This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Solar observation" – news ·newspapers ·books ·scholar ·JSTOR(July 2015) (Learn how and when to remove this message)

Solar observation is the scientific endeavor of studying theSun and its behavior and relation to the Earth and the remainder of theSolar System. Deliberate solar observation began thousands of years ago. That initial era of direct observation gave way to telescopes in the 1600s followed by satellites in the twentieth century.

Stratigraphic data suggest that solar cycles have occurred for hundreds of millions of years, if not longer; measuringvarves inprecambrian sedimentary rock has revealed repeating peaks in layer thickness corresponding to the cycle. It is possible that the early atmosphere on Earth was more sensitive tosolar irradiation than today, so that greater glacial melting (and thicker sediment deposits) could have occurred during years with greater sunspot activity.^[1][2]This would presume annual layering; however, alternative explanations (diurnal) have also been proposed.^[3]

Analysis oftree rings revealed a detailed picture of past solar cycles:Dendrochronologically datedradiocarbon concentrations have allowed for a reconstruction of sunspot activity covering 11,400 years.^[4]

Solar activity and related events have been regularly recorded since the time of theBabylonians. In the 8th century BC,^[5] they described solar eclipses and possibly predicted them from numerological rules. The earliest extant report of sunspots dates back to the ChineseBook of Changes,c. 800 BC. The phrases used in the book translate to "A dou is seen in the Sun" and "A mei is seen in the Sun", wheredou andmei would be darkening or obscuration (based on the context). Observations were regularly noted by Chinese and Korean astronomers at the behest of the emperors, rather than independently.[5]

The first clear mention of a sunspot in Western literature, around 300 BC, was by theancient Greek scholarTheophrastus, student ofPlato andAristotle and successor to the latter.^[6] TheRoyal Frankish Annals record that beginning on 17 March AD 807 a large sunspot that was visible for eight days and incorrectly conclude that it was atransit ofMercury.^[7]

The earliest surviving record of deliberate sunspot observation dates from 364 BC, based on comments byChinese astronomerGan De in astar catalogue.^[8] By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.^[9]

A large sunspot was observed at the time ofCharlemagne's death in AD 813.^[10] Sunspot activity in 1129 was described byJohn of Worcester andAverroes provided a description of sunspots later in the 12th century;^[11] however, these observations were also misinterpreted as planetary transits.^[12]

The first unambiguous mention of the solar corona was byLeo Diaconus, a Byzantine historian. 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.

— Leo Diaconus^[13]

The earliest known record of asunspot drawing was in 1128, byJohn of Worcester.^[14]

In the third year of Lothar, emperor of the Romans, in the twenty-eighth year of King Henry of the English...on Saturday, 8 December, there appeared from the morning right up to the evening two black spheres against the sun.

— John of Worcester,The Chronicle of John of Worcester, cited in Albert Van Helden, 1996.^[15]

Another early observation was of solar prominences, described in 1185 in theRussian Chronicle of Novgorod.^[13]

In the evening there as an eclipse of the sun. It was getting very gloomy and stars were seen ... The sun became similar in appearance to the moon and from its horns came out somewhat like live embers.

— Russian Chronicle of Novgorod^[13]

