The Maunder Minimum - Some Questions, Some Answers
Peter O. Taylor
(For an alternate explanation, see: http://verplant.org/history-geophysics/Maunder.htm)
"For the space of three years now (the sun) has remained without spots, which at other times, were so frequently to be seen." So wrote the famous Dutch astronomer, Huygens, to his contemporary Hevelius, on 16 September 1658. Strangely, it would be late in the nineteenth century before two well-known solar astronomers, Gustav Spoerer and E.W. Maunder, would describe fully the lull in solar activity that lasted for almost seventy-years, spanning the entire reign of Louis XIV and encompassing the times of Milton and Newton.
During what we now consider to be normal circumstances, the number of sunspots regularly rises and falls in accordance with an average cycle length of approximately eleven years. During cycle "minimum" the numbers often fall to zero, while at "maximum" daily counts have soared to over three-hundred.
Although Spoerer's work (1887) preceded Maunder's, it is the latter who is generally credited with the most complete description of the strange behavior of the sun in the 17th century. In his second paper on the subject, Maunder (1922) reported the following findings: During the period from 1645 to 1715, only a few spots were seen. For nearly half of this time, 1672 to 1704, no spots were observed on the sun's Northern Hemisphere; and for sixty years, until 1705, there was never more than one group visible at any one time. Thus it appears that the total number of spots for the entire period would have been less than we have come to expect in any one active year since that time.
Did this prolonged lull actually occur, or was it a lack of sophisticated equipment, or skilled observers, that only made it seem so? Is there other evidence, from associated, or seemingly unrelated studies, that could confirm its existence? Was anyone even looking? These are some of the fundamental
questions that have been asked during the many years that have elapsed since Maunder's time.
Well-known solar physicist, John Eddy, in his analysis of the Maunder Minimum (1976), answered many of these questions. Eddy concluded that the modest equipment that is required for serious sunspot observation was readily available in the 17th century. In fact, drawings of the sun from that era show almost all of the detail that modern drawings show. And of course, the 17th century supported many accomplishments in other areas of astronomy, many by active solar investigators. For example, the first known division in Saturn's ring-system was discovered in 1675, along with five of its moons (1655 to 1684), implying a telescopic resolution of nearly one arc-second.
Other examples include Romer's discovery in 1675 of the velocity of light from observations of the orbits of Jupiter's satellites; and transits of Venus and Mercury were recorded, demonstrating a certain familiarity with sunspots and their motions.
Eddy mentions many of the noted astronomers of the day. To name but a few: Flamsteed, Hooke, Halley, Huygens, Hevelius, Romer, Cassini, Grimaldi, and so on; many of whom regularly observed the sun and recorded their observations.
Some supporting evidence for the minimum can be deduced from observations of "naked-eye" sunspot sightings. Reports of sunspots seen without visual aid can be traced back to 28 B.C. and before. These sightings were made when the sun was obscured by heavy haze or smoke, or was low in the sky during early evening or morning hours. (Note: we now realize that this practice can be highly dangerous, and must be avoided.) As naked-eye sightings are relatively rare, the likelihood of their discovery is statistically greatest during times of high sunspot activity. Although a number were observed prior to, and later than the Maunder Minimum, apparently none were recorded during the period from 1639 to 1720. However, at least two other similar "gaps" do exist in S. Kanda's compilations.
Additional evidence comes from a generally unrelated field. It concerns the abundance of carbon 14 in tree rings. For technical reasons, we would expect to find a relative abundance of the element when solar activity has been at low levels. During the period of Maunder's minimum this is exactly the case. Increasing amounts do occur that peak in the year 1690.
Further, both the naked-eye-sunspot and carbon 14 indices correlate extremely well with each other and with long-term auroral activity, a phenomenon closely associated with periods of high sunspot number, and historically viewed with wonder. Aurorae were very rare during the period; far less common than in seventy-year periods previous to, and following the Minimum.
In fact, in his extensive analysis, Eddy found virtually no evidence to dispute the minimum. Thus it appears that the sun did undergo major changes, with possible terrestrial effects, during the 17th century. We are not certain what mechanism could cause such an event to occur. Many explanations have been suggested, ranging from the interaction of secondary and tertiary (or even additional) sunspot cycles, to complex explanations of the physics of the solar dynamo itself.
Perhaps Eddy best summarizes the scenario when he concludes, "the reality of the Maunder Minimum and its implications of basic solar change may be but one more defeat in our long and losing battle to keep the sun perfect, or, if not perfect, constant, and if inconstant, regular".
Peter O. Taylor
AAVSO Solar Division Chairman