solar-physics-timeline

Herschel's experimental setup for the detection of invisible solar radiation. Sunlight passes through a prism, forming the usual rainbow spectrum. A row of thermometers is positioned on a table beyond the red end of the spectrum. Thermometer 1, aligned with the spectrum, registers a rise in temperature, while the control thermometers 2 and 3 do not.

In the 1660's Isaac Newton had shown that sunlight can be separated into separate chromatic components via refraction through a glass prism. In 1800, William Herschel extended Newton's experiment by demonstrating that invisible "rays" existed beyond the red end of the solar spectrum. He did so by detecting the temperature rise in thermometers placed beyond the red end of the visible solar spectrum.

Herschel boldly conjectured that these invisible caloric rays, later named infrared radiation, were fundamentally no different from visible light, and could not be seen simply because the eye is not sensitive to them. Herschel also sought caloric rays beyond the violet end of the spectrum, but to no avail. However, the following year, Johann Wilhelm Ritter (1776–1810) used an experimental setup similar to Herschel's, but placed beyond the violet end of the spectrum a piece of paper soaked in silver chloride; the subsequent blackening of the paper beyond the visible violet demonstrated the existence of ultraviolet radiation. The following year, and using similar photochemical means, William Hyde Wollaston (1766-1828) independently rediscovered ultraviolet radiation.

References and further readings

Herschel, W. 1800, Experiments on the Refrangibility of the Invisible Rays of the Sun, Philosophical Transactions of the Royal Society of London 90, 284-292
Meadows, A.J. 1970, Early Solar Physics, Pergamon Press.

1802: Black lines in sunlight

Wollaston's experimental setup for the prismatic observation of the solar spectrum. Wollaston believed that the lines labeled here B, C and E marked natural color boundaries, although he also noticed other dark lines (f,g) that did not appear to delineate colors.

Reproduced from Philosophical Transactions of the Royal Society of London, vol. 92 (1802), p. 380 (Plate XIV)

While investigating the refractive properties of various transparent substances, the English chemist and physicist William Hyde Wollaston noticed dark lines in the spectrum of the Sun as viewed through a glass prism following the method of Isaac Newton. Beyond suggesting that these dark lines marked the boundaries of "natural colors," Wollaston did not pursue the matter much further. Yet this marked the first step towards solar spectroscopy, which was to revolutionize Solar Physics in the second half of the century.

References and further readings

Wollaston, W. H. 1802, A Method of Examining Refractive and Dispersive Powers by Prismatic Reflection, Philosophical Transactions of the Royal Society of London 92, 365-380
Meadows, A.J. 1970, Early Solar Physics, Pergamon Press.

1817: Solar spectroscopy is born

Reproduction of Fraunhofer's original 1817 drawing of the solar spectrum. The more prominent dark lines are labeled alphabetically; some of this nomenclature has survived to this day.

Denkschriften der K. Acad. der Wissenschaften zu München 1814-15, pp. 193-226

In what was to later lead to some of the more important advances in solar physics, Joseph von Fraunhofer (1787–1826) independently rediscovered the "dark lines" in the solar spectrum noticed 15 years earlier by William Hyde Wollaston. Fraunhofer pursued the matter mainly because he saw the possibility of using the lines as wavelength standards to be used to determine the index of refraction of optical glasses. Other physicists, however, were quick to realize that the Fraunhofer lines could be used to infer properties of the solar atmosphere, as similar lines were also observed in the laboratory in the spectrum of white light passing through heated gases.

In the hands of David Brewster (1781-1868), Gustav Kirchhoff (1824-1887), Robert Wilhelm Bunsen (1811–1899), and Anders Jonas Ångström (1814-1874) to name a few, spectroscopy turned into a true science which revolutionized not only solar physics, but also astronomy at large. Even today, most information gathered on the Sun and stars is obtained through spectroscopic means.

References and further readings

Meadows, A.J. 1970, Early Solar Physics, Pergamon Press.

1838: The solar constant

Pouillet's pyrheliometer.

The solar constant is a measure of the sun's luminosity, and is defined as the amount of solar energy per second falling on one square meter of the Earth's outer atmosphere, when the Earth is at a distance of exactly one astronomical unit (149,598,500 km) from the Sun. Although various scientists had attempted to calculate the Sun's energy output, the first attempts at a direct measurement were carried out independently, and more or less simultaneously, by the French physicist Claude Pouillet (1790–1868) and British astronomer John Herschel (1792–1871). Although they each designed different apparatus, the underlying principles were the same: a known mass of water is exposed to sunlight for a fixed period of time, and the accompanying rise in temperature recorded with a thermometer. The energy input rate from sunlight is then readily calculated, knowing the heat capacity of water. Their inferred value for the solar constant was about half the accepted modern value of 1367 ± 4 Watt per square meter, because they failed to account for of absorption by the Earth's atmosphere.

Pouillet's instrument (shown here) depicts water contained in the cylindrical container a, with the sun-facing side b painted black. The thermometer d is shielded from the Sun by the container, and the circular plate e is used to align the instrument by ensuring that the container's shadow is entirely projected upon it. Reproduced from A.C. Young's The Sun (revised edition, 1897).

References and further readings

Young, C.A. 1897, The Sun (revised ed.), Appleton and Co., chap. 8
Hufbauer, K. 1991, Exploring the Sun, The Johns Hopkins University Press.

1843: The sunspot cycle

Variation in observed sunspot numbers during the time period 1800-2000. The red curve is the Wolf sunspot number, and the purple line a count of sunspot groups based on a reconstruction by D.V. Hoyt. The green crosses are auroral counts, based on a reconstruction by K. Krivsky and J.P. Legrand.

Early sunspot observers noted the curious fact that sunspots rarely appear outside of a latitudinal band of about ± 30° centered about the solar equator, but otherwise failed to discover any clear pattern in the appearance or disappearance of sunspots. In 1826, the German amateur astronomer Samuel Heinrich Schwabe (1789–1875), set himself the task of discovering intra-mercurial planets, whose existence had been conjectured for centuries. Like many before him, Schwabe realized that his best chances of detecting such planets lay with the observation of the apparent shadows that they would cast while crossing the visible solar disk during their transit. The primary difficulty with this research program was the ever-present danger of confusing such planets with small sunspots. Accordingly, Schwabe began recording very meticulously the position of any sunspot visible on the solar disk on any day that weather would permit solar observation. In 1843, after 17 years of observations, Schwabe had not found a single intra-mercurial planet, but had discovered something else of great importance: the cyclic increase and decrease with time of the average number of sunspots visible on the Sun, with a period that Schwabe originally estimated to be 10 years, only one year shorter than the actual value.

