§ 457. The distances were observed for several months, and it was found that the distance between one star and the central point diminished, while that between the other and the central point increased. The observations were corrected for the proper motion, for aberration, and even an allowance was made for the effect of the temperature on the micrometer screw. These being allowed for the distances obtained on the successive nights of observation, should have been exactly equal. This, however, was not the case. The observed and computed differences differed, and those differences seemed to follow a certain law, and it soon became manifest that the errors could be greatly reduced by admitting a certain amount of parallactic motion of the central point of the components of the double star. The parallax obtained thus was 0."3309, which gives us the distance of the binary system of stars from our sun 670,000 × 95,000,000 of miles; a distance so great that light can traverse the interval only after a flight of nine years and three quarters, at the velocity of twelve millions of miles a minute. The diameter of the orbit described by the two stars of 61 Cygni round each other is about 50 times that of the earth's orbit, or 2 times that of Uranus. Their periodic time is about 540 years. Such is the universe in which we exist, and which we have at length found the means to subject to measurement.

$458. Once having learned the distance of a fixed star, if it were possible to measure its apparent diameter, it would be easy to ascertain its magnitude. But the fixed stars show no discs even with the highest powers. They increase in brilliancy so as to be painful to the eyes, and are surrounded by a haze or dawn like that of morning, but they remain mere points. A telescope which brings them two thousand times as near fails to give them any sensible diameter.

After years of incessant labor the parallax of the great stara Centauri, the third star of the heavens in brightness, has lately been ascertained to be a little more than nine tenths of a second, indicating a distance so enormous, that if our sun were so large as to fill the whole orbit of the earth, the sun when seen through a powerful telescope

would have a radius of only nine tenths of a second. It is a magnificent double star; one star being of a deep and brownish orange, the other of a fine yellow color; each star being of the first magnitude. Their distance is at present (1850) about 9", but it is rapidly diminishing, and in no great lapse of time, they will probably occult one another, their angular motion being comparatively small. Their apparent distance was formerly much greater, how much we cannot say for want of observations, but probably the major axis of their mutual orbit is short of a minute of space. They therefore afford strong indications of being very near our system. Added to which their proper motion is very considerable, and participated in by both, which proves their connection as a binary system. An additional presumption in favor of their proximity may be drawn from their situation in the nearest region of the milky way, among a great number of large stars.

The parallax of the double star & Lyrae has also been found by M. Struve of Dorpat. Its distance from the solar system is 771,400 radii of the earth's orbit, a space passed through by light in twelve years.

CHAPTER XX.

THE SUN AND PLANETS.

The Mass and Dimensions of the Sun. Its Atmosphere. Its light and Heat. The Solar Spots. Proposed explanations of their Appearance. The Sun's Rotation. The Centre of Gravity of the Solar System. Determination of the Orbits, Masses and Densities of the Planets.

§ 459. The sun always presents a round luminous disc, and since the telescope has shown its rotation, this is a proof that its form is nearly or quite spherical. It is the only fixed star so near that we can learn its diameter and the appearance of its surface. Its mass is 800 times that of all the planets taken together.

It is only 329,000 times

that of the earth, though its solid contents are to those of the earth as 1,300,000 to 1. Hence it attracts the earth only 329,600 times as forcibly as the earth attracts it. In speaking of its size, we include the luminous atmospheres, which are at least several thousand miles deep. The solid nucleus of the sun may have a much greater specific gravity than has here been assigned to the whole orb. Its diameter is 111 times that of the earth. Its actual diameter is 900,000 miles. If its centre were to coincide with the centre of the earth, its volume would not only include the orbit of the moon but would extend nearly as far again. It could contain within its circumference more than 130,000 globes as large as the earth, and a thousand globes as large as Jupiter. Its surface seems to have a state of constant ebullition and commotion or rather instability. Agitated apparently by powerful causes, it often accumulates into masses like waves, whose summits now round, now in ridges, constitute bright places, and which seen through a telescope, are finely mottled, and far from uniformly bright, resembling a mackerel sky.

