I thought that it might be interesting to trace the roots of modern solar astronomy and revisit some of the milestones of discovery from the start of recorded history, through the advent of the telescope age over four hundred years ago and the advances facilitated by the space age.
Prior to the invention and the first astronomical usage of the telescope in 1610, the only means of investigating the Sun and solar features, were made using naked eye observation and the subsequent recording of events such as solar eclipses, sunspots and the changes in the seasons. There have been lots of theories and hypotheses proposed over the years with some later proving to be way off the mark and so I have only concentrated on the theories and discoveries [in date order] that have served to make significant advances in our knowledge and understanding of the Sun, solar astronomy and astronomy in general.
With the advances in high resolution scanning and creation of the numerous digital libraries it is amazing as to how many of the monumental and priceless published works can now be read straight from the computer screen. I have compiled a digital library that contains over 150 books and substantive papers since the start of the seventeenth century and they can be viewed on this page on my website. This remains a dynamic document that grows as I discover more of these priceless works. http://thesolarexplorer.net/index.php?option=com_content&view=article&id=4&Itemid=8
Here are the significant events in date order:
The oldest eclipse records are from a clay tablet found in Syria with two dates stated at 3 May 1375 B.C. and 5 March 1223 B.C. and by the eighth century BC the Babylonians were keeping systematic records of solar eclipses. The oldest records of sunspots are from the Chinese Book of Changes compiled around 800 B.C.
The first mathematical attempt to determine the Sun-Earth distance was made by Aristachus of Somos around 300 B.C., he used the first quarter moon and geometry and estimated that the Sun was about 20 Earth Moon distances from the Earth (about 5 million miles).
Around 200 B.C., Eratosthenes used data acquired during lunar eclipses, attempted to measure the Earth-Sun distance and came up with a value of 804,000,000 stadia (about 83 million miles).
The first recorded mention of the solar corona was recorded by Leo Diaconus a Byzantine historian from the solar eclipse of 22 December 968 A.D.
John Worcester from a sighting on 8 December 1128 made what is possibly the first surviving sunspot drawing. He was a monk and his record forms a part of the Worcester Chronicles.
The Russian Chronicle of Novograd records for the 11 May 1185 eclipse “The Sun became similar in appearance to the moon and from its horns came out somewhat like live embers” – the first description of solar prominences?
In 1543 Nicholas Copernicus presented the new planetary model placing the Sun at the centre of the solar system with all planets including the Earth orbiting it together with axial rotation and precession of its spin axis.
The first telescope observations were made in 1610 and four astronomers almost simultaneously observed the Sun and recorded sunspots. Thomas Harriot the English astronomer made the first record on 8 December 1610 but did not publish his results, Johann Goldsmid [Fabricus] published his results in 1611 and interpreted sunspot movement as evidence of solar axial rotation. Christopher Scheiner published 3 letters in 1612 to infer physical properties of the Sun followed by Galileio Galilei in 1613 in his letters on sunspots. The drawings produced by Galileio were so accurate and all taken at about the same time of the day and as such it has since been possible to make an animated moving sequence of them showing solar rotation dating back to 1611. This work is part of the Galileio project that can be viewed online.
Christopher Scheiner continued his investigations and published his massive work Rosa Ursina in 1630 using his very accurate observations to infer that the Suns axis of rotation was inclined to the elliptical plane [that of the Earth’s orbit around the Sun].
On 7 November 1631 Pierre Gassendi made the first observations of Mercury transiting the face of the Sun.
Jeremy Horrocks in 1639 made the first observations and record of the transit of Venus across the face of the Sun.
René Descartes tabled a model in his book Principia Philosophiae published in 1644 that the Sun was just one of many stars. This was soon followed by the period 1645 to 1715 during which despite diligent observations by the newly opened observatories in Paris [1671] and Greenwich, London [1675], very few sunspots were seen during this period that is now referred to as the Maunder minimum. This was coincidental with far reduced sightings of aurora and indicates a general lowering of solar activity during this period.
The Royal Society, a fellowship of the world’s most eminent scientists was founded in 1660 and is now the oldest scientific academy in continuous existence. It was responsible for instigating many of the foundations of modern science. Its journal Philosophical Transactions commenced publication in 1665 and included many of the astronomical discoveries after.
