1645-1715: Sunspots vanish
Sunspots observations continued in the seventeenth century,
with the most active observers being the German
Johannes Hevelius (1611-1687)
and the French 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 is 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 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.
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.
1666: The colors of sunlight
Between 1664 and 1666, stimulated by the writings
of
René Descartes
and Robert Boyle (1627-1691),
Isaac Newton (1642-1727)
launched into a series of experiments aimed at
investigating the physical nature of "white light".
In the course
of one of these experiments, Newton let a narrow
beam of sunlight through glass prims, separating
its chromatic component into it's "rainbow" spectrum.
Use of a second inverted prism could then recombine
these colors into the original "white light".
Sketch made by
Newton in his
notebook, illustrating his
experimental setup used
to separate sunlight into its chromatic spectrum.
A narrow beam of sunlight enters a dark room through
a small hole, traverses a glass prism and its
refracted chromatic components projected onto a screen.
[Reproduced from:
P. Whitfield, Landmarks in Western Science, Routledge, 1999]
Newton explained correctly the production of his artificial rainbow by suggesting that white light is made of a variety of fundamental colors, which are refracted to varying degrees when they cross an interface between two different transparent media, here glass and air. Newton would have never guessed how his curious discovery would, some two centuries later, lead to a true revolution in physics and astronomy.
Hall, A.R., Isaac Newton, Adventurer in thought, Cambridge University Press reprint, 1996.
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 (MEarth/MSun=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
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
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 perhaps 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 opening 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).
Reproduction of one of Herschel original diagram 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 [from: Phil. Trans. 1801, vol. 91, pp. 265-318 (plate 18)].
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
By the closing decade of the eighteenth century,
the increasingly powerful reflecting telescopes built by
the German-born English astronomer
William Herschel (1738-1822)
had revealed the existence of a number of diffuse cloud-like structures,
dubbed Nebulae. Inspired by these observations, the
French astronomer and mathematician
Pierre Simon de Laplace (1749-1827)
put forth his nebular hypothesis, according to which
the sun and solar system formed from the gravitational collapse
of an initially slowly rotating, large but diffuse gas cloud.
Drawing of Nebulae by William Herschell. Herschell 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. Reproduced from W. Herschel, Philosophical Transactions of the Royal Society of London 101 (1811), 269-336 (p. 336, Plate IV).
Laplace's cosmological ideas were described in a popular work, published in 1796 and entitled Exposition du systè me du monde. 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.
Herschel, W. 1811, Astronomical Observations Relating to the Construction of the Heavens..., Philosophical Transactions of the Royal Society of London 90, 284-292Hoskin, M. (ed.) 1997, The Cambridge Illustrated History of Astronomy, Cambridge University Press, chap. 6
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. Herschell 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.
Meadows, A.J. 1970, Early Solar Physics, Pergamon Press.
While investigating the refractive properties of
various transparent substances, the English chemist and
physicist
William Hyde Wollaston (1766-1828)
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 revolutionalize Solar Physics in the second half of the
century.
Meadows, A.J. 1970, Early Solar Physics, Pergamon Press.
1800: The Sun's invisible radiation
Herschel's experimental setup for the detection of invisible
solar radiation. Sunlight passes through a prism (CD), forming
the usual rainbow spectrum (E). A row of thermometers is positioned
on a table (AB) 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.
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
1802: Black lines in sunlight
References and further readings:
Wollaston, W. H. 1802,
A Method of Examinimg Refractive and Dispersive Powers, by Prismatic Reflection
Philosophical Transactions of the Royal Society of London
92, 365-380
1817: Solar spectroscopy is born
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 (1766-1828).
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 being observed in
the laboratory in the spectrum of white light passing through heated gases.
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
[from: Denkschriften der K. Acad. der Wissenschaften zu München
1814-15, pp. 193-226]. Compare this to
Wollaston's drawing.