References and further readings

Stix, M. 1989, The Sun, Springer.

1845: The first solar photograph

The first photographic technique was developed in the 1830's by J. N. Niépce (1765–1833) and Louis Daguerre (1789–1851), which relied on the exposure of a thin iodine layer deposited on a silver substrate, subsequently fixed in a mercury bath. The images so produced became known as "daguerrotypes". This imaging technique was very soon applied to astronomy, through the enthusiastic support of the French astronomer and politician Francois Arago (1786–1853) and the British astronomer John Herschel, who first coined the term "photography", as well as "positive" and "negative" images.

Reproduction of the first daguerrotype of the Sun. The original image was a little over 12 centimeters in diameter.

Reproduced from G. De Vaucouleurs, Astronomical Photography, MacMillan, 1961 [plate 1]

The first successful daguerrotype of the Sun, shown here, was made on 2 April 1845 by the French physicists Louis Fizeau (1819–1896) and Léon Foucault (1819–1868), both being perhaps better known for their various pioneering measurements of the speed of light. The exposure was 1/60 of a second. This image shows the umbra/penumbra structure of sunspots, as well as limb darkening.

Daguerre's photographic process was soon supplanted by a new technique (developed starting in 1851) based on a colloidal suspension on a glass substrate, in essence the direct ancestor of modern photographic film. In 1858, daily photographic recording of the solar disk using a solar telescope especially designed for photography began at Kew, England, under the leadership of Warren De la Rue (1815–1889). Photographic techniques were soon thereafter applied to the study of prominences, solar granulation, and solar spectroscopy, with some of the more spectacular results of the period obtained by Jules Janssen (1824–1907) at Meudon, near Paris. The first photograph of a solar prominence was captured by Charles A. Young (1834–1908) in 1870.

The first useful Daguerrotype of a solar eclipse was secured on 28 July 1851 by the photographer/astronomer Berkowski at the Königsberg observatory (then in Prussia, now Kaliningrad in Russia). De la Rue's group also obtained many fine photographs of the 18 July 1860 total eclipse in Spain. Eclipse photographic techniques were further improved by the introduction of radial gradient filters, designed to differentially attenuate the innermost, brightest portion of the corona. The resulting photographs allow to discern details of coronal structure out to many solar radii; see for example slide 9 and slide 10 of the HAO slide set.

References and further readings

De Vaucouleurs, G. 1961, Astronomical Photography, New York: MacMillan.
Lankford, J. 1984, The impact of photography on astronomy, in The General History of Astronomy, vol. 4A, ed. O. Gingerich, Cambridge University Press, pps. 16–39.

1848: The sunspot number

Sunspot drawings by Johann Hieronymus Schroeter (1745–1816), an active solar observer between 1785 and 1795. Schroeter's sunspot drawings were a primary source for Wolf's reconstruction of activity cycle number 4 (1785–1798).

As Schwabe's discovery of the sunspot cycle gained recognition, the question immediately arose as to whether the cycle could be traced farther into the past on the basis of extant sunspot observations. In this endeavor the most active researcher was without doubt the Swiss astronomer Rudolf Wolf (1816–1893). Faced with the daunting task of comparing sunspot observations carried out by many different astronomers using various instruments and observing techniques, Wolf defined the relative sunspot number (r) as follows:

r=k(f+10g)

where g is the number of sunspots groups visible on the solar disk, f is the number of individual sunspots (including those distinguishable within groups), and k is a correction factor that varies from one observer to the next (with k=1 for Wolf's own observations, by definition). This definition is still in use today, but r is now usually called the Wolf (or Zürich) sunspot number. Wolf succeeded in reliably reconstructing the variations in sunspot number as far back as the the 1755–1766 cycle, which has has since been known conventionally as "Cycle 1," with all subsequent cycles numbered consecutively thereafter.

References and further readings

Hoyt, D.V. & Schatten, K.H. 1997, The Role of the Sun in Climate Change, Oxford University Press.
Hoyt, D.V. & Schatten, K.H. 1998, Group sunspot numbers: a new solar activity indicator, Solar Physics, 181, 491–512.

1852: The sunspot cycle is linked to geomagnetic activity

The correlation between sunspot number and geomagnetic activity index.

Diagram reproduced from A.C. Young's 'The Sun' (revised edition, 1897)

In 1852, within a year of the publication of Schwabe's results in Kosmos, Edward Sabine (1788–1883) announced that the sunspot cycle period was "absolutely identical" to that of geomagnetic activity, for which reliable data had been accumulated since the mid-1830s. In fact, three other researchers arrived at the same conclusion independently, and more or less simultaneously: Rudolf Wolf and Jean-Alfred Gautier (1793–1881), both in in Switzerland, and Johann von Lamont (1805–1879) in Germany. This marked the beginning of solar-terrestrial interaction studies.

References and further readings

Hoyt, D.V. & Schatten, K.H. 1997, The Role of the Sun in Climate Change, Oxford University Press.
Kivelson, M.G., and Russell, C.T. (eds.) 1995, Introduction to Space Physics, Cambridge University Press, chap. 1.

1858–1859: The Sun's differential rotation

Early nineteenth century solar astronomers were increasingly intrigued at the fact that determinations of the solar rotation period obtained by tracking sunspots, carried out over the preceding two centuries, varied between anywhere from 25 to 28 days. This difference, while small, was significantly larger than the accuracy with which good observers could track sunspot motion.

Spörer's Law of sunspot migration. The thick lines shows the latitude at which most sunspots are found (vertical axis, equator is at zero), as a function of time (horizontal axis). The dashed line is the Wolf sunspot number, showing the rise and fall of the solar cycle.

The resolution of this puzzle came in 1858, when Richard C. Carrington (1826–1875) in England and, shortly thereafter, Gustav Spörer (1822–1895) in Germany independently made two key discoveries. First, the latitude at which sunspots are most often seen decreases systematically from about 40° to 5° latitude as the sunspot cycle proceeds from one minimum to the next. Second, sunspots located at higher latitudes are carried around the sun more slowly than spots at lower latitudes. From this, Carrington concluded that the Sun rotates "differentially", yet another argument in favor of the fluid or gaseous nature of the Sun's outer layers. The aforementioned historical discrepancies in the solar rotation period are then explained by the fact that different astronomers simply observed the Sun at different epochs of the cycle.

