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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
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 has 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 (orbital period squared
proportional to mean distance cubed) is first laid out.
Using his planetary model and Brahe's observations, Kepler produced
in 1627 the
Rudolphine Tables
of planetary positions. 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.
In the first decade of the seventeenth century, four
astronomers more or less 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.
Reproduction of one of Galileo's sunspot drawings. The
umbrae/penumbrae structure is clearly depicted on this
June 23 1612 drawing.
To Harriot
belongs the oldest recorded sunspot
observation, on December 8 1610, as evidenced by
entries in his notebooks,
but 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 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 marking 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, in particular 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]
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 by 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
where the Sun is but one
of many star, each of which having formed at the center
of a primeaval vortex. Descartes viewed
sunspots as floating aggregates of etheral 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
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 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.
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 (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
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
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)].
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
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.
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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