The History Of Solar Observations

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Many aspects of the Sun remain a mystery even today, although recent observations have given us important insights into how the Sun works. Historical observations of the Sun have been the foundation for many hypotheses on solar-terrestrial relations. Some of the most important observations of the Sun in the early days were made of dark small regions that were later known as sunspots. Some of the earliest known references made to such solar features in the western hemisphere can be dated as far back as the ancient Greeks, to Theophrastus of Athens around 350 bc, but there are also records of sunspot observations in China from 28 bc. Sunspots, however, were not in good accord with the beliefs of most ancient Greeks, which was that the Sun was perfect and without blemishes. Nor were the sunspot concepts particularly popular with the Catholic Church during the Middle Ages. In the early Middle Ages, sunspots were sometimes mistaken as the passing of Mercury in front of the solar disk. Sunspots did therefore not receive much attention until the invention of the telescope by Galileo in the early 17th century, when their existence was demonstrated beyond doubt. The first telescopes consisted of an arrangement of two lenses, but more recent telescopes also use various arrangements of mirrors.

The invention of the telescope allowed better solar observations, and the existence of sunspots became too evident to ignore. According to Bray and Loughhead (1964), the first telescopic studies of the sunspots were started in 16117 by four astronomers: Fabricius, Galileo, Scheiner and Harriot. Fabricius deduced from sunspot observations that the Sun must rotate with a period of around 27 days. The solar rotation was of course calculated from a terrestrial frame of reference, and the solar rotation rate is slightly higher viewed from a galactic frame of reference. Galileo deduced that the dark regions were part of the Sun because their shape and size did not behave according to expectations, had they been planets. But these sunspots have ever since been an enigma. There is still no definite answer as to how and why they appear, although several hypotheses have been proposed.

Most of the historical solar observations were made through telescopes, and the first observations were intermittent because the Sun could not be seen during night-time or overcast conditions. The first telescopes were situated at various locations, often chosen to be in the proximity of the enthusiast astronomer and not necessarily where visibility ("atmospheric seeing'') was best. Some observatories were also moved, or the telescopes improved. In 1858, some of the first operational photoheliographs (photographs of the Sun) were made at the Kew observatory near London, but the telescope was moved to Spain in 1860. The telescope was re-erected at Cranford8 in 1861 and moved to Greenwich in 1873, where the observations commenced in 1874 and have continued to the present day.

The Royal Observatory installed a telescope at the Cape of Good Hope in 1875. A new enlarger was fitted on this telescope in 1889, giving images of 8 inches as opposed to 4 inches with the previous telescope configuration. The telescope's 4-inch lens was replaced with new ones in 1910 and 1926, presumably to obtain improved solar images. In 1949, the telescope was moved to Herstmonceux Castle in Sussex. Other observatories operated over a short period, such as the Durham observatory which was in operation from 1853 to 1861. The Zurich observatory has been in operation from 1855 to the present day.

The Mount Wilson observatory was established in 1904 when the Snow horizontal telescope at Yerkes Observatory was moved to the 1700-m high summit in California. In 1907 a 60-foot solar tower telescope was erected, and in 1912, a 150-foot tower was built. Hale and Adams were the first to make high-dispersion

7 An earlier date has also been given: December 10, 1610 according to Helland-Hansen and Nansen (1920), p. 147.

8 Middlesex, U.K. (Bray and Loughhead, 1964, p. 6).

photographs of the sunspot spectra at Mount Wilson (1906), and established that sunspots were cooler than the surrounding photosphere as opposed to being regions with higher absorptivity. They found certain line spectra of metals with stronger intensity than elsewhere from the solar surface and other spectral lines were weakened, consistent with laboratory experiments demonstrating that cooler gases are associated with more intense metallic lines than hotter gases (Kuiper, 1953, pp. 8-9).

One of the problems with telescopic observations from Earth's surface is the effect of atmospheric conditions on the image. A relocation of an observatory to a location which has less (more) clouds may result in a change in the quality of the observations. For instance, longer cloudy periods may result in faint and small-scale solar features going undetected. Winds, turbulence and haze can degrade the solar image quality, and clouds block the Sun and reduce the observing time. Poor observational conditions can result in undetected small sunspots near the Sun's limb. Small and short-lived sunspots may also go unnoticed due to extended periods of cloudiness. Many observatories are needed to track the evolution of solar events, and more recently a network of observatories around the world has joined in a collaborative effort to observe the Sun. With the recent solar satellites, it is possible to obtain uninterrupted observations of the Sun from just one platform.

2.3.1 The importance of good observations

Most of our knowledge about the Sun and our climate is derived from data of some form, be it actual observations or model data. These furthermore provide the framework for analytical analysis and physical models. It is therefore important to look at the data pool and assess its quality. The data quality is of utmost importance in science, as biases can have profound effects on analytical tests.

2.3.2 Criteria for good observations

A Norwegian meteorologist, Godske (1956), once proposed five criteria for making measurements: (i) the measurements must be unambiguous; (ii) repeated measurements of the same condition must give the same answer; (iii) one must know exactly what is measured; (iv) the instruments must be adapted to the conditions they measure; and (v) the observation must not alter the system. These conditions must be fulfilled if the observations are to be used in empirical studies of solar-terrestrial relations. The first two criteria reflect the quality of the measurements, and if these cannot be quantified in an objective way, there is no way that the data can be used to derive objective conclusions. The third point may seem obvious, but there may also be subtle aspects to this criterion. For instance, if the temperature is measured in direct sunlight, the observation is not of the air temperature, but that of the sun-exposed thermometer itself. If this criterion is not fulfilled, then it will be impossible to determine whether there is a real relationship between the two objects being studied. The fourth criterion is related to the third, and if (v) is not satisfied, then this implies a violation of (iii) for subsequent observations.