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 but a few, spectroscopy turned into a true science
which revolutionized not only solar physics, but also astronomy
at large. Still 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.
 |
Pouillet's pyrheliometer. Water is contained in the cylindrical container a, with the sun-facing side b painted black. The thermometer d is shielded from the Sun by the contained, 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). |
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.
Young, C.A. 1897, The Sun (revised ed.), Appleton and Co., chap. 8Hufbauer, K. 1991, Exploring the Sun, The Johns Hopkins University Press.
1843: The sunspot cycle
Early sunspots 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 and disappearance of sunspots.
In 1826, the German amateur astronomer
Samuel Heinrich Schwabe (1789-1875),
set himself about 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 upon crossing the visible solar disk during
conjunction; 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 sunspot visible
on the Sun, with a period that Schwabe originally estimated to be 10 years.
Variation in observed sunspot numbers during the time period 1800-present. 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.
Stix, M. 1989, The Sun, Springer.
1845: The first solar photograph
The first photographic technique was developed in the 1830's
by J. N. Niepce (1765-1833) and Louis Daguerre (1789-1851),
and 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 (1792-1871, son of
William Herschel), who first
coined the term "photography", as well as "positive" and "negative" images.
The first successful daguerrotype of the Sun, reproduced below,
was made on 2 April 1845 by the French
physicists Louis Fizeau (1819-1896) and Léon Foucault (1819-1868)
(the two 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.
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].
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 record of the solar disk using a solar telescope especially designed for photography began at Kew, in 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 Kalinigrad 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.
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.
As
Schwabe's
discovery of the sunspot cycle gained recognition,
the question immediately arose as to whether the cycle could
be traced farther in the past on the basis of extant sunspot
observations. In this endeavour 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:
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 used 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
as the The 1755--1766 cycle, which has has since been known
conventionally as
"Cycle 1", with all subsequent cycles numbered consecutively thereafter;
at this writing (January 2000), we are in the rising phase of cycle 23.
Hoyt, D.V. & Schatten, K.H. 1998, Group sunspot numbers:
a new solar activity indicator, Solar Physics, 181, 491-512.
Kivelson, M.G., and Russell, C.T. (eds.) 1995, Introduction to Space
Physics, Cambridge University Press, chap. 1.
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 (see diagram below).
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
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 mean of measuring the
surface differential rotation,
one moreover that is not restricted to latitudes
where sunspots are present:
measurement of the wavelength shift of spectral lines between the approaching
receding solar limb,
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
(1834-1908). 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 determinations
at latitudes about twice higher than the sunspot belts, demonstrating
that the Sun's polar regions rotate about 30% more slowly than
its equator.
Interestingly,
Christoph Scheiner had already noted
in his 1630
Rosa Ursina
that the rotation period inferred from tacking 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
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 absurb".
For this reason, most historians of science
continue to attribute the discovery
of solar differential rotation to Carrington and Spörer.
Eddy, J.A., Gilman, P.A., and Trotter, D.E. 1977, Science,
198, 824-829
Further General Readings
on the history of solar physics and astronomy.
Other Web Sites
with material on the history of solar physics.
-Last revised 15 December 2000 by
paulchar@ucar.edu.
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)
References and further readings:
Hoyt, D.V. & Schatten, K.H. 1997, The Role of the Sun in
Climate Change, Oxford University Press.
1852: The sunspot cycle is linked to geomagnetic activity
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
(1816-1893) 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.
The correlation between sunspot number and geomagnetic
activity index. Diagram reproduced from A.C. Young's
The Sun (revised edition, 1897).
References and further readings:
Hoyt, D.V. & Schatten, K.H. 1997, The Role of the Sun in
Climate Change, Oxford University Press.
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.
References and further readings:
Mitchell, W.M. 1916, The History of the Discovery of Solar Spots,
Popular Astronomy, 24, 22-ff.
-Written by paulchar@ucar.edu.
