The Sun Recorded Through History, historia nauki
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2
Naked-Eye Sunspots
Humans have been looking at the Sun for millennia. Nevertheless, on most
occasions one only manages to be dazzled. But there are some situations in
which the intensity of the light of the Sun diminishes and we can observe the
solar disk with the naked eye. Some examples are the Sun observed through
fog or during a dust-storm, or even the Sun observed through the smoke
during a forest fire. On these occasions, we can see the solar disk without
being dazzled. If there was a sunspot of great size just at that moment, then
we might distinguish it inside the solar disk (see Figure 2.1).
There are several myths concerning the vision of spots or blemishes on
the Sun in antiquity. For example, a supposed Aztec pre-Columbian myth has
a god-Sun (creator of the world) with dirty marks on his face. This seems
to be slightly more convincing than some supposed drawings of sunspots in
Egyptian civilization. One can also mention a beautiful story from the area
of the Zambesi in Africa that seems to be more credible. The Moon envies
the Sun and throws mud at its face. Luckily, this does not happen very often
since the Sun is vigilant. But every ten years approximately, the Sun loses
concentration and gets dirtied by the mud (Brody, 2002).
Undoubtedly written observations are more trustworthy than myths and
orally transmitted legends.
1
A 3000-year-old Babylonian tablet might con-
tain a reference to observations of sunspots (Sayce, 1877). Surviving Late
Babylonian astronomical diaries are very detailed (Sachs and Hunger, 1988),
but a clear allusion to sunspots has not been found. It should be empha-
sized that: (1) no more than 5% of the original texts have survived and
(2) systematic observations such as eclipses or planetary movements were
recorded in abundance, but other sporadic events such as aurorae or meteors
are very scarce. One can speculate that naked-eye sunspot observations in
1
Early writing systems are dated in the late 4th millennium BC. However, they
were not a sudden invention and were based on previous traditions of symbol
systems (Houston, 2004).
J.M. Vaquero, M. Vazquez,
The Sun Recorded Through History
,
57
Astrophysics and Space Science Library 361, DOI 10.1007/978-0-387-92790-9 2,
c
Springer Science+Business Media, LLC 2009
58
2 Naked-Eye Sunspots
Fig. 2.1.
Large sunspot groups in the solar disk on 27 October 2001 (courtesy
of Catania Astrophysical Observatory). The amateur astronomer J. Ruiz saw, that
day, four naked-eye sunspots using a filter.
China go back at least to the twelfth century BC (Hsu, 1972) and, probably,
the classical Greeks also observed sunspots.
Bicknell (1968) has suggested that a naked-eye sunspot was observed by
Anaxagoras of Clazomenai (500–428 BC) in 467 BC, but it is just a suppo-
sition. Sarton (1947) noted that there are no clear references to sunspots in
classical literature. However, there are some texts that might suggest system-
atic solar observations for weather prediction. There are several references
to possible naked-eye sunspot observations in the surviving fragments of
De
Signis Tempestatum
(“On weather signs”) by Theophrastus (371–287 BC).
The work consists of several chapters describing signs of different weather
meteors. Three fragments have references to possible sunspots (Hardy, 1991):
(1) “If the Sun has a black mark when it rises, or if it rises out of clouds
it is a sign of rain” (in the section “Signs of Rain”), (2) “If the Sun rises
with a burning heat but does not shine brilliantly, it is a sign of wind. If the
Sun has a hollow appearance, it is a sign of wind or rain
...
also black spots
on the Sun or Moon indicate rain, red spots wind” (in the section “Signs of
Wind”) and (3) “If the Sun rises brilliantly but without scorching heat and
without showing any special sign on his orb, it indicates fair weather” (in the
section “Signs of Fair Weather”). We consulted the
Opera
of Theophrastus
(Theophrasti Eresii, 1866), but an English translation of
De Signis Tempes-
tatum
can be consulted (Theophrastus, 1916).
Nevertheless, Theophrastus might not be an “observer of sunspots” in the
strict sense since he only expounded a simple rule of weather prediction using
2.1 The Human Eye as a Detector of Light
59
sunspots. Later, we can find other similar texts in Greek literature. Examples
may be verses 822–824 of
Phaenomena
(Appearances) by Aratus (315–240
BC) or some verses of
Georgics
by Virgil (70–19 BC). Modern knowledge of
major maxima and minima of solar activity (Usoskin et al., 2007) can help
us. During the 4th century BC solar activity was low. Thus, it is improbable
that Theophrastus’s contemporaries saw naked-eye sunspots. However, solar
activity during the 5th century BC was very high and it is quite probable
that ancient Greeks observed sunspots (including Anaxagoras). Moreover, it
is well known that the pre-Socratics were keen observers.
This chapter is devoted to naked-eye sunspot observations, and especially
to their possible use for the reconstruction of solar activity during the last
two millennia. Firstly, we will study the human eye as a detector of light.
