No area of science has made recourse to historical information more often than astronomy, and Halley's 1715 eclipse compendium is a wonderful example, as we shall see. First, though, I must sketch in a little of the background.
Halley was only 19 years old when he first made observations of sunspots, publishing the results in his second scientific paper.
That was in 1676. Using the naked eye, the ancient Chinese had observed large sunspots many centuries before that, when dust storms blew in from central Asia, blanketing parts of northern China. Such solar blemishes had similarly been noticed from Europe, but it was only when telescopes appeared in the seventeenth century that continuous monitoring of these dark markings on the Sun's surface was feasible.
Using a telescope an image of the Sun can be projected onto a screen (as in Figure 1-13). By following the movement of specific sunspots from day to day Halley and his contemporaries determined that, near its equator, the Sun takes about 25 days to spin. Not being a solid body, it does not rotate rigidly, but different speeds are apparent depending on the latitude, such that nearer its poles the Sun takes closer to 35 days to turn once.
Sunspot numbers have routinely been kept through to the present from Galileo's time, a hundred years earlier than the eclipse in question, and it was studies of these numbers that revealed the apparent 11-year periodicity in solar activity. There is evidence that the overall climate of the Earth follows the same cycle. In the early decades of the twentieth century another British astronomer, Edward Maunder, noted there had been a deficit of sunspots during the latter half of the seventeenth century; this is now known as the "Maunder Minimum." This coincides with a pronounced cooling of the climate known as the "Little Ice Age." The River Thames froze over, for example, and fairs were held on London's ice-covered waterway. This correlation may just have been a coincidence, but it seems to warrant more than merely a suspicion that the two phenomena are related.
This makes one wonder how else the Sun might be varying in its properties, sunspot numbers being just one diagnostic, and how the Earth's climate might alter in response to any such change. If the Sun had expanded, say, then one might also expect it to cool a little, and so not emit so much energy in the form of sunlight. There would then be a concomitant drop in the mean temperature of our planet. Certainly, astronomers observe other stars pulsating in and out, their power output varying radically, and because our climate balances on a knife edge a fairly slight alteration in the Sun's power could have major repercussions.
Studies of stellar evolution indicate that since it "switched on" over 4.5 billion years ago, the energy output of the Sun has increased by about 30 percent. In fact, this understanding is so well established that the jargon phrase "Early Faint Sun Paradox" is bandied about within circles of scientists interested in the evolution of the terrestrial environment, and in particular those studying how life developed on our planet. The point here is this: If the Sun were initially so much fainter, as is believed to be the case, then the Earth would have been a frigid world. Under such circumstances, how did even the simple mono-cellular slime, which was the sole occupant of the planet between about 3,800 and 570 million years ago, manage to evolve and survive?
This increase in solar output has not terminated. In our earlier description of solar evolution it was noted that the Sun is expected to continue to behave in a similar fashion to the present for another five billion years or so. Over that time, though, its power output is expected to double. If that increase were steady and uniform then over five thousand years (a suitable time scale for human civilization) the solar energy reaching the Earth might increase by one or two parts in a million. Such changes are dwarfed by other natural variations, like the way in which the Earth's orbit evolves and the orientation of its spin axis shifts. But what if the
Sun's output oscillates significantly or alters abruptly on timescales of only decades or centuries? Astronomers certainly see other stars acting in this way.
Such modern concerns as the anthropomorphic "greenhouse effect" would obviously be affected by changes in the solar output, so we'd better be sure we know how the Sun behaves over extended periods. Perhaps historical measures can assist. At last, then, we come to the significance of Halley's compilation of reports of the 1715 eclipse.
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