Timeline of solar astronomy 1613 - Galileo Galilei uses sunspot observations to demonstrate the rotation of the Sun 1619 - Johannes Kepler postulates a solar wind to explain the direction of comet tails 1802 - William Hyde Wollaston observes dark lines in the solar spectrum 1814 - Joseph Fraunhofer systematically studies the dark lines in the solar spectrum 1834 - Hermann Helmholtz proposes gravitational contraction as the energy source for the Sun 1843 - Heinrich Schwabe[?] announces his discovery of the sunspot cycle and estimates its period to be about ten years 1852 - Edward Sabine shows that sunspot number is correlated with geomagnetic field[?] variations 1859 - Richard Carrington[?] discovers solar flares 1860 - Gustav Kirchhoff and Robert Bunsen discover that each chemical element has its own distinct set of spectral lines and use this fact to explain the solar dark lines 1861 - F.G.W. Sporer[?] discovers the variation of sunspot latitudes during a solar cycle 1863 - Richard Carrington[?] discovers the differential nature of solar rotation 1868 - Pierre-Jules-Cesar Janssen[?] and Norman Lockyer discover an unidentified yellow line in solar prominence[?] spectra and suggest it comes from a new element which they name "helium" 1893 - Edward Maunder[?] discovers the 1645-1715 Maunder sunspot minimum 1904 - Edward Maunder plots the first sunspot "butterfly diagram 1906 - Karl Schwarzschild explains solar limb darkening 1908 - George Hale[?] discovers the Zeeman splitting of spectral lines from sunspots 1942 - J.S. Hey[?] detects solar radio waves 1949 - Herbert Friedman[?] detects solar X-rays 1960 - Robert Leighton[?], Robert Noyes[?], and George Simon[?] discover solar five-minute oscillations by observing the Doppler shifts of solar dark lines 1961 - H. Babcock[?] proposes the magnetic coiling sunspot theory 1970 - Roger Ulrich[?], John Leibacher[?], and Robert Stein[?] deduce from theoretical solar models that the interior of the Sun could act as a resonant acoustic cavity[?] 1975 - Franz-Ludwig Deubner[?] makes the first accurate measurements of the period and horizontal wavelength of the five-minute solar oscillations