In 1687 Isaac Newton presented a calculation as to the mass of the Sun in his Principia Mathematica. Newton underestimated the Sun-Earth distance because of a Parallax error and hence underestimated the Sun-to-Earth mass ratio by more than a factor of ten. Newton later used improved estimates of the solar parallax and brought his estimate to within a factor of two of the modern value in his second edition in 1713.
An observation report by Stephen Gray described seeing “a flash of lightning” near a sunspot on the 27 December 1705. This significance of this sighting was not realised until after 1859 with two independent sightings of the first solar flare. Gray had probably seen the same thing but it was not recognised as such at the time.
In 1762 Peter Daval made calculations from the parallax measurements of the Sun taken by Mr Short to calculate the Sun-Earth distance at 94,380,685 English miles.
By 1796 and after numerous nebular observations by William Herschel the French mathematician and astronomer Pierre Simon de Laplace put forward his nebular hypothesis that the Sun and the solar system formed from “the gravitational collapse of an initially slowly rotating large but diffuse gas cloud.”
In 1800 William Herschel extended Isaac Newton’s glass prism experiment and proved the existence of rays beyond the red end of the visible spectrum he used a thermometer place beyond the red end of the visible spectrum to detect temperature rise there by proving the existence of infrared radiation. A year later Johann Wilhelm Ritter placed paper soaked in silver chloride beyond the violet end that blackened proving the existence of ultraviolet radiation.
In 1802 William Hyde Wollaston noticed dark lines in the spectrum of the Sun as viewed through a glass prism while he was investigating the refractive properties of various transparent substances. He suggested that these lines marked the boundaries of natural colours but pursued them no further. This marked the first step towards solar spectroscopy that would later revolutionise Solar Physics.
In 1817 Joseph von Fraunhofer independently rediscovered the dark lines in the solar spectrum and saw the possibility to use them as wavelength standards used to determine the index of refraction for optical glass while other physicists realised that these Fraunhofer lines could be used to infer the properties of the solar atmosphere as similar lines were being observed in the laboratory in the spectrum of white light passing through heated gases. Spectroscopy soon turned into a true science which revolutionised astronomy.
The solar constant a measure of the Sun’s luminosity defined by conversion as the amount of energy incident per second one square meter of the outer terrestrial atmosphere, when the Earth is at a distance of one astronomical unit [149,598,500km] John Herschel in 1838 was one of three scientists that used the energy input rate from sunlight to heat water and the inferred value of the solar constant was about half that of the accepted modern value 1367 ± 4 Watts per square meter as they failed to account for the absorption of the Earth’s atmosphere.
Early observers of sunspots had noticed that these spots rarely appeared outside of the longitude band of about ± 30° of the solar equator. In 1826 Samual Heinrich Schwabe commenced making daily observations of sunspots [weather permitting] while he was attempting to find new planets inside the orbit of Mercury. In 1843 he had found no planets but discovered that the solar cycle increased and decreased over time with a period that he estimated to be about 10 years – he had discovered the solar cycle.
The first photographic technique was developed in the 1830’s by J. N. Niepce and Louis Daguerre, and it relied on the exposure of a thin iodine layer deposited on a silver substrate and this was subsequently fixed in a mercury bath. This imaging technique was very soon applied to astronomy, by the French astronomer Francois Arago, and the British astronomer John Herschel who first coined the term “photography”, as well as “positive” and “negative” images. The first successful photograph of the Sun was made on 2 April 1845 by the French physicists Louis Fizeau and Léon Foucault, they were 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 and limb darkening.
As Schwabe’s discovery of the sunspot cycle gained recognition, investigations were made as to whether the cycle could be traced farther in the past based on the extant sunspot observations. Rudolf Wolf took on this task by 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. Wolf succeeded in 1848 in reliably reconstructing the variations in sunspot number as far as the 1755–1766 cycle, now known as “Cycle 1”, with all subsequent cycles numbered consecutively thereafter.
On the 28 July 1851 Berkowski made the first photograph of a solar eclipse at the Royal Observatory in Königsberg, Prussia.
In 1852 Edward Sabine announced that the sunspot cycle period was “absolutely identical” to that of the geomagnetic activity data being accumulated since the mid-1830’s marking the beginning of solar-terrestrial interaction studies.
