The strange history of the spots of Mercury

Go figure

Schiaparelli began observing Mercury at its greatest elongation east of the Sun on February 6, 1882, corresponding to the planet’s appearance as an evening star. At this date, he succeeded in distinguishing a “large system of spots” on the almost dichotomized disc. These spots, he noted, strangely combined to form the number 5. He designated each part of the number with the letters w, a, b, k, and i. This number 5 made a deep impression on Schiaparelli, and it was to haunt him whenever Mercury headed east of the Sun (as it did in May, when he once again distinguished 5). On the other hand, each time the planet moved west from the Sun – becoming a morning star – Schiaparelli seemed to see the same prominent dark spot, which he called q.

He made his bravest string of sightings in August, when he tracked the planet’s tiny gibbous disk just 3.5° west of the Sun. This feat of observational daring, he later admitted, proved extremely damaging to his retinas. He found that “the planet appears almost perfectly round, with somewhat less than uniform light; but despite the fact that the apparent diameter was reduced to 4″ or 5″ in diameter, the positions of the observable marks could be judged with more certainty than at other times. This time, he seemed to recover the dark spot q. In September, the next time Mercury raced east of the Sun, it again discovered the 5. Schiaparelli’s ideas were now beginning to come to fruition, and he finally believed that the timely appearances of the observed marks confirmed that the Mercury’s orbital period and rotational period were the same: 88 Earth days.

On October 20, 1882, he wrote to his close friend and confidant François Terby, an amateur astronomer in Louvain, Belgium. Schiaparelli requested that, if he were to die before he could publish, Terby should publicize Schiaparelli’s work “so that this fine result may not be lost to science”. A passionate classical Schiaparelli communicated his result to Terby in Latin verses, which read (translated):

Cyllenius [Mercury]rotating on its axis like Cynthia [the Moon], The eternal night sustains, and also the day: One of the faces is burned by the perpetual heat, The other part, hidden, is deprived of the sun….

More prosaically said, one hemisphere of Mercury always faces the Sun, while the other always faces – just like the Moon in relation to the Earth. However, as with the Moon, Mercury would appear to wobble (or librate) around the fixed line between it and the Sun. This effect must have been rather considerable, given the eccentricity of Mercury’s orbit, and it provided Schiaparelli with some cover as he found the positions of its spots to be quite variable over time. Yet even libration could not explain all of the observed variation. In the end, Schiaparelli was forced to cite the existence of a substantial atmosphere around the tiny planet, and sometimes even bright white clouds.

Although he made up his mind on Mercury’s 88-day rotation and revolution period, Schiaparelli still held back from publishing until he could confirm his findings with a larger telescope. He eventually used a 19-inch Merz-Repshold refractor, which was installed at Brera in 1886. But observations with this larger range did not prove noticeably better than those made with the smaller Merz. Finally, at the end of 1889, Schiaparelli published a memoir in which he summarized his observations and published his famous planisphere. In December, he made a rare trip outside Milan to give a talk at the Quirinal Palace in Rome to a popular audience that included the King and Queen of Italy. During the lecture, Schiaparelli provocatively suggested the possibility that liquid water – and life itself – could flourish in the “twilight zone” between sides perpetually lit by the sun and perpetually shaded by the Mercury night.

Schiaparelli lived until 1910, remaining sure of his results until the end. A host of later observers also lined up to confirm his findings. Above the others, the Greek-French astronomer EM Antoniadi, whose long study of Mercury in the 1920s with the 33-inch telescope of the Observatory of Meudon near Paris seemed to definitively confirm Schiaparelli’s map, his period of rotation and its clouds. Researchers have come to regard Mercury’s 88-day rotational period as one of the best-established facts in all of planetary science. And yet it was all an illusion.

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