Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 37, 2002)

Science Nugget: Sep 13, 2002

A Little Bit of (Naked Eye) Sun Spot History

Introduction

The week 23 Nugget described The Yohkoh Galileo Project; a project to archive all the data collected by the Yohkoh mission in an easy to access, easy to use format. The project name was inspired by the solar observations of Galileo Galilei, whose meticulous drawings are still of scientific use today. Galileo however was not the first to document sun spots. This nugget delves a little into some historical sun spot observations of interest and some of the early thoughts as to what they were.

It is only in the last 200 years that astronomers came to the conclusion that sun spots are concentrations of strong magnetic fields piercing the solar photosphere. Visually, they look like dark blemishes on the solar disk. Most sun spots 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). However, because of their possible astrological significances, reports of naked-eye sun spot observations are indeed to be found in many ancient chronicles and court chronologies.

Chinese Records

The Chinese have among the earliest records of astronomical observations, with systematic and detailed histories since the 3rd century B.C. and less well documented observations going back several thousand years. The two oldest records of a sun spot observation are found in the Book of Changes, probably the oldest extant Chinese book, compiled in China around or before 800 B.C. 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. Some sun spots records describe these darkenings as like a star, bird, melon, egg, fruit and even a man. In the legend times, the god of the sun was a black bird with tree legs on the splendid disc. The old Chinese character for "sun" is also a black point in a circle. Astronomers at the court of the Chinese emperors made regular notes of sun spots. 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.

From the year 28 B.C., comes the report "Heping reign period, 1st year, 3rd month, day Guiwei, the rising sun was yellow; a black gas was at the center of the sun, like a coin".

During the period from 28 B.C. to 1638 A.D. there are 112 descriptions of outstanding sun spots in the official Chinese histories.

Where were the Greeks?

The Greeks were renowned for their astronomical observations, but there are few recorded observations of sun spots in the west until the 1600's; although one of the earliest of all recorded observations was by Theophrastus (374-287 B.C.) in the fourth century B.C. Part of the reason for this was because of the dominating views of Aristotle (384-322 BC) concerning the incorruptibility of the heavens. Aristotelian philosophy maintained the Sun was a perfect sphere (philosophically speaking) and without blemish. This meant that sun spots were "physically impossible", so that sightings were ignored or ascribed to transit of Mercury or Venus across the solar disk. Indeed, Johannes Kepler (1571-1630) misinterpreted a spot seen on May 18, 1607 as being exactly this. Later, "the perfection of the heavens" became part of orthodox Christian theology, similar to the belief that the earth was the center of universe and all the planetary bodies revolved around it.

The Europeans Are Coming

The 12th century drawing below, from the Chronicles of John of Worcester (one of the many monks who contributed to the Worcester Chronicles), represents what may be the first surviving sun spot drawing, from a sighting on Saturday, 8 December 1128.

The accompanying text translates: "...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 fact that the Worcester monks could apparently distinguish the umbrae and penumbrae of the sun spots they observed suggests that the spots must have been exceptionally large.

It wasn't until the early 1600's that sun spot observations began in earnest, coinciding with the invention of the telescope. The first telescopic observation of the sun probably belongs to Thomas Harriot (1560-1621) on Dec 8th, 1610. He was one of four prominent observers at this time using telescopes to look at the sun. They were Johann Goldsmid (1587-1616, known as Fabricius) in Holland, Thomas Harriot in England, Galileo Galilei (1564-1642) in Italy, and the Jesuit Christoph Scheiner (1575-1650) in Germany. Fabricius is credited with associating sun spots with the axial rotation of the sun, a view which Galileo made the convincing case for. Galileo's views on sun spots contributed significantly the sequence of events that landed him in front of the Roman Inquisition in 1633, but growing animosity on the part of the Jesuits who, in particular through their chief astronomer Christopher Clavius (1538-1612), also contributed to Galileo's downfall. [Academic jealousy is nothing new!]

Fabricius' de Maculis in Sole observatis et apparente earum cum Sole conversione, Narratio - Account of Spots observed on the Sun and of their apparent rotation with the Sun. Reproduction of one of Galileo's sun spot drawings. The umbrae/penumbrae structure is clearly depicted on this June 23 1612 drawing. A picture of Christoph Scheiner. Scheiner's original opinion was that sun spots were small planets closely orbiting the Sun.