2.3.2.1 The quality of the sunspot record

Because of the limitations of the atmospheric visibility, observations were later made from high-altitude balloons, aircraft, and spacecraft. The quality of sunspot observations before 1849 has been questioned9 (R. M. Wilson, 1998). When using solar data in connection with climate studies, it is extremely important to ensure that the sunspot record is not "contaminated" by the climate itself. For instance, if observations are made from a few observatories, as they were in the early record, then long overcast periods may result in undetected sunspots. Any atmospheric contamination can lead to a circular argumentation in solar-terrestrial studies. It may nevertheless be possible to support the direct sunspot observations by independent isotopic data from tree rings and ice cores as long as the climate does not affect these too. Therefore, the data quality places some limitations on empirical studies of solar-terrestrial links based on long data records. The quality of measurements of terrestrial variables also has a tendency to deteriorate with age.

It is not a trivial task to obtain long-term records of solar activity from historical observations. One difficulty is associated with the solar rotation, which causes periodic disappearance of the features on the Sun's surface. It is also known that the same feature may have different appearance for different observers, which may lead to some confusion.10

There exist two "official" sunspot records, the American and Zurich observations since 1950. A comparison between these is shown in Figure 2.3. The curves are very similar, but there are also some differences, such as a tendency for the American sunspot number tending to be higher in the first cycle shown in this plot, and mostly lower values in the second cycle. These discrepancies indicate that the sunspot number is associated with some degree of uncertainty. The sunspot numbers are usually determined from visual observations with a refractor of modest size and using a fairly low magnification. Wolf used a Fraunhofer refractor with an 8-cm aperture, a focal length of 110 cm, and a magnification coefficient of 64. A similar set-up is still used today at Zurich.

It is important to make certain that any changes in observation practices over time do not affect the results and hence give misleading impressions about long-term trends. The systematic improvements made to the telescopes ever since 1611 may, for instance, result in instruments that are capable of getting higher resolution and can capture more small sunspots. Furthermore, the gradual extension of the observational network may imply an improvement over time of sunspot observations. Thus, there is a risk that the historical sunspot record may suggest that the total number of sunspots has increased over time when many spots in the early record could have been missed due to the record being inhomogeneous. There is an east-west asymmetry in the number of sunspots observed discussed by Kuiper (1953). A zonal symmetry in the sunspot occurrence is expected unless the sunspots themselves are reclining with respect to the radial axes. The east-west asymmetry in the sunspot

9 "Poor" before 1818, "fair" between 1818 and 1848, and "good" from 1849 to present.

Zurich and American monthly mean sunspot number

Zurich and American monthly mean sunspot number

Figure 2.3. A comparison between two different estimates of the sunspot number. Both from ftp.ngdc.noaa.gov.

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Time

Figure 2.3. A comparison between two different estimates of the sunspot number. Both from ftp.ngdc.noaa.gov.

statistics (see Section 4.7.2) may be interpreted as an indication of not seeing all sunspots. Historically, we have only been able to observe one side of the Sun at any instant of time, and therefore approximately only half the number of sunspots at any time. As the Sun rotates, the hidden sunspots come into view whereas the sunspots in the west disappear behind the limb (see Figure 4.14).

Another source of inhomogeneity may be long-term changes to atmospheric transparency. Haze in the atmosphere or clouds may hide some sunspots, and if the amount of haze or clouds have increased, for instance, as a result of a warmer climate, then this can result in a systematic "under-count" of sunspots. According to Orlove et al. (2000), the Incas in the Andes may have used the visibility of Pleiades for centuries to make predictions about the next season's crop. In some Andean villages, the star Pleiades is celebrated in the month of June, and some forecasts were based on the size of Pleiades. A large apparent size was associated with a good harvest. There are indications of the upper stratospheric clouds being affected by El Nino events, which again affect the

2000

Time visibility of the stars so that they are dimmer around the onset of El Nino. This observation may have implications for the study between sunspots and climate. It may nevertheless be possible to correct for such atmospheric interference by using a number of independent stars as a baseline, the differences between the brightness of the subject and the baseline can be used to remove much of the atmospheric bias.

High-quality astronomical observations are not easy to obtain, and there are various factors that may degrade the observations. For instance, scintillation from stellar studies (10 Hz) have suggested that the amplitude of the twinkling intensity depends on the telescope aperture and high-altitude atmospheric winds. There may also be an image degradation and rapid image motion caused by local conditions near the telescope, such as surface winds. Furthermore, there may be slow image motions caused be atmospheric disturbance and telescope heating. Kuiper (1953) "guesstimated" that usually there are excellent observing conditions only 1% of the time. Presently, some of these effects may be corrected numerically through computer post-processing of the data.

Parasitic light is scattered light from Earth's atmosphere and optical instruments. Usually one assumes that the light scattering is independent of time and position, so that this effect can thus be corrected for by looking at the sky where there are no objects. Such corrections may be appropriate for usual astronomical studies, but it is important to ask whether they are good enough for deducing slow and small long-term changes that may be related to changes in the terrestrial climate.

The various concerns regarding the sunspot record, in addition to similar problems with climatic observations, may suggest that the sunspot number is not sufficiently reliable for studying long-term trends in solar-terrestrial relationships. It is therefore important to use caution when analysing these data records. Empirical studies may also be backed up by independent evidence, for example from so-called proxy data such as palaeo records.

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