In 1858 Richard C Carrington and shortly afterwards Gustav Sporer independently made two key discoveries, the first was that the latitude of sunspots mainly are seen to decrease systematically from about 40° to 5° latitude as the sunspot cycle proceeds from one minimum to the next. The second was that the sunspots at higher latitudes travel around the Sun more slowly and so Carrington concluded that there was differential rotation on the Sun. This provided further argument of the gaseous or fluid nature of the Sun’s outer layers.
Richard C Carrington was carrying out his daily monitoring of sunspots on the 1 September 1859 when he noticed two rapidly brightening patches of light near the middle of a sunspot group that he was studying. In the following minutes these patches dimmed as they moved across the active region. This event was also independently witnessed by R Hodgson. This was the first clear description of a solar flare. His monumental book on his sunspot observations was published in 1863.
In the late 1850s Robert W Bunsen and Gustav Kirchoff expanded on Fraunhofers spectral lines and compared them to laboratory observations of the spectrums in heated pure gases. They were able to demonstrate in 1859 that the Sun contained a large number of chemical elements that were mostly metals.
The total solar eclipse of 18 July 1860 was the most thoroughly observed up to that time was mainly recorded by drawings later evaluation of a peculiar feature on the SW limb when compared to modern coronographs reveal what is likely the first record of a coronal mass ejection in progress!
Hydrogen was recognised spectroscopically by A Ångstrӧm in 1862.
In 1868 J. Norman Lockyer and Jules Janssen had the same idea to use a spectroscope to observe the Sun through a very restricted wavelength centred on the red part of the visible spectrum. They were able to observe prominences for the first time in broad daylight and dispel the idea that these features were part of the Moon’s atmosphere. Lockyer had seven prisms mounted on a wheel behind his eyepiece to achieve the required band width for these observations. This was equivalent to modern hydrogen alpha solar telescopes.
During one solar eclipse they investigated a prominence spectrum and noted an emission line in the yellow part of the spectrum that did not correspond to any known chemical element, they continued investigations with the new observing method and Lockyer suggested that this line was due to an element thus far unknown on Earth that he subsequently named Helium [from the sun god Helios in ancient Greek mythology]. Later in 1895 William Ramsey was able to isolate Helium in a laboratory.
The new method of observing the Sun’s outer atmosphere led to rapid advances in solar physics and Lockyer and Janssen received a commemorative medal bearing their images from the Academie des Sciences de Paris.
In 1869 Charles A. Young and William Harkness independently noticed a feint emission line in the green part of an otherwise featureless coronal spectrum. They ascribed it in 1876 to a mysterious chemical element “Coronium”. This mystery would last for the next 66 years. Young was the first to photograph a solar prominence in 1870.
By the 1860’s through the spectroscopic work of Kirchoff and others they had offered strong evidence that the solar atmosphere was in a gaseous form and hot.
Jonathan H. Lane published a paper in the American Journal of Science and Arts in 1870 that presented the first mathematical model of the solar interior. He assumed that the Sun’s interior was gaseous and chemically homogeneous throughout, that is was in a state of hydrostatic equilibrium with inward gravitational pull being balanced by an outward gradient of gas pressure. He also assumed thermal equilibrium enforced by convection motions leading to stratification and described a rise in density and temperature when moving from the surface toward the sun’s centre. He used the Sun’s mass and radius to get the density profile of the surface and the perfect gas law to estimate the sun’s outer temperature but his results were five times too high by modern standards inferred through spectroscopic means.
By 1881 it had become increasingly clear that the earth’s atmosphere was absorbing a significant proportion of the sun’s luminosity. Later attempts to determine the sun’s luminosity were mover to the highest practical altitudes. Samuel Langley led an expedition to Mount Whitney California and calculated a value for the solar constant of 2903 Watt per square metre and twice that of modern calculated values. Later his assistant Charles Abbot obtained 1465 W/m² using the same original data.
The Fabry-Perot interferometer consists of two parallel flat semi-transparent mirrors separated by a fixed distance. This arrangement is called an etalon, was designed by Charles Fabry and Albert Perot in 1897.
In 1899 the Kirstian Birkland set out to study the Aurorae Borealis and concluded that they are caused by the episodic arrival of beams of charged particles from the Sun that were deflected and guided to high geographic latitudes by the Earth’s magnetic field. He later went on to produce artificial Aurorae in his laboratory.