The rapid development of spectroscopic techniques in the second half of the nineteenth century offered another means of measuring the surface differential rotation, one that is not restricted to latitudes where sunspots are present: measurement of the wavelength shift of spectral lines between the approaching and receding solar limbs, as a consequence of the Doppler effect. This was first carried out by Hermann Vogel (1841–1907) in 1871, and a few years after by Charles Young. These results were accurate enough to demonstrate that sunspots rotate at very nearly the same rate as the sun's photosphere. By the late 1880's Nils Dúner (1839–1914) had secured accurate spectroscopic rotational period calculations at latitudes about twice higher than the sunspot belts, demonstrating that the Sun's polar regions rotate about 30% slower than its equator.

Interestingly, Christoph Scheiner had already noted in his 1630 Rosa Ursina that the rotation period inferred from tracking sunspots at different heliocentric latitudes showed a systematic increase with latitude. However, in Scheiner's Aristotelian framework, the Sun could only be a solid, rigidly rotating sphere, and therefore he interpreted his data as a proof that sunspots were not markings on the solar surface, but instead cloud-like structures floating above it, since a fluid Sun was "physically absurd". For this reason, most historians of science continue to attribute the discovery of solar differential rotation to Carrington and Spörer.

References and further readings

Mitchell, W.M. 1916, The History of the Discovery of Solar Spots, Popular Astronomy, 24, 22–ff.
Eddy, J.A., Gilman, P.A., and Trotter, D.E. 1977, Science, 198, 824–829.

1859: First observation of a solar flare

Reproduction of a drawing by R.C. Carrington, showing the location of the flare he observed while making a drawing of an active region.

Reproduced from his 1860 paper in Monthly Notices of the Royal Astronomical Society (vol. 20, p. 13)

On 1 September 1859, the amateur astronomer Richard C. Carrington was engaged in his daily monitoring of sunspots when he noticed two rapidly brightening patches of light near the middle of a sunspot group he was studying (indicated by A and B on the drawing shown here). In the following minutes the patches dimmed again while moving with respect to the active region, finally disappearing at positions C and D. This unusual event was also independently observed by R. Hodgson (1804–1872), another British astronomer.

This serendipitous observation represents the first clear description of a solar flare, corresponding to a sudden and intense heating of solar atmospheric plasma caused by reconnection of magnetic fields. What Carrington observed would today be called a two-ribbon flare. Only the largest flares are bright enough to be seen in visible light. They are readily seen in X-rays, however (see slide 15 of the HAO slide set). An earlier, plausible observational report of a white light flare has been found in the (unpublished) notebooks of the English scientist and amateur astronomer Stephen Gray (1666–1736), who on 27 December 1705 observed what he described as a "flash of lightning" near a sunspot.

Both Carrington and Hodgson noted that magnetic monitoring instruments registered strong disturbances at about the same time, but it is not possible to tell for sure whether these were due to the flare they actually saw. It is more likely that they were caused by other generalized solar disturbances of which the flare was but one manifestation.

References and further readings

Carrington, R.C. 1860, Monthly Notices of the Royal Astronomical Society, 20, p. 13.
Lang, K.R. 2000, The Sun from Space, Springer, chap. 6

1859: The chemical composition of the Sun

Reproduction of part of the map of the solar spectrum published in 1863 by Kirchhoff, showing the identification of a large number of spectral lines with various chemical elements. Note numerous clear matches for Iron (Fe).

In the late 1850s the chemist Robert Wilhelm Bunsen (1811–1899) and theoretical physicist Gustav Kirchhoff, both at Heidelberg, took on the issue of spectral line identification where Joseph von Fraunhofer had left it some 40 years earlier. By simultaneous observations of the solar spectrum and laboratory flame spectra, they showed that (bright) emission lines in heated gases coincide with (dark) absorption lines seen when observing white light shining through the same gas (when cool). This established the empirical basis needed for the identification of the dark lines seen in the solar spectrum. By careful comparison with emission lines seen in the laboratory for various pure gases, Kirchhoff could demonstrate the existence in the Sun of a large number of chemical elements, mostly metals, also present on Earth. Hydrogen was identified spectroscopically in 1862 by A. Ångström (1814–1874), but it is only much later, in the 1920's, that Hydrogen was recognized as the most abundant solar constituent.

Following this and other groundbreaking work by David Brewster (1781–1868) and Ångström, spectroscopy continued to progress throughout the second half of the eighteenth century. In the solar context, some of the most active and innovative observers were J. Norman Lockyer (1836–1920), Jules Janssen, Hermann Carl Vogel, William Huggins (1824–1910), Angelo Secchi (1818–1878), Charles Young, and Samuel Langley (1834–1906). Even at that time, spectroscopy was still an empirical science without a sound physical basis, as quantum mechanics lay half a century in the future.

References and further readings

Meadows, A.J. 1984, The Origins of Astrophysics, in The General History of Astronomy, vol. 4A, ed. O. Gingerich, Cambridge University Press, pps. 3–15.

1860: First observations of a coronal mass ejection

The total solar eclipse of 18 July 1860 was probably the most thoroughly observed eclipse up to that time. These drawings include depictions of a peculiar feature in the SW (lower right) portion of the corona. Based on comparison with modern coronal observations, it is quite likely that these represent the first record of a Coronal Mass Ejection in progress.

Drawing by G. Tempel

Drawing by von Feilitzsch

Drawing by F.A. Oom

Drawing by E.W. Murray

Drawing by F. Galton

Drawing by C. von Wallenberg

Drawings of the 1860 eclipse
(Reproduced from Ranyard, C.A, 1879, Mem. Roy. Astron. Soc., 41, 520, chap. 44.)

Today coronal mass ejections are known to represent one of the more energetic and geo-effective manifestations of solar activity, with up to 10 billion tons of material being ejected into interplanetary space at speeds reaching up to 1000 kilometer per second. For more detail on CMEs see slide 13 and slide 14 of the HAO slide set.

References and further readings

Eddy, J.A. 1974, A Nineteenth-century Coronal Transient, in Astronomy and Astrophysics, 34, 235–240.

1881: The solar constant, again

Samuel Langley's base camp on California's Mt Whitney, July 1881. Some of Langley's instruments failed to arrive or arrived damaged, with the crucial spectral bolometer back in working order only by the end of August. The expedition was cut short on 8 September due to worsening observing conditions caused by the breakout of a series of wildfires elsewhere in California a few days earlier. Nonetheless, valuable data were collected.

Reproduced from: Eddy, J.A. 1990, J. Hist. Astron., 21, p. 115

By the second half of the nineteenth century, after various solar observing expedition to mountaintops, it was becoming increasingly clear that the Earth's atmosphere absorbs a significant portion of the sun's luminosity. Consequently, attempts at determining the solar constant were moved to the highest practical altitudes.

