The Maunder minimum is the name given to a period of extreme solar inactivity that occurred between 1645 and 1710. Of particular interest is that this period of inactivity corresponds closely to one of the coldest periods of the so-called Little Ice Age in Europe, a time of long, cold winters that caused severe hardships in the pre-industrial revolution world. During a thirty-year period during this time, scientists only observed about 50 sunspots, instead of an average number of about 45,000 spots. This inactivity has led scientists to extensively study the possible influences of solar activity on terrestrial climate, as well as examine other stars for evidence of activity cycle behavior similar to the sun’s behavior.
Some of the first telescopic observations were made by Italian astronomer and physicist Galileo Galilei (1564–1642) in 1611. He noted the presence of dark blemishes on the sun’s surface; these were the now well-known sunspots. (Several of Galileo’s contemporaries saw sunspots as well, but Galileo is the most famous and usually gets the credit for discovering sunspots.) Today scientists know that the number of sunspots rises and falls in a roughly 11-year cycle; this is one of the most obvious manifestations of the solar activity cycle.
Although sunspots were observed telescopically in 1611, it was not until 1843 that German astronomer Samuel Heinrich Schwabe (1789–1875) noticed a periodic rise and fall in their numbers. That it took over 200 years for astronomers to notice something so seemingly obvious is some cause for wonder, but it may be partly explained by the nearly complete absence of sunspots for 70 of those years, between 1645 and 1715.
For reasons not yet understood, the solar cycle operated at a greatly reduced amplitude during that time. Evidence suggests it did not cease entirely, but the sunspot number—an index representing the total level of sunspot activity at a given time—during the late 1600s was reduced by a factor of 10 to 20 from its typical value during normal cycles. This perplexing aspect of the sunspot record was formally pointed out by German astronomer Friederich Wilhelm Gustav Sporer (1822–1895) and English astronomer Edward Walter Maunder (1851–1928) in 1890, and it is now known as the Maunder minimum.
The existence of the Maunder minimum is interesting on purely astrophysical grounds, because it suggests that the regular rise and fall of sunspots observed from 1715 all the way through to the present day may not be a permanent, or even typical, aspect of solar behavior. It is possible to create a rough reconstruction of the sunspot record prior to the invention of the telescope, using indirect indicators of solar activity, and there is evidence for other Maunder minimumlike periods intermittently from about AD 1250 through 1715. The solar cycle as observed today, is therefore not the state in which the sun spends all—or even most—of its time. Having only observed one Maunder minimum, scientists have no idea whether the sun spends 10%, 50%, or 90% of its time in such a state.
Even the normal 11-year cycle seems to have longer-term behavior. Different cycles have different strengths, with some of them showing more sunspot activity than others. The strengths of the cycle peaks seem to follow a roughly 80-year period of very strong cycles, slightly weaker ones, then back to stronger ones, and so forth. With detailed sunspot records extending only a few hundred years, it is difficult to confirm or disprove this hypothesis. Combined with evidence for multiple periods of nearly complete inactivity, it becomes impossible to say whether the solar activity cycle, so extensively studied in the last 30 years, is normality or an aberration.
The seemingly erratic behavior of the solar cycle has led a number of astronomers to spend the better parts of their careers studying activity cycles on other stars, the idea being that if those stars show activity cycles or Maunder minimumlike characteristics, scientists might be better able to understand Earth’s own star. Most of this pioneering work has been carried out at the Mt. Wilson Observatory, near Los Angeles, California. Observations of solarlike stars have been underway at Mt. Wilson since 1963, and the program has accumulated a vast database of solar activity data. The result has been the discovery of a veritable zoo of activity cycles. Some stars have well-behaved cycles with periods comparable to the sun’s 11-year cycle; these are of particular interest for comparison to the sun. Other stars have highly variable cycles, while still others vary wildly but with no discernible, regular period. Finally, there are stars that show a complete absence of any activity cycle. Some of them appear to show no cyclic activity at all, while others exhibit tantalizing evidence of having turned off midway through the 30 years they have been observed from Earth. Whether or not these stars are truly in a Maunder minimum phase has not been answered, because it is very difficult to tell if they have low-amplitude cycles or no cycles at all, and it is even more difficult to study their finer characteristics in detail. However, there is no doubt that pronounced,
Little Ice Age —A period of long, severe winters in Europe that occurred roughly between 1300 and 1715, corresponding closely to a period of erratic solar activity.
Maunder minimum —A period between 1645 and 1715 when the solar activity cycle operated at a greatly reduced level.
Solar activity cycle —The periodic, roughly 11-year rise and fall in the number of active features, such as sunspots, prominences, and flares, in the Sun’s atmosphere; it is thought to be caused by the periodic tangling of the Sun’s magnetic field by its rotation and the motion of heat-transporting convective bubbles of gas beneath its surface.
Sunspot —Cooler and darker areas on the surface of the Sun. They appear dark only because they are cooler than the surrounding surface. Sunspots appear and disappear in cycles of approximately 11 years.
Sunspot number —An international estimate of the total level of sunspot activity on the side of the sun facing the Earth, tabulated at the Zurich Observatory. Observations from around the world are sent to Zurich, where they are converted into an official sunspot number. Since the Sun rotates, the sunspot number changes daily.
fairly regular activity cycles like the sun’s are not universal either for the Sun or its stellar cousins.
In 1991, a pair of Danish meteorologists published a paper in which they pointed out a remarkably strong correlation between the length of the solar activity cycle and the global mean temperature in the northern hemisphere. Not all activity cycles are the same length, with longer cycles (12 to 14 years) seeming to indicate cooler global temperatures than the short (nine to 10 year) cycles. It is very difficult to assess the effect of even recent solar cycles on global climate, let alone those from the Maunder minimum period, because of the relatively short time span for which detailed observations exist, and because climate records become sparse to nonexistent as one looks back more than one hundred years or so.
In 2002, research showed that the rotation of the sun slowed during the Maunder minimum. Scientists think this abnormality could have something to do with the decreased number of sunspots because a slower revolving star, such as the Sun, means less heat emerging from it (and impinging on orbiting planets, such as the Earth). Despite the ongoing controversy, for which there is decidedly no definitive answer as of the year 2006, there is no doubt the Maunder minimum years were a time of significant misery in Europe, with the long, harsh winters leading to shortened growing seasons, failed crops, and widespread famine. Whether, or to what degree, the Sun is responsible for this, is an important question for atmospheric scientists and astronomers to tackle over the next few decades.
Brody, Judit. The Enigma of Sunspots: A Story of Discovery and Scientific Revolution. Edinburgh, UK: Floris Books, 2002.
Hill, Steele. The Sun. New York: Abrams, 2006.
Soon, Willie. The Maunder Minimum and the Variable Sun-Earth Connection. River Edge, NJ: World Scientific, 2003.