Giordano Bruno andJohannes Kepler suggested the idea that the sun rotated on its axis.^[16] Sunspots were first observed telescopically on 18 December 1610 (Gregorian calendar, not yet adopted in England) by English astronomerThomas Harriot, as recorded in his notebooks.^[17] On 9 March 1611 (Gregorian calendar, also not yet adopted in East Frisia) they were observed byFrisian medical student Johann Goldsmid (latinised nameJohannes Fabricius) who subsequently teamed up with his fatherDavid Fabricius, a pastor and astronomer, to make further observations and to publish a description in a pamphlet in June 1611.^[18] The Fabricius' usedcamera obscura telescopy to get a better view of the solar disk, and like Harriot made observations shortly after sunrise and shortly before sunset. 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. Several scientists such asJohannes Kepler,Simon Marius, andMichael Maestlin were aware of the Fabricius' early sunspot work, and indeed Kepler repeatedly referred to it his writings. However, like that of Harriot, their work was otherwise not well known.Galileo Galilei almost certainly began telescopic sunspot observations around the same time as Harriot, given he made his first telescope in 1609 on hearing of the Dutch patent of the device, and that he had managed previously to make naked-eye observations of sunspots. He is also reported to have shown sunspots to astronomers in Rome, but we do not have records of the dates. The records of telescopic observations of sunspots that we do have from Galileo do not start until 1612, for when they are of unprecedented quality and detail as by then he had developed the telescope design and greatly increased its magnification.^[19] LikewiseChristoph Scheiner had probably been observing the spots using an improvedhelioscope of his own design. Galileo and Scheiner, neither of whom knew of the work of Harriot or Fabricius vied for the credit for the discovery. In 1613, inLetters on Sunspots, Galileo refuted Scheiner's 1612 claim that sunspots were planets inside Mercury's orbit, showing that sunspots were surface features.^[18][20]

Although the physical aspects of sunspots were not identified until the 20th century, observations continued.^[21] Study was hampered during the 17th century due to the low number of sunspots during what is now recognized as an extended period of low solar activity, known as theMaunder Minimum. By the 19th century, then-sufficient sunspot records allowed researchers to infer periodic cycles in sunspot activity. In 1845,Henry andAlexander observed the Sun with athermopile and determined that sunspots emitted less radiation than surrounding areas. The emission of higher than average amounts of radiation later were observed from the solarfaculae.^[22]Sunspots had some importance in the debate over the nature of theSolar System. They showed that the Sun rotated, and their comings and goings showed that the Sun changed, contrary to Aristotle, who had taught that all celestial bodies were perfect, unchanging spheres.

Sunspots were rarely recorded between 1650 and 1699. Later analysis revealed the problem to be a reduced number of sunspots, rather than observational lapses. Building uponGustav Spörer's work, the wife-and-husband team ofAnnie Maunder andEdward Maunder suggested that the Sun had changed from a period in which sunspots all but disappeared to a renewal of sunspot cycles starting in about 1700. Adding to this understanding of the absence of solar cycles were observations ofaurorae, which were absent at the same time, except at the very highest magnetic latitudes^[23]

The lack of asolar corona duringsolar eclipses was also noted prior to 1715.^[24]

The period of low sunspot activity from 1645 to 1717 later became known as the "Maunder Minimum".^[25] Observers such asJohannes Hevelius,Jean Picard andJean Dominique Cassini confirmed this change.^[20]

After the detection ofinfra-red radiation byWilliam Herschel in 1800 and ofUltraviolet radiation byJohann Wilhelm Ritter, solar spectrometry began in 1817 whenWilliam Hyde Wollaston noticed that dark lines appeared in the solar spectrum when viewed through a glassprism.Joseph von Fraunhofer later independently discovered the lines and they were namedFraunhofer lines after him. Other physicists discerned that properties of the solar atmosphere could be determined from them. Notable scientists to advance spectroscopy wereDavid Brewster,Gustav Kirchhoff,Robert Wilhelm Bunsen andAnders Jonas Ångström.^[26]

Samuel Heinrich Schwabe (1789–1875). German astronomer, discovered the solar cycle through extended observations of sunspots

Rudolf Wolf (1816–1893), Swiss astronomer, carried out historical reconstruction of solar activity back to the seventeenth century

The cyclic variation of the number of sunspots was first observed bySamuel Heinrich Schwabe between 1826 and 1843.^[27]Rudolf Wolf studied the historical record in an attempt to establish a history of solar variations. His data extended only to 1755. He also established in 1848 a relative sunspot number formulation to compare the work of different astronomers using varying equipment and methodologies, now known as theWolf (or Zürich) sunspot number.

