In 1826 Samuel Heinrich Schwabe (1789-1875) was looking for a hypothetical planet (Vulcan) inside the orbit of Mercury. As such a planet would be difficult to see because being near the Sun it would always be obscured by daylight, Schwabe hoped to observe the planet as a dark spot passing across the face of the Sun. To conduct the search properly he had to track and record sunspots to make sure they were not mistaken for the new planet. For seventeen years on every clear day he recorded the sunspots. He did not find the planet but included his findings in his Solar Observations During 1843 in which he suggested that the number of sunspots waxed and waned on a ten-year cycle.
In 1828 the naturalist Baron Friedrich Wilhelm Alexander von Humboldt (1769-1859) initiated an international study of terrestrial magnetism, based at the observatory in Gottingen University (1734) and under the direction of Carl Friedrich Gauss (1777-1855). In 1851 Humboldt published Schwabe’s work in his own Cosmos. Immediately, Johann von Lamont (1805-79) and Edward Sabine (1788-1883), who were working independently on magnetic observations, found that in periodic terms the variations in the declination of the compass needle on Earth strongly corresponded with the sunspot cycle.
Johann Rudolf Wolf (1816-93), who investigated sunspots before Schwabe, introduced the Zurich sunspot number, an empirical system of gauging solar activity by counting sunspots and sunspot groups. The length of the sunspot cycle was found to vary, but the average value was near eleven years.
In September 1859 Richard Christopher Carrington (1826-75) and Richard Hodgson (1804-72) saw a bright outburst of white light near a sunspot. This was followed seventeen hours later by a very powerful magnetic storm, which strongly suggested the solar-terrestrial connection.
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