Walter and Annie Maunder were plotting the latitudes of sunspots and in 1904 they produced the first butterfly diagram. Over each successive solar rotation they used a vertical line segment spanning the range where sunspots were observed and these were drawn on a time-latitude diagram. This diagram clearly shows that the sunspots for a new cycle start at higher latitudes and that they drift toward the sun’s equator as the cycle proceeds. These butterfly diagrams are still used today to compare the different solar cycles.
It was believed since Kirchoff that the solar atmosphere was made of a layer of cool gas, illuminated from below by the hotter solar interior. In the second part to the nineteenth century yielded results incompatible with a simple inert and absorbing atmosphere. Progress by Arthur Schuster who in a series of papers written between 1903 and 1905 investigated the passage of radiation through an atmosphere that can not only absorb, but also emit and scatter and re-emit the light traversing it.
A full model of the solar atmosphere based on radiative equilibrium was developed by Karl Schwarzchild in his 1906 paper showed that the observed limb solar darkening profile pointed to a state of radiative equilibrium, rather than the adiabatic stratification expected to result from convective equilibrium. Schwarzschild’s paper opened the door to the physical interpretation of stellar spectra, and more generally to the construction of realistic structural and evolutionary models of the sun and stars.
By 1908 the magnetic nature of sunspots was confirmed by George Ellery Hale and collaborators by measuring Zeeman splitting in magnetically sensitive lines in the spectra of sunspots and the detection of polarization of the split spectral components. He had proved that sunspots are the seats of strong magnetic fields. This was not only the first detection of a magnetic field outside the Earth but the sunspots had magnetic strengths over a thousand times greater than that of the Earth’s own magnetic field. These discoveries lead to the discovery of lower temperatures within the sunspots when compared to the temperature of the photosphere.
By 1910 Enjar Herzsprung and Henry Norris Russell had produced a scatter graph of the stars showing the relationship between the stars’ absolute magnitudes or luminosities verses their spectral type or classification and effective temperatures. They plotted each star on a graph and this represents a major step towards the understanding of stellar evolution or the lives of stars.
In 1910 British astrophysicist Arthur Eddington suggested the existence of the solar wind, without naming it, in a footnote to his article on Comet Morehouse he postulated that the ejected material consisted of electrons while in his study of this comet he supposed them to be ions.
In 1919, Frederick Lindemann also suggested that particles of both polarities, protons as well as electrons, come from the Sun. Eugene Parker realised that the heat flowing from the Sun in Chapman’s model and the comet tail blowing away from the Sun in Biermann’s hypothesis had to be the result of the same phenomenon, which he termed the “solar wind”.
Hale in 1919 went on to show that large sunspots pairs almost always show the same magnetic polarity pattern in each solar hemisphere, show opposite polarity patterns between the North and South solar hemispheres, and these polarity patterns are reversed from one sunspot cycle to the next, indicating that the physical magnetic cycle has a period of twice the sunspot cycle period.
The theory of the solar internal structure was further developed by Arthur Stanley Eddington between 1916 and the publication of his book “The Internal Constitution of the Stars” in 1926. The mass-luminosity relation predicted by Eddington’s stellar structure was compared to the then available data for various types of stars and showed good agreement and provided strong empirical evidence to support his theory. One vital piece was still missing the internal source for the Sun’s energy.
Albercht Unsöld established in 1928 the preponderance of Hydrogen in the Sun on the basis of a few spectral lines. Henry Norris Russell concluded in 1929 that Hydrogen was the dominant constituent in the solar atmosphere followed by Helium with metals present in very small quantities.
Much of the progress made in understanding the Sun’s outer atmosphere had been made diring the brief times of total solar eclipses. In 1931 Bernard Lyot designed and used an instrument that is now known as a coronograph. This was a telescope equipped with an occulting disk designed to block out the solar disk. Lyot managed to take the first full daylight photographs of the solar corona.
In the mid 1930’s spectroscopic coronal observations by Lyot revealed feint coronal emission lines with much broader wavelengths than expected. He assumed this broadening was the result of a thermal nature and inferred coronal temperatures of around 600,000 K. This was met with due caution at the time.
By 1941 acceptance of the very high coronal temperatures was confirmed by the spectroscopic work of Walter Grotrian and Bengt Edlén. The coronal green lines tentatively named by Young in 1869 as “coronium” were associated with high ionization stages of Iron (Fe) and Nickle (Ni). This required coronal temperatures initially estimated at over 250,000 K that soon rapidly rose to 1-2 million K.