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 centuryRichard Carrington and Spörer independently noted the migration of sunspot activity towards the solar equator as the cycle progresses. This pattern is best visualized in the form of the so-called butterfly diagram, first constructed byEdward Walter Maunder andAnnie Scott Dill Maunder in the early twentieth century (see graph). Images of the Sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce a time-series diagram.

Half a century later, the father-and-son team ofHarold andHorace Babcock showed that the solar surface is magnetized even outside of sunspots; that this weaker magnetic field is to first order adipole; and that this dipole undergoes polarity reversals with the same period as the sunspot cycle (see graph below). These observations established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole.

The Sun was photographed for the first time, on 2 April 1845, by French physicistsLouis Fizeau andLéon Foucault. Sunspots, as well as thelimb darkening effect, are visible in theirdaguerrotypes. Photography assisted in the study of solar prominences,granulation and spectroscopy. Charles A. Young first captured a prominence in 1870. 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.^[29]

Early estimates of the Sun's rotation period varied between 25 and 28 days. The cause was determined independently in 1858 by Richard C. Carrington andSpörer. They discovered that the latitude with the most sunspots decreases from 40° to 5° during each cycle, and that at higher latitudes sunspots rotate more slowly. The Sun's rotation was thus shown to vary by latitude and that its outer layer must be fluid. In 1871Hermann Vogel, and shortly thereafter byCharles Young confirmed this spectroscopically.Nils Dúner's spectroscopic observation in the 1880s showed a 30% difference between the Sun's faster equatorial regions and its slower polar regions.^[29]

The first modern, and clearly described, accounts of a solar flare and coronal mass ejection occurred in 1859 and 1860 respectively. On 1 September 1859, Richard C. Carrington, while observing sunspots, saw patches of increasingly bright light within a group of sunspots, which then dimmed and moved across that area within a few minutes. This event, also reported by R. Hodgson, is a description of a solar flare. The widely viewed total solar eclipse on 18 July 1860 resulted in many drawings, depicting an anomalous feature that corresponds with modern CME observations.^[26]

For many centuries, the earthly effects of solar variation were noticed but not understood. E.g., displays ofauroral light have long been observed at high latitudes, but were not linked to the Sun.

In 1724,George Graham reported that the needle of amagnetic compass was regularly deflected frommagnetic north over the course of each day. This effect was eventually attributed to overhead electric currents flowing in the ionosphere and magnetosphere byBalfour Stewart in 1882, and confirmed byArthur Schuster in 1889 from analysis of magnetic observatory data.

In 1852, astronomer and British major generalEdward Sabine showed that the probability of the occurrence of magnetic storms on Earth was correlated with the number ofsunspots, thus demonstrating a novel solar-terrestrial interaction. In 1859, a greatmagnetic storm caused brilliant auroral displays and disrupted globaltelegraph operations.Richard Carrington correctly connected the storm with asolar flare that he had observed the day before in the vicinity of a large sunspot group—thus demonstrating that specific solar events could affect the Earth.

Kristian Birkeland explained the physics of aurora by creating artificial aurora in his laboratory and predicted thesolar wind.

Early in the 20th century, interest in astrophysics grew in America, and multiple observatories were built.^[30]: 320 Solar telescopes (and thus, solar observatories), were installed atMount Wilson Observatory in California in 1904,^[30]: 324 and in the 1930s atMcMath–Hulbert Observatory.^[31] Interest also grew in other parts of the world, with the establishment of theKodaikanal Solar Observatory in India at the turn of the century,^[32] theEinsteinturm in Germany in 1924,^[33] and the Solar Tower Telescope at the National Observatory of Japan in 1930.^[34]