The American scientist Samuel Langley carried out the most elaborate attempt at determining the solar constant at the time, during an expedition to Mt Whitney, California, in July 1881. Using his recently invented bolometer (an instrument based on the varying electrical resistivity of metals with temperature), as well as other instruments, Langley carried out measurements at different wavelengths and at different altitudes, demonstrating the strong variation with wavelength of the absorption by Earth's atmosphere. However, the solar constant value he calculated at the time, 2903 Watt per square meters, is nearly a factor of two larger than the modern value (1367 W/m2), something apparently due to errors in the data reduction procedure, since Langley's later assistant Charles Abbot (1872–1973) obtained 1465 W/m2 with the original Mt Whitney data.

References and further readings

Hufbauer, K. 1991, Exploring the Sun, The Johns Hopkins University Press.
Eddy, J.A. 1990, Journal for the History of Astronomy, 21, p. 115.
Foukal, P.V. 1990, Solar Astrophysics, John Wiley and Sons.

1908: The magnetic nature of sunspots

The magnetically-induced Zeeman splitting in the spectrum of a sunspot.

Reproduced from the 1919 paper by G.E. Hale, F. Ellerman, S.B. Nicholson, and A.H. Joy (in The Astrophysical Journal, vol. 49, pps. 153-178)

The study of sunspots and their 11-year cycle was finally put on a firm physical footing by the epoch-making work of George Ellery Hale (1868-1938) and collaborators in the opening decades of the twentieth century. In 1907–1908, by measuring the Zeeman splitting in magnetically sensitive lines in the spectra of sunspots, and detecting the polarization of the split spectral components, Hale provided the first unambiguous and quantitative demonstration that sunspots are the seats of strong magnetic fields (see also slide 4 and slide 5 of the HAO slide set). Not only was this the first detection of a magnetic field outside the Earth, but the inferred magnetic field strength, 3000 Gauss, is over a thousand times greater than the Earth's own magnetic field. It was subsequently realized that the pressure provided by such strong magnetic field would also lead naturally to the lower temperatures observed within the sunspots, as compared to the photosphere.

References and further readings

Hale, G.E. 1908, On the probable existence of a magnetic field in sunspots, The Astrophysical Journal, 28, pps. 315–343
Stix, M. 1989, The Sun, Springer.

1919: The Sun's magnetic cycle

A diagram taken from the 1919 paper by G.E. Hale, F. Ellerman, S.B. Nicholson, and A.H. Joy, illustrating what is now known as Hale's polarity laws. This presented solid evidence for the existence of a well-organized large-scale magnetic field in the solar interior, which cyclically changes polarity approximately every 11 years.

The Astrophysical Journal, vol. 49, pps. 153–178

In the decade following his groundbreaking discovery of sunspot magnetic fields, George Ellery Hale and his collaborators went on to show that large sunspots pairs almost always (1) show the same magnetic polarity pattern in each solar hemisphere, (2) show opposite polarity patterns between the North and South solar hemispheres, and (3) these polarity patterns are reversed from one sunspot cycle to the next, indicating that the physical magnetic cycle has a period twice that of the sunspot cycle period. These empirical observations have stood the test of time and are known as "Hale's polarity Laws". Their physical origin is now known to be the operation of a large scale "hydromagnetic dynamo" within the solar interior, although the details of the process are far from adequately understood. Because the Sun's dynamo-generated magnetic field is ultimately responsible for all manifestations of solar activity (flares, coronal mass ejections, etc.), solar dynamo modeling remains a very active area of research in solar physics.

References and further readings

Hale, G.E., Ellerman, F., Nicholson, S.B., and Joy, A.H. 1919, The Astrophysical Journal, 49, pps. 153–178
Stix, M. 1989, The Sun, Springer.

1931: The Coronagraph

Bernard Lyot's coronagraph design. The occulting disk is at B, and the diaphragm and screen at D, E are needed to block stray light arising from diffraction at the primary lens and diaphragm A.

Reproduced from L'Astronomie, 66 (1952) (Fig. 113, p. 269)

Much of the remarkable progress made in understanding the Sun's outer atmosphere had been made through the use of observations carried out during total solar eclipses. The relative rarity of such eclipses, the cost and logistical difficulties of traveling to often remote location to observe them, the short duration of totality, as well as the frustrating vagaries of weather, motivated the search for a way to observe the corona at will and in full daylight. This was finally achieved in 1931 by the French solar physicist Bernard Lyot (1897–1952), who first designed an instrument now known as the coronagraph.

A coronagraph is nothing more than a telescope equipped with an occulting disk sized in such a way as to block out the solar disk. Although this may sound trivial, it turns out to be extremely difficult to achieve the needed accurate optical alignment and mechanical stability, without which stray light makes the viewing of the faint corona all but impossible. Lyot also managed to secure the first full daylight photographs of the corona. His success motivated others to replicate and modify his design, the most successful of these followers being Max Waldmeier at the ETH/Zürich, and Donald H. Menzel (1901–1976) at Harvard College Observatory. Here at HAO, we've designed and observed with many coronagraphs, such as Mk3/Mk4, K-Cor, and the coronagraph aboard the SMM satellite.

References and further readings

D'Azambuja, L. 1952, L'oeuvre de Bernard Lyot, L'Astronomie, 66, 265–277.
Hufbauer, K. 1991, Exploring the Sun, The Johns Hopkins University Press.

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Solar Physics Historical Timeline (1600 - 1799)

kolinski — Mon, 13 Feb 2023 20:25:54 +0000

Solar Physics Historical Timeline (1600 - 1799) kolinski Mon, 02/13/2023 - 13:25

Timeline

1600-1799

In this page

1609 - The Sun in focus

1610 - First telescopic observations of sunspots

1644 - The Sun as a star

1645-1715 - Sunspots vanish

1687 - The mass of the Sun

1774-1801 - The physical nature of sunspots

1796 - The nebular hypothesis

1609: The Sun in focus

An early convert to the Copernican system was Johannes Kepler (1571-1630). After ten years of laborious work using the accurate observations of planetary positions accumulated over 20 years by the astronomer Tycho Brahe (1546-1601), Kepler came to realize that the orbital paths of planets have the form of ellipses with the Sun at one focus, and that the radius vector joining a given planet to the Sun sweeps equal areas in equal time (today known as Kepler's first and second laws). In 1609 Kepler published his landmark Astronomia Nova, and in 1619 his Harmonice mundi, where what is now known as Kepler's third law is first laid out (the orbital period squared is proportional to the mean distance cubed). Using his planetary model and Brahe's observations, Kepler produced the Rudolphine Tables of planetary positions in 1627. These proved more accurate, by over an order of magnitude, than previous tables produced using the original planetary model of Copernicus.