In 1929 – Robert d’Escourt Atkinson and Fritz Houtermans used the measured masses of low-mass elements and applied Einstein’s discovery [1905] that E=mc2 to predict that large amounts of energy could be released by fusing small nuclei together.
Hans Bethe’s work in 1939 showed how nuclear fusion powers the stars – the source of the Sun’s energy was finally proven. Bethe won the 1967 Nobel Prize for physics for this work.
James Stanley Hey laid the basis for the development of radio astronomy while working on radar technology for astronomical research. In 1942 he discovered that the Sun radiates radio waves and also localized for the first time an extragalactic radio source in the constellation Cygnus.
In 1942 Hannes Alfvén suggests the existence of electromagnetic-hydromagnetic waves in a paper published in Nature. Alfvén waves in plasma are a low-frequency travelling oscillation of the ions and the Sun’s magnetic field.
Herbert Friedman an American pioneer in the application of sounding rockets (an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight) to solar physics and was the first to detect solar X-rays in 1949.
Horace W. Babcock invented and built a number of astronomical instruments, and in 1953 was the first to propose the idea of adaptive optics. He specialized in spectroscopy and the study of magnetic fields of stars. He proposed the Babcock Model, a theory for the magnetism of sunspots and in 1961 he proposed the magnetic cooling of sunspots theory.
In January 1959, the Soviet satellite Luna 1 first directly observed the solar wind and measured its strength.
Gail Moreton was using time lapse photography at the Lockheed Solar Observatory when he spotted the chromospheric signature of a large-scale coronal shock wave in 1959. These shockwaves now bear his surname.
In 1960 Robert Leighton, Robert Noyes and George Simon discover five-minute oscillations by observing the Doppler shifts of dark lines and they published in 1962. In 1970 Roger K. Ulrich, John Leibacher and Robert F. Stein deduce from theoretical solar models that the interior of the Sun could act as resonant acoustic activity. The solar oscillations can be observed on the surface of the Sun and can now be used to make precise measurements of the characteristics of the interior of the Sun. These two factors represent the birth of Helioseismology.
R Tousey made the first detection of a CME on 14 December 1971, using the Seventh Orbiting Solar Observatory (OSO-7). Initially it was thought that the camera may have failed but the next image showed that the brighter area had moved away from the Sun.
Ken Huggett, founded Solarscope Ltd on the Isle of Man in 1973 his company uses Laser optics, and specifically for the manufacture of high quality planar air-spaced, confocal, solid and tuneable Fabry-Perot etalon instrumentation. :
Skylab was launched on 14 May 1973 it was the U.S.’s first space station launched and operated by NASA it orbited the Earth from 1973 to 1979. Numerous scientific experiments were conducted aboard Skylab during its operational life, and crews were using an X-ray telescope and were able to confirm the existence of coronal holes on the Sun [areas where the Sun’s corona – its outer atmosphere is darker, and colder, and has lower-density plasma than average].
Del Woods founded the DayStar Filter Company in February 1975. DayStar developed several series of specialized filters for visual and imaging applications that became included in most professional solar observatories and those of amateurs.
The first accurate measurement of the period of horizontal wavelength of the five-minute solar oscillations was made by Franz-Ludwig Deubner in 1975.
The Solar Maximum Mission satellite (SMM) was designed to investigate solar phenomena and in particularly solar flares. It was launched on February 14, 1980 and it was notable in that its useful life compared with other similar spacecraft. It was intercepted and maintained on the Space Shuttle Challanger in 1984, and in the shuttle’s payload bay the satellite received maintenance and repairs. The Solar Maximum Mission ended on December 2, 1989, when the spacecraft re-entered the Earth’s atmosphere and burned up.
The term heliophysics was first coined in 1981 to denote the physics of the entire Sun: from centre to corona.
In 1981 NASA retrieves data from 1978 that shows a comet diving into the Sun.
In 1990, the Ulysses probe was launched to study the solar wind from high solar latitudes. All prior observations had been made at or near the Solar System’s ecliptic plane.
The Solar and Heliospheric Observatory (SOHO) was launched on December 2, 1995 to study the Sun with its 10 instruments and it has discovered over 2400 comets to date. It began normal operations in May 1996. This joint project between the European Space Agency (ESA) and NASA was originally planned as a two-year mission, SOHO currently continues to operate after over seventeen years in space and in November 2012, a mission extension lasting until December 2014 was approved.