Around 1900, researchers began to explore connections between solar variations and Earth's weather.Smithsonian Astrophysical Observatory (SAO) assignedAbbot and his team to detect changes in the radiation of the Sun. They began by inventing instruments to measure solar radiation. Later, when Abbot was SAO head, they established a solar station atCalama, Chile to complement its data fromMount Wilson Observatory. He detected 27 harmonic periods within the 273-monthHale cycles, including 7, 13, and 39-month patterns. He looked for connections to weather by means such as matching opposing solar trends during a month to opposing urban temperature and precipitation trends. With the advent ofdendrochronology, scientists such as Glock attempted to connect variation in tree growth to periodic solar variations and infer long-term secular variability in thesolar constant from similar variations in millennial-scale chronologies.^[35]

Until the 1930s, little progress was made on understanding the Sun's corona, as it could only be viewed during infrequent total solar eclipses.Bernard Lyot's 1931 invention of theCoronagraph – 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 astronomerGeorge Ellery Hale, as anMIT undergraduate, invented thespectroheliograph, with which he made the discovery of solarvortices. In 1908, Hale used a modified spectroheliograph to show that the spectra of hydrogen exhibited theZeeman effect whenever the area of view passed over a sunspot on the solar disc. This was the first indication that sunspots were basically magnetic phenomena, which appeared in opposite polarity pairs.^[36] Hale's subsequent work demonstrated a strong tendency for east-west alignment of magnetic polarities in sunspots, with mirror symmetry across the solar equator; and that the magnetic polarity for sunspots in each hemisphere switched orientation from one solar cycle to the next.^[37] This systematic property of sunspot magnetic fields is now commonly referred to as theHale–Nicholson law,^[38] or in many cases simplyHale's laws.

The introduction of radio revealed periods of extreme static or noise. Severeradar jamming during a large solar event in 1942 led to the discovery of solar radio bursts.

Many satellites in Earth orbit or in the heliosphere have deployed solar telescopes and instruments of various kinds forin situ measurements of particles and fields.Skylab, a notable large solar observational facility, grew out if the impetus of theInternational Geophysical Year campaign and the facilities ofNASA.Other spacecraft, in an incomplete list, have included theOSO series, theSolar Maximum Mission,Yohkoh,SOHO,ACE,TRACE, andSDO among many others; still other spacecraft (such asMESSENGER,Fermi, andNuSTAR) have contributed solar measurements by individual instruments.

Modulation of solar bolometric radiation by magnetically active regions, and more subtle effects, was confirmed by satellite measurements of the totalsolar irradiance (TSI) by the ACRIM1 experiment on theSolar Maximum Mission (launched in 1980).^[39] The modulations were later confirmed in the results of the ERB experiment launched on theNimbus 7 satellite in 1978.^[40] Satellite observation was continued byACRIM-3 and other satellites.^[41]

Direct irradiance measurements have been available during the last three cycles and are a composite of multiple observing satellites.^[41][42] However, the correlation between irradiance measurements and other proxies of solar activity make it reasonable to estimate solar activity for earlier cycles. Most important among these proxies is the record of sunspot observations that has been recorded since ~1610. 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 ofcosmogenic isotopes – have been used to infer solar magnetic activity, and thus likely brightness, over several millennia.

Totalsolar irradiance has been claimed to vary in ways that are not predicted by sunspot changes or radio emissions. These shifts may be the result of inaccurate satellite calibration.^[43][44] A long-term trend may exist in solar irradiance.^[45]

The Sun was, until the 1990s, the only star whose surface had been resolved.^[46] Other major achievements included understanding of:^[47]

• X-ray-emitting loops
• Corona and solar wind
• Variance of solar brightness with level of activity and verification of this effect in other solar-type stars
• The intenseFibril state of the magnetic fields at the visible surface of a star like the sun
• The presence of magnetic fields of 0.5×10⁵ to 1×10⁵ gauss at the base of the conductive zone, presumably in some fibril form, inferred from the dynamics of rising azimuthal flux bundles.
• Low-levelElectron neutrino emission from the Sun's core.^[47]

The most powerful flare observed by satellite instrumentation began on 4 November 2003 at 19:29 UTC, and saturated instruments for 11 minutes. Region 486 has been estimated to have produced an X-ray flux ofX28. Holographic and visual observations indicate significant activity continued on the far side of the Sun.