References and further reading

Thoren, V.E. 1989, Tycho Brahe, in The General History of Astronomy, vol. 2A, eds. R. Taton and C. Wilson, Cambridge University Press, pps. 3-21.
Gingerich, O. 1989, Johannes Kepler, in The General History of Astronomy, vol. 2A, eds. R. Taton and C. Wilson, Cambridge University Press, pps. 54-78.
Gingerich, O., and Voelkel, J.R. 1998, Journal for the History of Astronomy, 29, 1-34. Physics.

1610: First telescopic observations of sunspots

Reproduction of one of Galileo's sunspot drawings. The umbrae/penumbrae structure is clearly depicted on this June 23. 1612 drawing.

In the first decade of the seventeenth century, four astronomers nearly simultaneously turned the newly invented telescope toward the Sun, and noted the existence of sunspots. They were Johann Goldsmid (1587-1616, a.k.a. Fabricius) in Holland, Thomas Harriot (1560-1621) in England, Galileo Galilei (1564-1642) in Italy, and the Jesuit Christoph Scheiner (1575-1650) in Germany.

To Harriot belongs the oldest recorded sunspot observation, on December 8 1610, as evidenced by entries in his notebooks. However, he did not pursue these observations in any systematic or continuous manner at the time. Fabricius was the first to publish his results in 1611, and correctly interpreted the apparent motion of sunspots in terms of axial rotation of the Sun. Galileo and Scheiner, however, were the most active in using sunspots to attempt to infer physical properties of the Sun. To Galileo belongs the credit of making a convincing case that sunspots are indeed features of the solar surface, as opposed to intra-Mercurial planets (Scheiner's original position). Galileo's views were first laid out in detail in his 1613 Letters on Sunspots, written in response to Scheiner's own views on the matter (first published in 1612 under the pseudonym of Apelles in the form of three letters to Mark Welser (1558-1614), Augsburg Magistrate, patron of science, and scientific correspondent of both Scheiner and Galileo). Some years later Scheiner, in his massive 1630 treatise on sunspots entitled Rosa Ursina, accepted the view of sunspots as markings on the solar surface and used his accurate observations to infer the fact that the Sun's rotation axis is inclined with respect to the ecliptic plane (i.e., the plane of the Earth's orbit around the Sun).

The existence of ephemeral blemishes on the Sun's surface was in stark conflict with the then prevailing Ptolemaic/Aristotelian-based cosmology endorsed by the Roman catholic Church (after suitable modification to avoid open contradiction with the Scriptures). Galileo's views on sunspots contributed significantly the sequence of events that landed him in front of the Roman Inquisition in 1633. Officially, Galileo was condemned for disobedience to the Church, in the context of his open endorsement of the Copernican heliocentric planetary model. Growing animosity on the part of the Jesuits who, particularly through their chief astronomer Christopher Clavius (1538-1612), had been originally quite supportive of Galileo's early telescopic discoveries, also contributed to Galileo's downfall.

References and further reading

Galileo, G. 1610, Sidereus Nuncius, trans. A. van Helden 1989, The University of Chicago Press.
Galileo, G. 1613, Letters on Sunspots [in S. Drake (trans.) 1957, Ideas and Opinions of Galileo, Doubleday].
Galileo, G. 1632, Dialogues concerning the two chief world systems, trans. S. Drake, 2nd edition 1967, University of California Press.
Mitchell, W.M. 1916, The history of the discovery of the solar spots, in Popular Astronomy, 24, 22-ff.
Shea, W.R. 1970, Galileo, Scheiner, and the interpretation of Sunspots, Isis, 61, 498-519.
Drake, S. 1978, Galileo at work: his scientific biography, Chicago: The University of Chicago Press [1995 Dover reprint]

1644: The Sun as a star

Detail of a diagram from the 1644 Principia philosophiae of René Descartes, depicting his conception of the cosmos as an aggregate of contiguous vortices, most with a star at their center. S is the Sun.

The Copernican system replaced the Earth with the Sun as the center of the universe, but otherwise maintained a clear distinction between the Sun and the "fixed" stars, distributed on the fixed, outermost sphere of the copernican cosmos. This last concession to humanity's cosmic centrality was rejected by the generation of Copernicans following Kepler and Galileo. Prominent among them was René Descartes (1596–1650) who, in his 1644 book Principia philosophiae, put forth a model of the cosmos in which the Sun is but one of many stars, each formed at the center of a primeval vortex. Descartes viewed sunspots as floating aggregates of ethereal matter, accreted along the Sun's rotational axis, where centrifugal forces are negligible.

References and further reading

Aiton, E. J. 1989, The Cartesian Vortex Theory, in The General History of Astronomy, vol. 2A, eds. R. Taton and C. Wilson, Cambridge University Press, pps. 207-221.

1645-1715: Sunspots vanish

This very anachronistic plot shows the variation in observed sunspot numbers during the time period 1600-1800. The red curve is the Wolf sunspot number, and the purple line a count of sunspot groups based on a reconstruction by D.V. Hoyt. The green crosses are auroral counts, based on a reconstruction by K. Krivsky and J.P. Legrand.

Sunspots observations continued in the seventeenth century, with the most active observers being the German Johannes Hevelius (1611-1687) and the French Jesuit Jean Picard (1620-1682). Very few sunspots were observed from about 1645 to 1715, and when they were, their presence was noted as a noteworthy event by active astronomers. At that time a systematic solar observing program was underway under the direction of Jean Dominique Cassini (1625-1712) at the newly founded Observatoire de Paris, with first Picard, and later Philippe La Hire, carrying out the bulk of the observations. Historical reconstructions of sunspot numbers indicate that the dearth of sunspots was real, rather than the consequence of a lack of diligent observers. A simultaneous decrease in auroral counts further suggest that solar activity was greatly reduced during this time period.

This period is now known as the Maunder minimum, after the solar astronomer E.W. Maunder, who, following the pioneering historical investigations of Gustav Spörer (1822-1895), was most active and steadfast in investigating the dearth of sunspot sightings by astronomers active in the second half of the seventeenth century. The documented occurrence of exceptionally cold winters throughout Europe during those years may be causally related to reduced solar activity, although this remains a topic of controversy.

