As I continue my discussion of Landscheidt’s paper, Swinging Sun, 79-Year Cycle, and Climate Change, please refer here for my first three parts (or click the “Landscheidt” category tab on the left of the page).
We are now actually getting to the Introduction of the paper. But first, let me refer back to a portion of the abstract: Rare activity-deficient periods like the Maunder Minimum, which according to Eddy et al. are related to changes in the Earth’s climate…
He tosses this guy Eddy’s name out there, and then in the first sentence of the introduction does it again: Eddy (1976, 1977) has focused attention on periods of exceptionally weak solar activity like the Maunder Minimum (1645 to 1715) and the Spoerer Minimum (1460 to 1550). These grand minima, confirmed by Carbon-14 data, seem to be related to long-term changes in world climate. Their influence on solar-terrestrial phenomena is obvious. In addition, they furnish new evidence of long-duration variations in solar activity.
Well, I figured a good place to go from here would be to read the papers by John A. Eddy. So, I tracked down the 1976 issue of Science magazine in which he published “The Maunder Minimum.” (18 June 1976, Volume 192, Number 4245. Pages 1189-1202 of the bound periodicals.) I have also tracked down the article referenced from 1977, but I will get to that one later.
What follows is my summary of Eddy’s paper, which hopefully serves as a good backdrop for our Landscheidt discussion, and provides a better understanding of what this Maunder Minimum period is all about.
In the 1890s, Gustav Spoerer and E.W. Maunder examined evidence that helped them conclude that from 1645 – 1715, almost no sunspots occurred. Eddy’s article explains what they looked at, and then he delves into further evidences to support their conclusion.
In an 11-year cycle that waxes/wanes, spots on the sun are counted and recorded daily. Astronomers use annual means to smooth out short-term variations and average out variations caused by the sun’s rotation. A typical number of sunspots in a minimum year = 6, but there may be days/weeks with zero spots. A monthly mean of zero is uncommon, and only in 1810 has there been an annual mean, to two-digit accuracy, of zero. On the flip side, there may be hundreds of spots present during maximum, and throughout the maximum period several will be seen daily.
History of Records
Sunspot numbers are readily available from 1700, but not all counts are of the same quality. Sunspots seen with the naked eye are noted as early as the 4th Century B.C. With the invention of the telescope in 1610, they could be seen well enough to consider counting. It wasn’t until Heinrich Schwabe alluded to the possibility of an 11-year cycle in 1843 that the presence of a solar cycle was noted. Rudolf Wolf, director of the Observatory at Bern, studied and verified the cycle. In 1848, Wolf organized a number of observatories to record sunspots on a regular basis. He also went through and reconstructed old numbers from literature and observatory archives and found enough information to reconstruct daily drawings back to the year 1818. He was further able to reconstruct “monthly averages” to 1749, and “annual averages” to 1700. Eddy grades the reliability of these periods as: (a) 1848 – current => reliable; (b) 1818 – 1847 => good; (c) 1749 – 1817 => questionable; (d) 1700 – 1748 => poor.
It is interesting to Eddy that Wolf stopped his exercise at 1700. He suggests the possibility that Wolf ran into strange results at that point (no recorded sunspot activity), and simply attributed it to poor or lacking information. Wolf perhaps felt vindicated that the cycle had been proven back to that point and didn’t consider the possibility of an actual change in the activity of the sun. But should we assume the solar cycle perpetuates unperturbed?
Prolonged Sunspot Minimum
In the period from 1887-89, Spoerer published papers suggesting an extraordinary interruption – a 70 year period of little sunspot activity. After his death, E.W. Maunder picked up the mantle and presented his findings initially in 1894 to little fanfare. Eddy also notes a paper by Agnes Clerke claiming a marked dearth of aurorae during that period. But it was Maunder who pressed on and finally suggested that the reason it took so long after the telescope was invented in 1610 to recognize the 11-year solar cycle pattern was due in part during the early years to a practical cessation of solar activity.
The bullet-point conclusions that Maunder/Spoerer made were:
- Practically no sunspots were recorded as being witnessed from 1645 – 1715
- Not a single sunspot in the sun’s northern hemisphere was noted from 1672 – 1704
- There was never more than one sunspot group seen at a time from 1645 – 1705
- From 1645 – 1715, the total handful of sunspots seen were mostly single spots at low solar latitudes, lasting a single rotation or less. The entire period’s count was less than what we see in a single active year under normal condition.
Support for these conclusions are circumstantial (reviews of scientific journal quotes, for example). Eddy expands the evidence to further support the reality of the Maunder Minimum.
Is our “current sun” really the norm? Is the 11-year cycle a regular and persistent phenomenon? How reliable were the instruments used during that time? Was anyone even interested in observing the sun?
Eddy thinks not. And this also undermines the argument for planetary gravitational tides driving sunspot activity, and lends support to dynamo theory (which Landscheidt supports, but then makes the case that the dynamic activity is influenced by planetary mass in relation to the sun).
Solar Observations in the 17th Century
By 1612, telescopes distinguished umbrae (dark central regions of a sunspot) from penumbrae (grayish outer part of a sunspot). By 1625, solar faculae (bright patches or veiny areas near sunspots) were discovered. During the Maunder Minimum, Greenwich and Paris founded observatories. There was more than an ability to sketch the sun through projection of the solar image on a screen and record specific details. Basically, Eddy’s point is that there is no reason to believe that instruments of the time were incapable of seeing even small sunspots, and he utterly rejects that argument (suggested by William Herschel in 1801).