Sunspot and infrared spectral line measurements made in the latter part of the first decade of the 2000s suggested that sunspot activity may again be disappearing, possibly leading to a new minimum.^[48] From 2007 to 2009, sunspot levels were far below average. In 2008, the Sun was spot-free 73 percent of the time, extreme even for a solar minimum. Only 1913 was more pronounced, with no sunspots for 85 percent of that year. The Sun continued to languish through mid-December 2009, when the largest group of sunspots to emerge for several years appeared. Even then, sunspot levels remained well below those of recent cycles.^[49]

In 2006, NASA predicted that the next sunspot maximum would reach between 150 and 200 around the year 2011 (30–50% stronger than cycle 23), followed by a weak maximum at around 2022.^[50][51] Instead, the sunspot cycle in 2010 was still at its minimum, when it should have been near its maximum, demonstrating its unusual weakness.^[52]

Cycle 24's minimum occurred around December 2008 and the next maximum was predicted to reach a sunspot number of 90 around May 2013.^[53] The monthly mean sunspot number in the northern solar hemisphere peaked in November 2011, while the southern hemisphere appears to have peaked in February 2014, reaching a peak monthly mean of 102. Subsequent months declined to around 70 (June 2014).^[54] In October 2014, sunspot AR 12192 became the largest observed since 1990.^[55] The flare that erupted from this sunspot was classified as an X3.1-class solar storm.^[56]

Independent scientists of theNational Solar Observatory (NSO) and theAir Force Research Laboratory (AFRL) predicted in 2011 that Cycle 25 would be greatly reduced or might not happen at all.^[57]

• Willson, Richard C.; H.S. Hudson (1991). "The Sun's luminosity over a complete solar cycle".Nature.351 (6321):42–4.Bibcode:1991Natur.351...42W.doi:10.1038/351042a0.S2CID 4273483.
• Foukal, Peter; et al. (1977). "The effects of sunspots and faculae on the solar constant".Astrophysical Journal.215: 952.Bibcode:1977ApJ...215..952F.doi:10.1086/155431.
• Stetson, H.T. (1937).Sunspots and Their Effects. New York: McGraw Hill.Bibcode:1937sate.book.....S.
• Yaskell, Steven Haywood (31 December 2012).Grand Phases On The Sun: The case for a mechanism responsible for extended solar minima and maxima. Trafford Publishing.ISBN 978-1-4669-6300-9.
• NOAA / NESDIS / NGDC (2002)Solar Variability Affecting Earth NOAA CD-ROM NGDC-05/01. This CD-ROM contains over 100 solar-terrestrial and related global data bases covering the period through April 1990.
• Solanki, S.K.; Fligge, M. (2001). "Long-term changes in solar irradiance". In Wilson, A. (ed.).Proceedings of the 1st Solar and Space Weather Euroconference, 25-29 September 2000, Santa Cruz de Tenerife, Tenerife, Spain. Vol. 463. ESA Publications Division. pp. 51–60.Bibcode:2000ESASP.463...51S.ISBN 9290926937. ESA SP-463.{{cite book}}:|journal= ignored (help)
• Solanki, S.K.; Fligge, M. (2000). "Reconstruction of past solar irradiance".Space Science Reviews.94 (1/2):127–38.Bibcode:2000SSRv...94..127S.doi:10.1023/A:1026754803423.S2CID 117772676.
• Recent Total Solar Irradiance dataArchived 2013-07-06 at theWayback Machine updated every Monday

• Solar phenomena
• Vortices
• Space physics
• Solar System

• CS1 errors: generic name
• CS1 maint: numeric names: authors list
• Webarchive template wayback links
• Articles with short description
• Short description is different from Wikidata
• Articles needing additional references from July 2015
• All articles needing additional references
• Pages using multiple image with auto scaled images
• CS1 errors: periodical ignored