Secondly, we will study the criteria of visibility of sunspots. Then, we will be
ready to present the historical observations that have survived and the modern
programmes of observation carried out by amateur astronomers. Finally, we
will show the interest of this kind of observation for the reconstruction of the
history of the Sun during the last two millennia.
2.1 The Human Eye as a Detector of Light
In the context of this book, the human eye is the available detector. There-
fore it is of interest to review its main properties.
Oculus hoc est fundamentum
Opticum
by C. Scheiner (1619) is probably the first work about visual optics
and anatomy. It includes detailed observations of the pupil during accommo-
dation and of the refractive power of the lens and the aqueous and vitreous
humour (Southall, 1922). He was impressed by the analogy between the eye
and the camera obscura (Daxecker, 2004).
Sunlight enters the eye by passing through the cornea, where the image is
focused. The brightness of the image is controlled by the varying diameter of
the pupil, the aperture of the optical system. The average size of the exposed
pupil varies throughout life, but averages between 6 and 7 millimetres. The iris
regulates the amount of light passing through the pupil, acting like a camera
shutter. As the amount of light entering the eye diminishes, the iris muscle
pulls away from the centre, causing the pupil to dilate and allowing more light
to pass. The image is given a fine focus by the
lens
. Finally the image falls
on the retina, where the real power of the eye is located (Figure 2.2). At any
given instant, the retina can resolve a contrast ratio of 100:1.
The retina is a thin layer composed of five types of cells lining the back
of the eye. The cells are arranged in four layers. Using the outermost two,
the retina can turn parts of itself on and off as it analyses the image. The
photoreceptors are located behind. The ones that allow us to see colour are
the cones, with their concentration moving outwards from an area of the
retina called fovea centralis through the central part of the retina, called
the
macula
. A few degrees from the fovea appear the rods, outnumbering
60
2 Naked-Eye Sunspots
Retinal blood vessels
Retina
Cornea
Iris
Macula
Lens
Fig. 2.2.
Cross-section of the human eye.
the cones by 20 to 1, and acting as light collectors. These elements respond
to light by generating electrical impulses that travel out of the eye through
the optic nerve to the brain.
The sensitivity of the eye ranges over about 14 orders of magnitude from
a minimum threshold to a light level that could possibly cause damage. The
photopic (cone) threshold is almost four magnitudes above the minimum. The
next two magnitudes are called the mesopic range and it is here that both rods
and cones contribute to vision. The scotopic peak sensitivity (of rod cells) is at
about 500 nm, while photopic sensitivity peaks at around 550 nm (Figure 2.3).
Wavelengths shorter than 315 nm are absorbed by the cornea (causing injury)
and do not reach the retina.
As in the case of telescopes, eyes are not perfect optical systems. So the
relative intensity of the object is distributed across the retina as shown in
1
0.8
0.6
0.4
0.2
400
700
500
Wavelength (nm)
600
Fig. 2.3.
Photopic sensitivity of the human eye.
2.1 The Human Eye as a Detector of Light
61
1.0
0.8
0.6
0.4
0.2
0.0
4
3
Retinal Distance (minutes of arc)
2101234
Fig. 2.4.
The Point Spread Function of the retina of the eye.
Figure 2.4. At any given instant, the retina can resolve a contrast ratio of
100 to 1.
2.1.1 Solar Damage to the Eye
Solar retinopathy is a kind of damage to the eye’s retina, particularly to the
macula, induced by prolonged exposure to solar radiation. It usually occurs
due to staring at the Sun or viewing a solar eclipse. The main damage to the
eye is photochemical rather than thermal (Ham et al., 1976). Young people
are much more likely to suffer damage than their elders, because the eye
gradually becomes yellower with age, filtering out the harmful UV photons
(Istock, 1985).
In a letter to British philosopher John Locke (1632–1704), Isaac Newton
(1643–1727) describes the effects of looking at the reflection of the Sun in a
mirror, while standing in a darkened room.
2
Westfall (1980) comments that
“Newton left the sun alone after that”. Newton’s description of his symptoms
2
“The observation you mention [
...
] I once made upon my self with the hazzard
of my eyes. The manner was this. I looked a very little while upon ye sun in a
looking-glass with my right eye and then turned my eyes into a dark corner of
my chamber and winked to observe the impression made by the circles of colours
which encompassed it & how they decayed by degrees & at last vanished. This
I repeated a second and a third time. At the third time when the phantasm of
light & colours about it were almost vanished, intending my phansy upon them
to see their last appearance I found to my amazemt that they began to return &
by little & little to become as lively & vivid as when I had newly looked upon the
sun. But when I ceased to intende my phansy upon them they vanished again.
After this I found that as often as I went into the dark & intended my mind upon
them as when a man looks earnestly to see any thing which is dicult to be seen,
I could make the phantasm return without looking any more upon the sun. And
the oftener I made it return, the more easily I could make it return again. And
at length by repeating this without looking any more upon the sun I made such
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