Solar observation
[3] Analysis of tree rings revealed a detailed picture of past solar cycles: Dendrochronologically dated radiocarbon concentrations have allowed for a reconstruction of sunspot activity covering 11,400 years.
[5] The first clear mention of a sunspot in Western literature, around 300 BC, was by the ancient Greek scholar Theophrastus, student of Plato and Aristotle and successor to the latter.
[7] The earliest surviving record of deliberate sunspot observation dates from 364 BC, based on comments by Chinese astronomer Gan De in a star catalogue.
He wrote of the 22 December 968 total eclipse, which he experienced in Constantinople (modern-day Istanbul, Turkey):[13] at the fourth hour of the day ... darkness covered the earth and all the brightest stars shone forth.
And it was possible to see the disk of the Sun, dull and unlit, and a dim and feeble glow like a narrow band shining in a circle around the edge of the disk.The earliest known record of a sunspot drawing was in 1128, by John of Worcester.
[16] Sunspots were first observed telescopically on 18 December 1610 (Gregorian calendar, not yet adopted in England) by English astronomer Thomas Harriot, as recorded in his notebooks.
[17] On 9 March 1611 (Gregorian calendar, also not yet adopted in East Frisia) they were observed by Frisian medical student Johann Goldsmid (latinised name Johannes Fabricius) who subsequently teamed up with his father David Fabricius, a pastor and astronomer, to make further observations and to publish a description in a pamphlet in June 1611.
Johann was the first to realize that sunspots revealed solar rotation, but he died on 19 March 1616, aged 26 and his father a year later.
In 1845, Henry and Alexander observed the Sun with a thermopile and determined that sunspots emitted less radiation than surrounding areas.
Notable scientists to advance spectroscopy were David Brewster, Gustav Kirchhoff, Robert Wilhelm Bunsen and Anders Jonas Ångström.
Gustav Spörer later suggested a 70-year period before 1716 in which sunspots were rarely observed as the reason for Wolf's inability to extend the cycles into the 17th century.
Also in 1848, Joseph Henry projected an image of the Sun onto a screen and determined that sunspots were cooler than the surrounding surface.
[28] Around 1852, Edward Sabine, Wolf, Jean-Alfred Gautier and Johann von Lamont independently found a link between the solar cycle and geomagnetic activity, sparking the first research into interactions between the Sun and the Earth.
[29] In the second half of the nineteenth century Richard Carrington and Spörer independently noted the migration of sunspot activity towards the solar equator as the cycle progresses.
Solar eclipses were also photographed, with the most useful early images taken in 1851 by Berkowski and in 1860 by De la Rue's team in Spain.
The widely viewed total solar eclipse on 18 July 1860 resulted in many drawings, depicting an anomalous feature that corresponds with modern CME observations.
This effect was eventually attributed to overhead electric currents flowing in the ionosphere and magnetosphere by Balfour Stewart in 1882, and confirmed by Arthur Schuster in 1889 from analysis of magnetic observatory data.
In 1852, astronomer and British major general Edward Sabine showed that the probability of the occurrence of magnetic storms on Earth was correlated with the number of sunspots, thus demonstrating a novel solar-terrestrial interaction.
Smithsonian Astrophysical Observatory (SAO) assigned Abbot and his team to detect changes in the radiation of the Sun.
Later, when Abbot was SAO head, they established a solar station at Calama, Chile to complement its data from Mount Wilson Observatory.
Bernard Lyot's 1931 invention of the Coronagraph – a telescope with an attachment to block out the direct light of the solar disk – allowed the corona to be studied in full daylight.
[26] American astronomer George Ellery Hale, as an MIT undergraduate, invented the spectroheliograph, with which he made the discovery of solar vortices.
In 1908, Hale used a modified spectroheliograph to show that the spectra of hydrogen exhibited the Zeeman effect whenever the area of view passed over a sunspot on the solar disc.
Many satellites in Earth orbit or in the heliosphere have deployed solar telescopes and instruments of various kinds for in situ measurements of particles and fields.
Solar radio emissions at 10.7 cm wavelength provide another proxy that can be measured from the ground, since the atmosphere is transparent to such radiation.
Other proxy data – such as the abundance of cosmogenic isotopes – have been used to infer solar magnetic activity, and thus likely brightness, over several millennia.
Total solar irradiance has been claimed to vary in ways that are not predicted by sunspot changes or radio emissions.
[56] Independent scientists of the National Solar Observatory (NSO) and the Air Force Research Laboratory (AFRL) predicted in 2011 that Cycle 25 would be greatly reduced or might not happen at all.
