968: The first mention of the solar corona
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 solar eclipse,
the first explicit mention of what can be pretty ambiguously 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 slide set.
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
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 slide set.
Sunspot drawing in the Chronicles of John of Worcester, twelfth
century. Notice the depiction of the penumbra around each spot.
Reproduced from R.W. Southern, Medieval Humanism,
Harper & Row 1970, [Plate VII].
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
sighting exist in the historical records, starting
with Theophrastus (374-287 B.C.) in the fourth century B.C.
However, by far 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 accumulation of (relatively) cold gas held
suspended high in the solar atmosphere by the Sun's magnetic field
(see
Slide 6 and
sun_pictorialIntro/slide7.html">Slide 7
of the HAO slide set).
Large prominences are often visible at times of solar eclipses, in the
form of small reddish filaments or blobs in the lower corona.
The first fairly 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 as 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 cosmos of the late Christian medieval era was a fusion of ideas
combining the physics of
Aristotle
and 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 precession 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.
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 precession
which Copernicus thought necessary to properly reproduce ancient
observations.
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 at first 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
