The other 'important numerical facts relative to the sun are the following: Its diame ter calculated on the basis of the shorter distance hitherto received, is, in round numbers, 850,000 m., or more than 107 times the mean diameter of the earth; so that the volume or bulk of the sun exceeds that of the earth 1,200,000 times, and is 600 times greater than the bulk of all planets at present known, together. The mass of the sun, or quantity of matter it contains as measured by weight, exceeds that of the earth only 300,000 times; and thus it appears that the matter of the sun has only one-fourth the density of that of the earth. From this and other facts, it is inferred that the matter of the sun exists for the most part in a gaseous condition. Still his mass is 740 times greater than the masses of all known planets put together. The period of rotation of the sun upon its axis, which Galileo was the first to calculate from observations of the sun spots, and which takes place in the same direction as that of the earth, is about 25 days 8 hours. It appears, however, that this period varies according to the solar latitude of the spots from which it is calculated. The inclination of the axis of the sun to the eclip tic is about 7-f°, and the longitude of the ascending node is about 74° 30'.
2. The form or figure of the sun has been the subject of recent investigations. The polar and equatorial diameters of the sun's disk as observed, have been supposed to differ, though by a very small quantity only. The photographs of the sun do not quite agree in the amount of the value for the diameter with that given by observations.
The general laws by which the relation of our earth to the sun, as the source of light and heat, is governed, are of the most simple kind. The rays which emanate from the sun's disk into space proceed in diverging lines, and, on arriving at the earth, their intensity will be inversely proportional to the square of the sun's distance. This may be called the primary law; but the more obvious phenomena of solar heat and light are manifested to us under a secondary law depending on the obliquity of incidence of the sun's rays. See CLIMATE, EARTH, TEMPERATURE, etc.
3. Chemical Constitution of the Sun.—Astronomy has weighed and measured the sun long ago, and in our days chemistry, aided by physics, makes an analysis of it. The way in which this surprising result is arrived at is explained under SPECTRUM. The main fact on which the method rests is briefly this: that a substance, when compara tively cold, absorbs the very same rays which it gives out when heated. Hence it was inferred by Kirchhoff that if there was sodium or iron in a comparatively cold state in the solar atmosphere, above the source of light, these substances would produce black lines corresponding in spectral position with the bright lines which they give out when heated. On this principle the presence in the solar spectrum of hydrogen, magnesium, calcium, sodium, and metals of the iron group has been ascertained with something like certainty. There are less clear indications of other metals, such as zinc and lead; while
metals of the tungsten, antimony, silver, and gold classes have been searched for in vain. Of the metalloids, such as oxygen, carbon, nitrogen, sulphur, and the like, none had been detected till, in 1877, prof. Henry Draper of America announced the discovery of oxygen. The presence of these substances in the sun is haildly doubted, but their identification is difficult. A chief source of complication in research of this kind is the effect on the spectra of substances produced by differences of temperature and pressure. Excessive heat seems to dissociate the groups of atoms forming the molecules into sim pler groups, and thus produces a different spectrum difficult to recognize. The labors of Lockycr, Huggins, Janssen, Draper, and others are directed toward overcoming these , slid other obstacles.
4. Physical Constitution of the the first discovery by Galileo of those remarkable phenomena on the sun called patches with an area fre quently exceeding several times the surface of the earth—an immense variety of theories as to the probable constitution of the solar body has been brought forward by nearly every observer. Solar photography promises valuable aid in this research by enabling is to keep a permanent record of passing phenomena, ready at any time for deliberate measurement and comparison.
One of the most important discoveries in connection with sun-spots, science owes to Dr. Alexander Wilson of Glasgow, who, in the year 1769, observed certain general and remark able features of sun-spots, which enabled him to establish the significance of these phenom ena for a solution of the question as to the sun's physical constitution. These features are as follows: When a spot was near the middle of the sun, it was found to consist of a dark central part, called the nucleus or umbra, and around this was a comparatively brighter envelope, called the penumbra, and at such a time both parts were distinctly visible. But as the spot approached one border, the penumbra on the side nearest the observer became gradually more and more foreshortened, while the penumbra on the other side grew broader and broader, and at length, as the spot was disappearing—that is, passing the edge of the limb—the near side of the penumbra, as well as the dark cen tral part, entirely vanished, nothing remaining except the opposite penumbra. When a spot made its appearance on the other side of the border, Wilson noticed the same phe nomena in an opposite order, and soon discovered that they were nearly universal. It followed from these observations at once that every spot presents the appearance of a funnel-shaped opening in the sun's body, which, by the rotation of the latter, succes sively presented the described appearances. These observations have been abundantly confirmed by the photographic records of Messrs. De la Rue, Stewart, and Loewy.