§ 460. From the properties of the sun's rays it is known that this incandescent substance, which causes light and heat, is neither a solid nor a liquid, but a gas. The intensity of the sun's heat is proved by the facility with which the calorific rays traverse glass, a property which is found to belong to artificial fires in proportion to their intensity. The most vivid flames disappear, the most intensely ignited solids appear only as black spots on the disc of the sun when held between it and the eye. From this last fact it follows that the body of the sun, however dark it may appear when seen through its spots, may be in a state of complete ignition. Light and heat may arise from chemical changes taking place at the surface, from electricity, or from some cause unknown to us. The sun's di

rect light has been estimated to be equal to 5,563 wax candles placed at the distance of one foot from the eye. That of the moon is only equal to one candle placed at a distance of twelve feet.

§ 461. Its heat and light are sent forth equally in all directions. When a planet is turned toward it, we become

aware of these rays by their reflection; but they are as numerous throughout the whole system. We know its heat only by that intercepted by the earth, yet of that alone as much as is received in one year is sufficient to melt a stratum of ice forty-six feet deep covering the whole globe. Part of this heat is radiated back into space, but by far the greater part descends into the earth during summer, and returns thence in the course of the winter. The fixed stars are too distant to afford us sensible heat. The heat received on a given area exposed at the distance of the earth, and on an equal area at the visible surface of the sun, must be in the proportion of the area of the sky occupied by the sun's apparent disc to the whole hemisphere, or as 1 to about 300,000. A far less intensity of solar radiation collected in the focus of a burning glass suffices to dissipate gold and platina in vapor. The planets and the moon, though so near, shining only by reflected light, give no sensible heat, though chemical rays have been detected in moonlight. It would require 90,000 moons, as many as would fill the whole of our visible sky, to afford us light equal to that which we have in a cloudy day when the sun does not shine out.

§ 462. It is now believed that the sun consists of a solid nucleus enveloped by an elastic non-luminous atmosphere, supporting luminous strata which to us appear as the disc of the sun. We infer the existence of this atmosphere from the appearances which accompany the spots seen in the sun. These spots are perfectly and intensely black, but are surrounded by a penumbra of a nearly uniform half shadow, thus presenting the appearance of a deep pit. There is no gradual melting of one shadow into the other, of spot into penumbra, penumbra into full light. The idea conveyed is more that of the successive withdrawal of veils, the partial removal of definite films, than of the melting away of a mist, or the mutual diffusion of gaseous media. The only theory which at all accounts for them supposes the black part of the spot to be the opaque body of the sun laid bare, or at least an opening, and the surrounding penumbra to be the nonluminous atmosphere. Luminous matter often is seen piled

up round the edges of the spot, and often precedes the breaking out of a spot, indicating great agitation of the atmosphere.

The immense scale on which these spots take place, and the rapidity of their changes, confirm the idea that they take place in a mobile fluctuating gaseous atmosphere. A single second of angular measure seen from the earth, corresponds on the sun's disc to 465 miles. And a circle of this diameter (containing therefore nearly 220,000 square miles) is the least space which can be distinctly discerned on the sun as a visible area. A spot seen on the 29th of March, 1837, occupied an area of nearly five square minutes, equal to 3,780,000,000 square miles. The black centre of the spot, May 25, 1837, (not the tenth part of the preceding one), would have allowed the globe of the earth to drop through it, leaving a thousand miles clear of contact on each side of the tremendous gulf. That such a spot should close up in six weeks time, its borders must approach at the rate of more than 1,000 miles a day. 50 and even 150 spots have been seen at once on the sun. Sometimes several unite, and often a large one divides into several smaller ones, which soon vanish. They frequently appear in clusters, as at the time of the annular eclipse of the sun in 1836, when there were five near together.

§ 463. These spots have a motion of their own independent of the sun's motion. These two facts have suggested an explanation of them from analogous phenomena which take place in the earth's atmosphere. The spots on the north side of the equator, move northward, and either join and disappear, or burst, one large one forming several small ones. Spots on the south side move southward, and exhibit the same changes.

On the earth hurricanes occur within the same latitudes, never cross the equator, and either gradually subside, or burst into smaller storms, and are dissipated. Hurricanes are moving cylinders of air which rotate with extreme rapidity, and are slowly translated, thus resembling the spots on the sun which have vorticose movement. These motions when left to themselves die away, the lower