POPULAR ASTRONOMY.

CHAPTER I.

THE SUN, THE CENTRAL ORB OF THE PLANETARY

SYSTEM.

DISCOVERIES OF THE ANCIENTS.-The Source of Life and Light and Heat. The Sun's Motion among the Stars.-His Orbit circular.Length of the Year.-Inequality of the Sun's Motion.-Explained by Hipparchus.-Solar Eclipses.-Their First Prediction.

DISCOVERIES OF THE MODERNS.-The Sun's Distance.-His Horizontal Parallax.-Importance of this Element.-Measured by the transit of Venus.-The Sun's actual Diameter and real Magnitude.-His Rotation. The Solar Spots.-Their Periodicity.-Speculations as to the Physical Constitution of the Sun.

THE Sun is beyond comparison the grandest of all the celestial orbs of which we have any positive knowledge. The inexhaustible source of the heat which warms and vivifies the earth, and the origin of a perpetual flood of light, which, flying with incredible velocity in all directions, illumines the planets and their satellites, lights up the eccentric comets, and penetrates even to the region of the fixed stars,—it is not surprising that in the early ages of the world, this mighty orb should have been regarded as the visible emblem of the Omnipotent, and as such should have received divine honours.

On the approach of the sun to the horizon in the early dawn, his coming is announced by the gray eastern twilight, before whose gradual increase the brightest stars, and even the planets, fade and disappear. The coming splendour grows

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and expands, rising higher and yet higher, until, as the first beam of sunlight darts on the world, not a star or planet remains visible in the whole heavens, and even the moon, under this flood of sunlight, shines only as a faint silver cloud.

This magnificent spectacle of the sunrise, together with the equally imposing scenes which sometimes accompany the setting sun, must have excited the curiosity of the very first inhabitants of the earth. This curiosity led to a more careful examination of the phenomena attending the rising and setting sun, when it was discovered that the point at which this great orb made his appearance was not fixed, but was slowly shifting on the horizon, the change being easily detected by the observation of a few days. Hence was discovered, in the primitive ages,

THE SUN'S APPARENT MOTION.-In case the sun is observed attentively from month to month, it will be found that the point of sunrise on the horizon moves slowly, for a certain length of time, toward the south. While this motion continues, the sun, at noon, when culminating on the meridian, reaches each day a point less elevated above the horizon, and the diurnal arc or daily path described by the sun grows shorter and shorter. At length a limit is reached; the point of sunrise ceases to advance toward the south, remaining stationary a day or two, and then slowly commences its return toward the north. This northern movement continues; each day the sun mounts higher at his meridian passage, the diurnal arc above the horizon grows longer and longer, until, again, a northern limit is reached, beyond which the sun never passes. Here he becomes stationary for one or two days, and then commences his return toward the south. Thus does the sun appear to vibrate backward and forward between his southern and northern limits, marking to man a period of the highest interest; for within its limits the spring, the summer, the autumn, and the winter, have run their cycles, and by their union have wrought out the changes of the year.

The length of this important period was, doubtless, first determined by counting the days which elapsed from the time when the sun rose behind some well-defined natural object in the horizon until his return in the same direction to the same point of rising. Of course, these changes in the sun's place were studied with profound attention. They were among the first celestial phenomena discovered, and among the first demanding explanation. The stars were found never to change their points of rising, culmination, and setting. Their diurnal arc remained for ever the same, and the amount of time they remained above the horizon, depended on their distance from the north polar point.

Observation having thus revealed the fact that the sun was undoubtedly moving alternately north and south, a more critical research showed the equally important truth, that this great luminary was slowly shifting its place among the fixed stars. This was not so readily determined; but, by noting the brilliant stars which first appeared in the evening twilight, after sunset, it was soon discovered that these stars did not long remain visible. Indeed, the whole starry heavens seemed, from night to night, to be plunging downward to overtake the setting sun, or rather, that the sun himself was mounting upward to meet the stars; and thus was discovered a solar motion in a direction opposed to the diurnal revolution of the heavens.

From month to month the sun was seen to advance among the stars, and at the end of an entire year, after all the former changes of northern and southern motion had been accomplished, the sun was found to return to the same group of fixed stars from whence he set out; and thus it became manifest that this revolution among the stars was identical in period with the changes from north to south; and hence these phenomena had, in all probability, a common origin.

Here was the first great problem offered for solution to the old astronomers. The facts and phenomena were carefully studied, and the reader may now exercise his own

power of thought in an effort to explain the facts recorded, before accepting the solution we are about to present.

An examination of the points of rising, of culmination, and of setting, of the fixed stars, showed them to be absolutely invariable; and in case these glittering points could leave behind them, in their diurnal revolution, lines of silver light, sweeping upward from their point of rising to the meridian, and downward to the point of setting, these lines of light would be seen to be parallel circles. All the stars north of the equinoctial (in the region of the earth we inhabit) describe diurnal circles, of which more than one half is above the horizon, while all the stars south of the same line sweep round in circles, of which less than half lies above the same plane. Any star, precisely on the equinoctial, half-way between the north and south poles, passes one half its revolution above, and the other half below the horizon.*

These facts being carefully noted, it was seen that in case the sun, on any day of its annual journey, chanced to coincide with a fixed star, that for that day the sun and star would describe the same diurnal circle, and would remain above the horizon an equal length of time. Thus along the sun's path it became possible to select a number of stars over which the sun passed, and which would by their position mark his route in the heavens. To aid in this investigation, as well as for some other purposes, the ancients erected a vertical staff on a level plane, and then noted where the shadow of the top of the staff fell at noon each day throughout the year. This instrument was called a gnomon,+ and its use revealed many important facts in the solar motion, and detected others hitherto overlooked. If on the same

All the stars, &c., perform, while above the horizon, diurnal arcs; their paths below the horizon being nocturnal arcs. The diurnal arc may therefore be accomplished during the night, and the nocturnal arc by day; the signification of these terms varying somewhat from their common acceptation.

This is usually understood to be some substance forming an angle: the metal which makes an angle with a dial-plate, to indicate the hours, is called the "gnomon."

day we note carefully the length of the shadow of the gnomon a little while before and after noon, we shall find the shadow slowly decrease in length as the sun rises to its culmination; and immediately after passing the meridian the shadow commences to increase in length. Mark the point where the shortest shadow fell, and the line joining this point with the foot of the gnomon is a north and south line, and on this line all the noon shadows will fall throughout the entire year.

By a careful examination it was discovered that the noon shadow on the day of the winter solstice, or southern limit, always fell on the same point. The same was true of the noon shadow on the day of the summer solstice, or northern limit. These points were exactly opposite each other on the sun's apparent orbit (or path among the stars). It was further discovered, that selecting any day in the year, the noon shadow for that day invariably fell on the same point as it had done on preceding years; and hence it became manifest that the sun's track among the stars did not change from year to year.

The question now arose as to the figure of the sun's path: was that figure a circle? and did the sun move with uniform velocity? As all the stars described diurnal circles; as this curve was the simplest as well as the most beautiful of curves; as its curvature was everywhere the same; as it had neither beginning nor ending, it was early adopted as the celestial curve, shadowing forth, even in its form, the ceaseless journeys of the revolving worlds. It was assumed, then, that the sun swept round the sphere of the heavens once a year, with uniform velocity, in a circular orbit, of which the earth was in the centre.

This hypothesis accounted fully for all the discovered phenomena, and justly ranks among the most important of the primitive discoveries.

The gnomon gave to the old astronomers a ready means not only of tracing the sun's path among the stars, but also of measuring the inclination of the plane of the ecliptic to