References and further readings

Eddy, J.A. 1976, The Maunder Minimum, Science, 192, 1189-1203.
Eddy, J.A. 1983, The Maunder minimum: a reappraisal, Solar Phys., 89, 195-207.
Ribes, J. C., and Nesme-Ribes, E. 1993, The solar sunspot cycle in the Maunder minimum AD1645 to AD1715, Astronomy and Astrophysics, 276, 549-563.
Hoyt, D.V. & Schatten, K.H. 1997, The Role of the Sun in Climate Change, Oxford University Press.

1687: The mass of the Sun

The mass of the Sun and its distance from the Earth are two very fundamental quantities that were only determined with reasonable accuracy in the eighteenth century. The first quantitative estimate of the Sun's mass is due to Isaac Newton (1642-1727). Newton presented the calculation in his "Principia Mathematica," making use of his newly formulated law of universal gravitation. Newton argued that stable planetary orbits resulted from a balance between centripetal and gravitational acceleration; In doing so he could finally provide a physical explanation for the three laws of planetary motions established empirically by Kepler. The ratio of Sun-to-Earth mass can be in principle determined without knowing the actual value of the universal gravitational constant. This only required a knowledge of orbital periods and radii. Newton, however, used too high a value for the solar parallax, thus grossly underestimating the Sun-Earth distance, and, consequently, underestimating the Sun-to-Earth mass ratio by more than a factor of ten (M_Sun/M_Earth = 28700 instead of 332945). In later editions of his Principia (in 1713 and 1726), Newton used improved estimates of the solar parallax, and brought his estimate to within a factor of two of the modern value.

References and further readings

Wilson, C. 1989, The Newtonian achievement in Astronomy, in The General History of Astronomy, vol. 2A, eds. R. Taton and C. Wilson, Cambridge University Press, pps. 234-274.
Hufbauer, K. 1991, Exploring the Sun, The Johns Hopkins University Press.

1774-1801: The Physical nature of sunspots

Reproduction of one of Herschel's original diagrams on the nature of sunspots. This hypothesis relies heavily on the asymmetric appearance of sunspots when seen near the solar limbs, as originally pointed out by A. Wilson in 1774.

Phil. Trans. 1801, vol. 91, pp. 265-318 (plate 18)

The physical nature of sunspots remained a topic of controversy for nearly three centuries. The universally opinionated Galileo proposed, with unusual reservation, that sunspots may be cloud-like structures in the solar atmosphere. Scheiner believed them to be dense objects embedded in the Sun's luminous atmosphere. In the late eighteenth century William Herschel (1738-1822; discoverer of the planet Uranus), following an hypothesis earlier put forth by A. Wilson in 1774, suggested that sunspots were openings in the Sun's luminous atmosphere, allowing a view of the underlying, cooler surface of the Sun (likely inhabited, in Herschel's then influential opinion).

References and further readings

Berry, A. 1898, A Short History of Astronomy (Dover Reprint), chap. 12.
Hufbauer, K. 1991, Exploring the Sun, The Johns Hopkins University Press.

1796: The nebular hypothesis and the Sun's origin

Drawing of Nebulae by William Herschel.

Reproduced from W. Herschel, Philosophical Transactions of the Royal Society of London 101 (1811), 269-336 (p. 336, Plate IV)

By the closing decade of the eighteenth century, the increasingly powerful reflecting telescopes built by the German-born English astronomer William Herschel had revealed the existence of a number of diffuse cloud-like structures, dubbed Nebulae. Herschel believed that these assorted Nebulae could be interpreted as different snapshots of an evolutionary sequence of gravitational collapse into one or more stars, along the lines proposed by Laplace.

Inspired by these observations, the French astronomer and mathematician Pierre Simon de Laplace (1749–1827) put forth his nebular hypothesis, in which the Sun and solar system formed from the gravitational collapse of an initially slowly rotating, large, diffuse gas cloud. Laplace's cosmological ideas were described in a popular work entitled Exposition du systè me du monde published in 1796. This marked a turning point in the history of science, since therein he categorically rejects the Biblical account of the creation of the Universe, and offers instead a physically-based theory that, in its main thrust if not in all details, remains valid to this day.

References and further readings

Herschel, W. 1811, Astronomical Observations Relating to the Construction of the Heavens, Philosophical Transactions of the Royal Society of London 90, 284–292.
Hoskin, M. (ed.) 1997, The Cambridge Illustrated History of Astronomy, Cambridge University Press, chap. 6.

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Solar Physics Historical Timeline (0 - 1599)

kolinski — Mon, 13 Feb 2023 18:48:08 +0000

Solar Physics Historical Timeline (0 - 1599) kolinski Mon, 02/13/2023 - 11:48

Timeline

0-1599

In this page

AD 968 - The first mention of the solar corona

1128 - The first sunspot drawing

1185 - The first description of solar prominences

1543 - The Sun moves to center stage

AD 968: The first mention of the solar corona

The first mention of the solar corona was by Leo Diaconus in 968 AD.

The solar corona is the hot, extended outer atmosphere of the Sun. It is far too faint to be seen against the blinding brightness of the solar disk itself, but becomes visible, and spectacularly so, at times of total solar eclipses when the solar disk is obscured by the Moon.

While the solar corona is visible at any total solar eclipse, the first explicit mention of what can be unambiguously interpreted to be the corona was made by the Byzantine historian Leo Diaconus (ca. 950-994), as he observed the total eclipse of 22 December 968 from Constantinople (now Istanbul, Turkey). His observation is preserved in the Annales Sangallenses, and reads:

"...at the fourth hour of the day ... darkness covered the earth and all the brightest stars shone forth. And is 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".

Compare this description to the modern eclipse photographs shown on slide 9 and slide 10 of the HAO Sun Pictorial. A much older possible description of the corona is said to be found on engraved oracle bones dating from the Shang dynasty in China (1766 to 1123 BC), but is far more ambiguous and open to interpretation than Diaconus' description.

References and further reading

Hetherington, B. 1996, A chronicle of pre-telescopic astronomy, John Wiley and Sons.

1128: The first sunspot drawing

Sunspot drawing in the Chronicles of John of Worcester, twelfth century. Notice the depiction of the penumbra around each spot.

R.W. Southern, Medieval Humanism, Harper & Row 1970, [Plate VII]

This drawing, from the Chronicles of John of Worcester (one of the many monks who contributed to the Worcester Chronicles), represents to the best of our knowledge the first surviving sunspot drawing, from a sighting on Saturday, 8 December 1128. Compare it with the sunspots seen on Slide 1 and Slide 3 of the HAO Sun Pictorial.

The accompanying text translates to something like:

"...from morning to evening, appeared something like two black circles within the disk of the Sun, the one in the upper part being bigger, the other in the lower part smaller. As shown on the drawing."

