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1223 BC - The oldest eclipse record
ca. 800 BC - The first plausible recorded sunspot observation
ca. 350 BC - Sun circling under a sheltering sky
ca. 250 BC - The distance to the Sun
The
oldest eclipse record is found on a clay tablet
uncovered in the ancient city of Ugarit,
(in what is now Syria), with two plausible
dates usually cited: 3 May 1375 BC or 5 March 1223 BC, the
latter being favored by most recents authors on the topic.
It is certainly clear that
by the eight century BC, the Babylonians were keeping a systematic record
of solar eclipses, and may even have been able to predict them fairly
accurately based on numerological rules.
Total eclipses of the Sun are arguably the most impressive astronomical
phenomenon that can be observed more or less regularly with the naked eye (see slide 9 and slide 10 of the HAO slide set The Sun: A Pictorial Introduction). They occur when the Moon reaches a
point in its orbit around the Earth that lies on the line joining the Earth
and Sun. By a remarkable coincidence, the Moon's angular diameter, as seen
from the Earth, is almost identical to that of the Sun. The Sun's disk is then
completely eclipsed, and daytime darkness falls upon the Earth for a few minutes
(This physical explanation of the phenomenon was only put forth much
later, in the first century BC). Like comets, solar eclipses were taken to be astrological omens of great
significance. It is therefore not surprising that such a spectacular event is
often mentioned in surviving written records and chronicles of ancient civilizations.
References and further reading:
Fotheringham, J.K. 1933, The Story of Hi and Ho, Journal of the
British Astronomical Association, 43, 248-257.
Zirker, J.B. 1995, Total Eclipses of the Sun, Princeton University
Press.
Littman, M., Willcox, F., and Espenak, F. 2000, Totality: Eclipses
of the Sun, 2nd ed., Oxford University Press.
The two oldest record of a sunspot observation are found in the
Book of Changes, probably the oldest extant Chinese book,
compiled in China around or before 800 BC.
The text reads "A dou is seen in the Sun", and A mei is
seen in the Sun". From the context, the words (i.e., chinese characters)
"dou"
and "mei" are taken to mean darkening or obscuration.
Astronomers at the court of the Chinese and Korean emperors made
regular notes of sunspots, most less elliptical than the one cited
above. It seems, however, that observations were not carried out
systematically for their own sake, but instead took place whenever
astrological prognostication was demanded by the emperor. The surviving
sunspots records, though patchy and incomplete, covers nearly 2000 years
and represents by far the most extensive pre-telescopic sunspot record.
Sunspots are concentrations of strong magnetic fields piercing the
solar photosphere. Visually, they
look like dark blemishes on the solar disk (see
slide 1 and
slide 3 of the HAO slide set).
Most sunspots are too small to be readily visible by naked eye observations,
but some reach a size sufficient to be visible without a telescope,
under suitable viewing conditions (for example, when the sun is partially
obscured by fog or thick mist, or clouds). Because of their possible
astrological significances, reports of naked-eye sunspot
observations are indeed to be found in many ancient chronicles and
court chronologies.
References and further reading:
Mossman, J.E., 1989, A comprehensive search for sunspots without the aid
of a telescope, 1981-1982, in Quarterly J. R. Astr. Soc.,
30, 59-73.
Stephenson, F.R. 1990, Historical evidence concerning the Sun:
interpretation of sunspot records during the telescopic and pre-telescopic
eras, in Phil. Trans. R. Soc. London, A330, 499-512.
Hetherington, B. 1996, A chronicle of pre-telescopic astronomy,
John Wiley and Sons.
ca. 350 BC: Sun circling under a sheltering sky
One of the major intellectual achievement of ancient Greece is
the physical model of the cosmos developed by
Aristotle (384-322)
An essential feature is the place occupied by the Earth at the center
of the Universe, with the Sun, planets and sphere of fixed stars
revolving about that center, the Sun occupying the fourth
sphere. In this geocentric model the Earth
is absolutely fixed, with the motions of precession and daily rotation
ascribed to the two outermost spheres of the model.
The Aristotelian cosmos.
The Earth sits motionless at the center
of the universe, and the outer sphere,
the Primum Mobile, is assumed to undergo a full revolution
in 24 hours.
This basic planetary arrangement formed the basis of
mathematical model of planetary motion developed four centuries later by
Claudius Ptolemy (ca. 100-170).
In Aristotle's
scheme there exist fundamental physical differences
between the terrestrial and celestial realms, as demarcated by
the Moon's sphere. Everything under the Moon is made of the four
elements earth, water, air and fire, themselves arranged concentrically
about the center of the universe. Pure circular motion prevail
throughout the heavens, which are are made of an incorruptible
fifth element (or "quintessence").
References and further reading:
Grant, E. 1977, Physical Science in the Middle Ages,
Cambridge University Press
Crowe, M.J. 1990, Theories of the World from Antiquity to the
Copernican Revolution, Dover.
Pedersen, O. 1993, Early Physics and Astronomy, revised ed.,
Cambridge University Press.
The first mathematically-based attempt at determining the Sun-Earth
distance is due to
Aristarchus of Samos
(ca. 310-230 BC).
The procedure followed by Aristarchus is illustrated on the diagram
below; form a triangle
by connecting the Earth (E), Sun (S) and Moon (M). At the first or third Moon
quarter,
the triangle so described in a right-angle triangle (a=90°). The
angle b can be measured by an observer on Earth,
which then allows the angle c to
be computed (c=90-b when a=90°). The ratio
of the Earth-Moon segment (EM) to the Earth-Sun segment (ES) is by definition
equal to sin(c) (in modern trigonometric parlance; Aristarchus expressed
this differently).
Aristarchus' geometric construction used to estimate the distance
to the Sun. The Earth-Sun-Moon triangle and sizes are not drawn to scale.
While sound in theory, in practice
this procedure is highly inaccurate
in the Earth/Sun/Moon case; this is because EM is much smaller than ES,
implying that b is very close to 90°, so that c is
in turn very small.
This has the consequence that a small measurement error on b
translates in a large variation in the ratio EM/ES
(again in modern parlance,
a measurement error db is amplified by a factor
1/(sin c)2,
which is large when c is very small).
Aristarchus measured
b=87°, while the true value is in fact 89° 50 minutes.
This may seem a small error, but because of the large error amplification
Aristarchus' value leads to EM/ES=19, instead of the true value
EM/ES=397. Nonetheless, Aristarchus' calculation was the first to
mathematically set the spatial scale of the cosmos.
References and further reading:
Van Helden, A. 1985, Measuring the Universe, University
of Chicago Press.
Hirschfeld, A.W. 2001, Parallax, Freeman.
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