Eddy also believes there was sufficient interest in observing the sun, and that there would have been a fairly consistent viewing of the sun. He lists many notable astronomers of the time, as well as their scientific articles on the sun and its spots. A quote from William Derham in 1711 suggests that the sun, in fact, was very consistently scrutinized precisely because of its lack of activity. Finding a spot was a big deal and worthy of publication. Scientists Scheiner and Hevelius, at least for a time, were known to make daily drawings of the sun and its sunspots.
Aurora Borealis/Australis activity is correlated with sunspot activity because of the interaction of charged particles emitted from solar flares with the earth’s magnetic field. These are easily seen, require no telescope, and are visible for hours over large geographical areas. An indicator of a lack of sunspot activity, then, is a sparsity of aurorae. Eddy first rules out the possibility of perpetual overcast skies for the 70 year period in quesiton.
The results are consistent with reduced sunspot activity. Fewer aurorae were recorded during the 70-year period of the Maunder Minimum than in the 70 years immediately preceding it, and the 70 years succeeding. In England/France/Germany/Denmark/Poland, where observations were usually made, we would normally expect anywhere from 300 to 1000 occurrences in a 70 year period in that region alone. From 1645 – 1715, only 77 aurora occurrences were recorded in the entire world. There is then a distinct “turning on” the aurora at the end of this period, not a gradual increase that would otherwise suggest a learning curve or recording curve.
Sunspots seen with the naked Eye
While not a reliable count, such spots suggest high activity. Large sunspots or groups can be seen at sunrise/sunset, or through haze or smoke. The best records are kept in the Orient, compiled by Sigeru Kanda of the Tokyo Astronomical Observatory in 1933. There were 143 naked eye sightings dating back to 28 B.C. through 1743, though most are after the 3rd century. There is about one sighting per decade, distributed regularly for a time. But there are periods where no such spots are recorded: 579 – 808; 1403 – 1519 then one sighting, then no more sightings from 1520 to 1604. Then, during the Munder Minimum.
Carbon-14 and the History of the Sun
C-14 is an isotope that disintegrates in a known period of time. By knowing the quantity at hand and the time period from when it originated, one can determine the initial concentration. How does this tie into the sun? Well, C-14 is formed in our atmosphere through interaction with cosmic rays. More rays, more C-14. Sunspot activity impedes these rays, so the more active the sun is, the fewer rays that reach the earth, and the less C-14 is produced. A quiet sun – no sunspots – provides a clear path for these rays, and initiates a higher C-14 level (not to be confused with Carbon Dioxide). Studies show that there was a prolonged increase in C-14 levels between 1650 – 1700, coincident with the Maunder Minimum. It also shows a period from 1460-1550 with high levels. This also corresponds with the naked eye observations and other studies, Eddy labeled this period the Spoerer Minimum. Likewise, we see clustering of naked eye spots at times where C-14 measures indicate greater activity.
Carbon-14, though, also varies interestingly over a 9000 year period based on a sinusoidal curve represented by earth’s magnetic field. The long-term C-14 levels appear driven by this curve, while shorter-term fluctuations about this curve are the result of solar activity. This long-term fluctuation from trough to peak is about 100 ppm. The earth’s magnetism peaked around 100 AD, where we expect (and see) a minimum of C-14 due to greater shielding of the earth against cosmic rays.
We see anywhere from a 10 – 50 year lag from solar changes to an impact on C-14 readings. A 10 ppm deviation caused by the sun – apart from other influences such as the magnetism of the earth – indicates entry into a Grand Minimum or Grand Maximum period. In recent times, we see a large negative deviation (25 ppm – indicative of a Grand Maximum) which indicates anomalously high solar activity. There is, however, a potential impact from fossil fuel combustion that may be introducing different carbon isotopes into the atmosphere. This could reduce the ratio of C-14, meaning the full level of the deviation isn’t attributed to solar activity. To the extent that it is due to the sun, it is an indicator that recent solar activity is anomalously high. I haven’t done further study on this point.
Absence of Corona at Eclipse
The shape of the corona seen at full eclipse varies with solar activity. With high activity, there are long, tapered streamers that extend outward. As activity wanes, these subside. At minimum, they are absent, except for horizontal ones at the solar equator. When absent, all we’d see at eclipse would be the dimmer, false corona caused by scattered light from dust and space materials between the moon and sun.
Accounts of total solar eclipses during the Spoerer and Maunder Minimums are consistent with an absent structured corona. While possible that observers were looking for and worried about other measurements, certainly not everyone would miss this show and it would be almost certain to be documented in some manner, especially considering we do see descriptions of the glow around the moon that perfectly describes the expected false corona. Contrast this to an eclipse in 1715, where sunspots numbered 26 and rising. A solar corona is well described.
Eddy does note, however, that it is somewhat unusual that this striking display isn’t well-documented in any previous era, including prior to the Little Ice Age. He openly questions whether or not there was some change in the sun’s activity after emerging from the Maunder Minimum, which is still an unanswered question.
Eddy observes that a 12th/13th Century Maximum and 16th/17th Century Minimum may be extrema of a larger cycle, which could be giving rise to a 22nd/23rd Century Maximum. I admit to being confused by the math. It would seem that this would indicate a 20th/21st Century Maximum, which actually seems to have been the case. He does say that solar activity has steadily increased since the end of the Maunder Minimum (which could be the driver in global warming?) The solar constant (total radiative output of the sun) also increased at a rate of 0.5% per Century during the 1900s, which is also expected to drive warming.
I will read Eddy’s next paper, and add anything new in my next post on this topic.