After this burst of discovery and observation, sun spot observation went dormant for many years. In a letter to Humphrey Marshall dated, London, February 14, 1773, a Mr B. Franklin comments about the sun spot observations Marshall had sent him

  ...The observations I communicated to our astronomers of the Royal Society, who are much pleased with them, and hand them from one to another; so that I have had little opportunity of examining them myself, they not yet being returned to me.
  Here are various opinions about the solar spots. Some think them vast clouds of smoke and soot arising from the consuming fuel on the surface, which at length take fire again on their edges, consuming and daily diminishing until they totally disappear. Others think them spots of the surface, in which the fire has been extinguished, and which by degrees is rekindled. It is remarkable, that, though large spots are seen gradually to become small ones, no one has observed a small spot gradually become a large one; at least I do not remember to have met with such an observation. If this be so, it should seem that they are suddenly formed of their full size; and perhaps, if there were more such constant and diligent observers as you, some might happen to be observing at the instant such a spot was formed, when the appearances might give some ground of conjecture by what means they are formed.
  The professor of astronomy at Glasgow, Dr. Wilson, has a new hypothesis. It is this; that the sun is a globe of solid matter, all combustible perhaps but whose surface only is actually on fire to a certain depth, and all below that depth unkindled, like a log of wood, whose surface to half an inch deep may be burning coal, while all within remains wood. Then he supposes, by some explosion similar to our earthquakes, the burning part may be blown away, leaving bare the unkindled part below, which then appears as a spot, and only lessened as the fluid burning matter by degrees flows in upon it on all sides, and at last covers or rekindles it.

The Wilson referred to in this letter was the Astronomer Royal, Alexander Wilson. In 1769 he discovered the "Wilson effect". Wilson had been observing a large sun spot as it approached the edge of the Sun and noticed that its appearance changed. The penumbra on the side remote from the limb appeared to contract and then disappear. When the same spot re-appeared on the other side of the Sun two weeks later the same behavior was shown on the penumbra on the opposite side of the spot. Wilson explained his observation with the hypothesis that the spots were saucer shaped depressions on the Sun's surface formed by the partial removal of luminous matter which he believed to cover the supposedly dark solid interior of the Sun.

A few years later, William Herschel (1738-1822; discoverer of the planet Uranus), following on from Wilson's hypothesis, suggested that sun spots 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). A drawing of Herschel's is shown below.

It would be at the beginning of the 19th century that the true nature of the sun and sun spots would start to be understood, through the field of spectroscopy and the work of Fraunhofer (1787-1826), Gustav Kirchhoff (1824-1887), Robert Wilhelm Bunsen (1811-1899), and Anders Jonas Ångström (1814-1874), to name but a few.

In 1857 Heinrich Schwabe was presented with the Gold Medal of the Royal Astronomical Society for his discovery of the sun spot cycle. He observed and recorded sun spots for 43 years and he found about a 10 year period in the appearance of the spots. This had not been noticed in 200 years of observing sun spots.

This work interested another man, Richard Carrington, so much that he also began to systematically observe sun spots between 1853 and 1861. He discovered that sun spots undergo a latitude drift. That is, that the average latitude of the spots decreases steadily from the beginning to the end of the solar cycle and get closer to the solar equator.

With the introduction of photographic studies of sun spots in the latter half of the 1800's, the old era of naked eye discovery on the Sun came to an end.

References:

Traditional Chinese Astronomical Records, Li CiYuan - IAU Commission 41 : Meetings : Astronomical Archives Session

Historical Sun Spot Drawing Resource Page: The Early Observers, 1128 to 1800 AD.

Great Moments of Solar Physics (1):

Sunspots

[Topical index] -o- [Chronological index]

September 13, 2002

Alisdair Davey ard@solar.physics.montana.edu


Fabricius' de Maculis in Sole observatis et apparente earum cum Sole conversione, Narratio - Account of Spots observed on the Sun and of their apparent rotation with the Sun.	Reproduction of one of Galileo's sun spot drawings. The umbrae/penumbrae structure is clearly depicted on this June 23 1612 drawing.	A picture of Christoph Scheiner. Scheiner's original opinion was that sun spots were small planets closely orbiting the Sun.