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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'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.
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.
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.
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 (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.
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
Meadows, A.J. 1970, Early Solar Physics, Pergamon Press.
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.
The solar constant is a measure of the sun's luminosity, and
is defined by convention as the amount of energy incident per second on one
square meter of the outer terrestrial
atmosphere, when the Earth is at a distance
of one astronomical unit (149,598,500 km) from the Sun.
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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).
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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.
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.
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.
References and further readings:
Stix, M. 1989, The Sun, Springer.
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.
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.
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 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:
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 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.
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.
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.
Kivelson, M.G., and Russell, C.T. (eds.) 1995, Introduction to Space
Physics, Cambridge University Press, chap. 1.
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.
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.
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 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.
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
On 1 September 1859, the amateur astronomer
Richard C. Carrington
(1826-1875) 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
below).
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.
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).
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
In the late 1850s the chemist
Robert Wilhelm Bunsen (1811-1899) and theoretical physicist
Gustav Kirchhoff
(1824-1887), both at Heidelberg,
took on the issue of spectral line identification
pretty much where
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 cool gas. 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.
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).
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 (1824-1907),
Hermann Carl Vogel (1841-1907),
William Huggins (1824-1910),
Angelo Secchi (1818-1878),
Charles Young
(1834-1908), 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.
The total solar eclipse of 18 July 1860 was probably the most thoroughly
observed eclipse up to that time. The three drawings are a sample
of drawings produced at that time which 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.
Click on the above to view full size diagrams
Drawings of the 1860 eclipse by G. Tempel (top left),
von Feilitzsch (top center),
F.A. Oom (top right), E.W. Murray (bottom right), F. Galton (bottom center),
and C. von Wallenberg (bottom right).
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 geoeffective- manifestation 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.
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.
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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.
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The American scientist
Samuel Langley (1834-1906) 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.
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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).
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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
The Sun: a Pictorial Introduction). Not only was this the first
detection of a magnetic field outside the Earth, but the inferred
magnetic field strength, 3000 Gauss, turned out 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.
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A diagram taken 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),
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.
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In the decade following his groundbreaking discovery
of sunspot magnetic fields,
George Ellery Hale (1868-1938)
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 of twice
the sunspot cycle period. These empirical observations have stood the test
of time and are since known
as Hale's polarity Laws. Their physical origin is now
now known to originate
with 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.),
to this day 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.
Much of the remarkable progress made in understanding the Sun's
outer atmosphere had been made through the use of observations
carried out at times of total solar eclipses. The relative rarity of such
eclipses, the cost and logistical difficulties of travelling to often
remote location to observe then, the short duration of totality,
as well as the frustrating vaguaries 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.
Lyot's first 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).
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 other
to replicate and modify his design, the most succesful of these
followers being Max Waldmeier at the ETH/Zürich, and Donald H.
Menzel (1901-1976) at Harvard College Observatory.
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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