The facts that the Worcester monks could apparently distinguish the umbrae and penumbrae of the sunspots they observed suggests that the spots must have been truly exceptionally large.

Large sunspots can be visible to the naked-eye under suitable viewing conditions, for example when the sun is partially obscured by clouds or mist, particularly at sunrise or sunset. Numerous such sightings exist in the historical records, starting with Theophrastus (374–287 B.C.) in the fourth century B.C. However, the most extensive pre-telescopic records are found in the Far East, especially in the official records of the Chinese imperial courts, starting in 165 B.C. In the West, the dominating views of Aristotle (concerning the incorruptibility of the heavens) meant that sunspots were "physically impossible", so that sightings were ignored or ascribed to transit of Mercury or Venus across the solar disk.

References and further reading

Van Helden, A. 1996, Galileo and Scheiner on sunspots, in Proc. Am. Phil. Soc., 140, 358–396.

1185: The first description of solar prominences

Prominences are large accumulations of (relatively) cooler gas held suspended high in the solar atmosphere by the Sun's magnetic field (see Slide 6 and Slide 7 of the HAO Sun Pictorial). Large prominences are often visible during total solar eclipses, in the form of small reddish filaments or blobs in the lower corona. A description of what can be possibly interpreted as prominences can be found in Chinese court records dating from 28 BC by Chinese astronomers during the reign of Emperor Cheng of the Western Han Dynasty. From then until the late Ming Dynasty in the mid-17th century, Chinese history books recorded more than 100 sunspots. Furthermore, they also took note of other phenomena concerning the sun, such as solar prominences and coronas. The first record of a solar prominence has been found in a tortoise shell inscription, which describes "three suddenly bursting fires eating a chunk of the sun". The first unambiguous description of prominences is usually taken to be that found in the Russian Chronicle of Novgorod, in the following description of the 1 May 1185 solar eclipse:

"In the evening there was 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."

References and further reading

Sviatsky, D. 1923, Astronomy in the Russian Chronicles, Journal of the British Astronomical Association, 33, 285–287.
Hetherington, B. 1996, A chronicle of pre-telescopic astronomy, John Wiley and Sons.

1543: The Sun moves to center stage

The Copernican planetary model. The Sun is at the center of all planetary motions, except for the Moon which orbits Earth. Under this arrangement the orbital speed of planets decreases steadily outwards, and the outer sphere of fixed stars is truly motionless. In Copernicus' original model the Earth has three motions: a daily 24-hr axial rotation, a yearly orbital motion about the Sun, and a third motion, somewhat related to procession which Copernicus thought necessary to properly reproduce ancient observations.

The cosmos of the late Christian medieval era was a fusion of ideas combining the physics of Aristotle with the planetary astronomy of Ptolemy. This is the world view that was destroyed in the sixteenth and seventeenth centuries. The first blow was dealt by Nicholas Copernicus (1473–1543), who published his landmark book "De Revolutionibus Orbium Coelestium" in 1543. There, Copernicus presented a new planetary model with the Sun placed in center, and letting all planets (including the Earth) orbit around the Sun. Copernicus also gave the Earth two additional motions: a daily axial rotation, and a procession of that spin axis. In doing so, Copernicus eliminated the need for the two outermost spheres of the ptolemaic model and produced a system where the speeds of revolution decrease gradually outward all the way to the fixed stars.

Copernicus ostensibly introduced his heliocentric model in order to do away with equants and various motions previously attributed to the sphere of fixed stars, but it appears clear that he believed in the physical reality of his heliocentric hypothesis. Because Copernicus' model could be construed as yet another mathematical device useful in astronomy, but without physical reality, his model could be used by astronomers without attracting the ire of philosophers and theologians committed to the centrality and fixity of the Earth. This situation was to change in the next century.

References and further reading

Kuhn, T.S. 1957, The Copernican Revolution, Harvard University Press.
Boas, M. 1962, The Scientific Renaissance 1450–1630, Harper & Row [Dover reprint available].
Gingerich, O. 1993, The Eye of Heaven, New York: American Institute of Physics.
Grant, E. 1994, Planets, Stars, & Orbs. The Medieval Cosmos, 1200–1687, Cambridge University Press

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Solar Physics Historical Timeline (1223 BC - 200 BC)

kolinski — Mon, 13 Feb 2023 16:19:19 +0000

Solar Physics Historical Timeline (1223 BC - 200 BC) kolinski Mon, 02/13/2023 - 09:19

Timeline

1223 BC-200 BC

In this page

1223 BC - The oldest eclipse record

800 BC - The first plausible recorded sunspot observation

350 BC - Sun circling under a sheltering sky

200 BC - The distance to the Sun

1223 BC: The oldest eclipse record

The oldest eclipse record is found on a clay tablet uncovered in the ancient city of Ugarit, in what is now Syria, with two plausible dates usually cited: 3 May 1375 BC or 5 March 1223 BC, the latter being favored by most recents authors on the topic.

Ancient ruins at Ugarit city, now Syria, where clay tablet with oldest eclipse record was discovered.

It is certainly clear that by the eight century BC, the Babylonians were keeping a systematic record of solar eclipses, and may even have been able to predict them fairly accurately based on numerological rules. Total eclipses of the Sun are arguably the most impressive astronomical phenomenon that can be observed more or less regularly with the naked eye (see slide 9 and slide 10 of the HAO slide set The Sun: A Pictorial Introduction). They occur when the Moon reaches a point in its orbit around the Earth that lies on the line joining the Earth and Sun. By a remarkable coincidence, the Moon's angular diameter, as seen from the Earth, is almost identical to that of the Sun. The Sun's disk is then completely eclipsed, and daytime darkness falls upon the Earth for a few minutes (this physical explanation of the phenomenon was only put forth much later, in the first century BC). Like comets, solar eclipses were taken to be astrological omens of great significance. It is therefore not surprising that such a spectacular event is often mentioned in surviving written records and chronicles of ancient civilizations.

References and further reading

Fotheringham, J.K. 1932, The Story of Hi and Ho, Journal of the British Astronomical Association, 43, 248-257.
Zirker, J.B. 1995, Total Eclipses of the Sun, Princeton University Press.
Littman, M., Willcox, F., and Espenak, F. 2000, Totality: Eclipses of the Sun, 2nd ed., Oxford University Press.

ca. 800 BC: The first plausible recorded sunspot observation

The two oldest record of a sunspot observation are found in the Book of Changes, probably the oldest surviving Chinese book, compiled in China around, or before, 800 BC.

