s Comet put on in 1986. I’m told that astronomers expect no better from the 2061 encounter.
