intention to shew how the sun's distance has been measured, as the proceeding is complicated, and requires a detailed statement, although the method adopted is merely the extension of that used by an ordinary land-surveyor, who measures the breadth of a river which he cannot cross, or the distance of a tower which he cannot reach. We may, however, give the simple proportion by which the sun's diameter is determined when the distance and angular diameter are known, for the distance is nothing more nor less than the radius of the circle on which the angle is measured. There are 1,296,000 seconds of arc in an entire cir1,296,000 cumference; there are, therefore, seconds in that part of the 1,296,000 circumference equal to the diameter, and 3.1416 X 2 part of the circumference equal to the radius; so that we have-The diameter of the body in miles: the distance in miles :: the angular diameter: 206,265". Now, the diameter of the sun, at the earth's mean distance, is a trifle over 32'-that is, a little over half a degree, so that we have 206,265":91,000,000 miles :: 1920": 842,702. More accurately, it is 852,584 miles. 3-1416 = 206,265" in that Having, then, this diameter, we can determine the size or volume of the sun; it is 1,200,000 times greater than our earth. Thanks to Newton's universal law of gravitation, we can determine his weight or mass from the motions of the planets around him, their orbits being curved because they are continually being deflected from a straight path by the action of the sun's attraction, just as a projectile shot from a cannon is deflected from its straight path by the action of the earth's attraction. In this way it has been found that the weight or mass of the sun is 300,000 times greater than that of our earth. Here, then, it is at once evident that the materials of which the sun is built up are lighter, bulk for bulk, than those of which our earth is composed, for, otherwise, being 1,200,000 times as large, it would be 1,200,000 times as heavy; but as it is only 300,000 times as heavy, it follows that, bulk for bulk, it is only one-quarter as dense as the earth. We shall shew further on how this great lightness may probably be accounted for. 300,000 1,200,000 = Our account of the sun would be incomplete did we not refer to the system of bodies which revolve round it, including the earth. We find planets-of which the Earth is one-differing greatly in size, and situated at various distances from the sun. We find again a ring of little planets, clustering in one part of the system, called asteroids, or minor planets: and we already know of at least two masses or rings of smaller planets still, some of them so small that they weigh but a few grains; these give rise to the appearances called meteors, bolides, or shooting-stars. We find also comets, some of which break in, as it were, upon us from all parts of space, and then, passing round our sun, rush back again; we find others so little erratic that they may be looked upon as members of the solar household. We have then Eight large Planets, as follow, in the order of distance from the sun : 1. Mercury; 2. Venus; 3. EARTH; 4. Mars; 5. Jupiter; 6. Saturn; 7. Uranus; 8. Neptune. One hundred and six small Planets revolving round the sun, between the orbits of Mars and Jupiter. Meteoric bodies, which at times approach near the earth's orbit, and occasionally reach the earth's surface. Comets. The Zodiacal Light.-A ring of apparently nebulous matter, the exact nature and position of which in the system are not yet determined. Let us next inquire into the various distances of the planets from the sun, bearing in mind, that as the orbits are elliptical, the planets are sometimes nearer to the sun than at other times. The average or mean distances are as follow; the sizes and times of revolution are also given : Such, then, is the sun and his system, taken as a whole-a point of view not to be passed over in any general description of the sun. We now pass on to the telescopic appearance of the great central luminary. II. WHAT THE SUN IS LIKE. Having now got an idea of the sun's place in the universe, and his size, mass, and density, compared with our own earth, we may begin to scrutinise him somewhat more closely. One of the first triumphs of the telescope was the discovery of spots in the sun-a discovery which sent a thrill through the world of schoolmen, for they imagined that the fundamental Aristotelian doctrine of the immutability and incorruptibility of the heavens was thereby contravened. It is doubtful whether this discovery is to be attributed to the great Galileo, or to Fabricius, or even to our own countryman Harriot; it is certain, however, that they observed sun-spots about the years 1610 and 1611. It is not surprising that the strangest explanations of these curious solar phenomena were hazarded in those early times with the imperfect instruments then in use. It was at length, however, suspected by Galileo that they really belonged to the sun, and were not planets revolving Fig. 3.-General Telescopic Appearance of the Sun, shewing spots and faculæ. round him; and from their motions he at once inferred that the sun turned on his axis like our own earth-some 25 days or so being required for a complete rotation. It needed, however, a period of Fig. 4-Position of the Sun's Axis, and apparent paths of the spots across the disc, as seen from the Earth at different times of the year. The arrows shew the direction in which the sun turns round. 150 years to make any great step in advance on the work done by Galileo and his contemporaries, and this step we owe to Dr Wilson of Glasgow, who in the year 1769 shewed it to be very probable |