The text reads "A dou is seen in the Sun", and "A mei is seen in the Sun". From the context, the words (i.e., Chinese characters) "dou" and "mei" are taken to mean darkening or obscuration. Astronomers at the court of the Chinese and Korean emperors made regular notes of sunspots. It seems, however, that observations were not carried out systematically for their own sake, but instead took place whenever astrological prognostication was demanded by the emperor. The surviving sunspots records, though patchy and incomplete, cover nearly 2000 years and represents, by far, the most extensive pre-telescopic sunspot record.

Sunspots are concentrations of strong magnetic fields piercing the solar photosphere. Visually, they look like dark blemishes on the solar disk (see slide 1 and slide 3 of the HAO slide set). Most sunspots are too small to be readily visible by naked eye observations, but some reach a size sufficient to be visible without a telescope, under suitable viewing conditions (for example, when the sun is partially obscured by fog or thick mist, or clouds). Because of their possible astrological significance, reports of naked-eye sunspot observations are found in many ancient chronicles and court chronologies.

References and further reading

Mossman, J.E., 1989, A comprehensive search for sunspots without the aid of a telescope, 1981-1982, in Quarterly J. R. Astr. Soc., 30, 59–73.
Stephenson, F.R. 1990, Historical evidence concerning the Sun: interpretation of sunspot records during the telescopic and pre-telescopic eras, in Phil. Trans. R. Soc. London, A330, 499-512.
Hetherington, B. 1996, A chronicle of pre-telescopic astronomy, John Wiley and Sons.

ca. 350 BC: Sun circling under a sheltering sky

The Aristotelian cosmos as modeled by Ptolemy. The Earth sits motionless at the center of the universe, and the outer sphere, the Primum Mobile, is assumed to undergo a full revolution in 24 hours.

One of the major intellectual achievements of ancient Greece is the physical model of the cosmos developed by Aristotle (384-322). An essential feature is the place occupied by the Earth at the center of the Universe, with the Sun, planets and sphere of fixed stars revolving about that center, the Sun occupying the fourth sphere. In this geocentric model the Earth is absolutely fixed, with the motions of procession and daily rotation ascribed to the two outermost spheres of the model.

This basic planetary arrangement formed the basis of a mathematical model of planetary motion developed four centuries later by Claudius Ptolemy (ca. 100-170). In Aristotle's scheme there existed fundamental physical differences between the terrestrial and celestial realms, as demarcated by the Moon's sphere. Everything inside the Moon is made of the four elements earth, water, air and fire, themselves arranged concentrically about the center of the universe. Pure circular motion prevails throughout the heavens, which are are made of an incorruptible fifth element (or "quintessence").

References and further reading

Grant, E. 1977, Physical Science in the Middle Ages, Cambridge University Press
Crowe, M.J. 1990, Theories of the World from Antiquity to the Copernican Revolution, Dover.
Pedersen, O. 1993, Early Physics and Astronomy, revised ed., Cambridge University Press.

ca. 200 BC: The distance to the Sun

Aristarchus' geometric construction used to estimate the distance to the Sun. The Earth-Sun-Moon triangle and sizes are not drawn to scale.

The first mathematic-based attempt at determining the Sun-Earth distance is due to Aristarchus of Samos (ca. 310-230 BC). The procedure followed by Aristarchus is illustrated in this diagram: form a triangle by connecting the Earth (E), Sun (S) and Moon (M). At the first or third Moon quarter, the triangle so described is a right-angle triangle (a=90°). The angle b can be measured by an observer on Earth, which then allows the angle c to be computed (c=90-b when a=90°). The ratio of the Earth-Moon segment (EM) to the Earth-Sun segment (ES) is by definition equal to sin(c) (in modern trigonometric parlance -- Aristarchus expressed this differently).

While sound in theory, in practice this procedure is highly inaccurate in the Earth/Sun/Moon case; this is because EM is much smaller than ES, implying that b is very close to 90°, so that c is in turn very small. This has the consequence that a small measurement error on b translates in a large variation in the ratio EM/ES (again in modern parlance, a measurement error db is amplified by a factor 1/(sin c)2, which is large when c is very small). Aristarchus measured b=87°, while the true value is in fact 89° 50'. This may seem like a small error, but because of the large error amplification, Aristarchus' value leads to EM/ES=19, instead of the true value EM/ES=397. Nonetheless, Aristarchus' calculation was the first to mathematically set the spatial scale of the cosmos.

References and further reading

Van Helden, A. 1985, Measuring the Universe, University of Chicago Press.
Hirschfeld, A.W. 2001, Parallax, Freeman.

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solar-physics-timeline

Solar Physics Historical Timeline (2000-present)

Category

Solar Physics Historical Timeline

1223 BC - 200 BC

0 - AD 1599

1600 - 1799

1800 - 1999

2000 - present

Category

Solar Physics Historical Timeline (1800-1999)

Timeline

In this page

1800: The Sun's invisible radiation

References and further readings

1802: Black lines in sunlight

References and further readings

1817: Solar spectroscopy is born

References and further readings

1838: The solar constant

References and further readings

1843: The sunspot cycle

References and further readings

1845: The first solar photograph

References and further readings

1848: The sunspot number

References and further readings

1852: The sunspot cycle is linked to geomagnetic activity

References and further readings

1858–1859: The Sun's differential rotation

References and further readings

1859: First observation of a solar flare

References and further readings

1859: The chemical composition of the Sun

References and further readings

1860: First observations of a coronal mass ejection

References and further readings

1881: The solar constant, again

References and further readings

1908: The magnetic nature of sunspots

References and further readings

1919: The Sun's magnetic cycle

References and further readings

1931: The Coronagraph

References and further readings

Category

Solar Physics Historical Timeline (1600 - 1799)

Timeline

In this page

1609: The Sun in focus

References and further reading

1610: First telescopic observations of sunspots

References and further reading

1644: The Sun as a star

References and further reading

1645-1715: Sunspots vanish

References and further readings

1687: The mass of the Sun

References and further readings

1774-1801: The Physical nature of sunspots

References and further readings

1796: The nebular hypothesis and the Sun's origin

References and further readings

Category

Solar Physics Historical Timeline (0 - 1599)

Timeline

In this page

AD 968: The first mention of the solar corona

References and further reading

1128: The first sunspot drawing

References and further reading

1185: The first description of solar prominences

References and further reading

1543: The Sun moves to center stage

References and further reading

Category

Solar Physics Historical Timeline (1223 BC - 200 BC)

Timeline

In this page

1223 BC: The oldest eclipse record