(326.) When the sun is in either tropic, it enlightens, as we have seen, the pole on that side the equator, and shines over or beyond it to the extent of 23° 28′ 40′′. The parallels of latitude, at this distance from either pole, are called the polar circles, and are distinguished from each other by the names arctic and antarctic. The regions within these circles are sometimes termed frigid zones, while the belt between the tropics is called the torrid zone, and the immediate belts temperate zones. These last, however, are merely names given for the sake naming; as, in fact, owing to the different distribution of land and sea in the two hemispheres, zones of climate are not co-terminal with zones of latitude.

(327.) Our seasons are determined by the apparent passages of the sun across the equinoctial, and its alternate arrival in the northern and southern hemisphere. Were the equinox invariable, this would happen at intervals precisely equal to the duration of the sidereal year; but, in fact, owing to the slow conical motion of the earth's axis described in art. 264, the equinox retreats on the ecliptic, and meets the advancing sun somewhat before the whole sidereal circuit is completed. The annual retreat of the equinox is 50"-1, and this arc is described by the sun in the ecliptic in 20′ 19′′9. By so much shorter, then, is the periodical return of our seasons than the true sidereal revolution of the earth round the sun. As the latter period, or sidereal year, is equal to 365 6h 9m 9s 6, it follows, then, that the former must be only 365d 5h 48m 49s 7; and this is what is meant by the tropical year.

(328.) We have already mentioned that the longer axis of the ellipse described by the earth has a slow motion of 118 per annum in advance. From this it results, that when the earth, setting out from the perihelion, has completed one sidereal period, the perihelion will have moved forward by 11"-8, which arc must be described before it can again reach the perihelion. In so doing, it occupies 4' 39'7, and this must therefore be added to the sidereal period, to give the interval between

two consecutive returns to the perihelion. This interval, then, is 365d 6h 13m 49s.3*, and is what is called the anomalistic year. All these periods have their uses in astronomy; but that in which mankind in general are most interested is the tropical year, on which the return of the seasons depends, and which we thus perceive to be a compound phenomenon, depending chiefly and directly on the annual revolution of the earth round the sun, but subordinately also, and indirectly, on its rotation round its own axis, which is what occasions the precession of the equinoxes; thus affording an instructive example of the way in which a motion, once admitted in any part of our system, may be traced in its influence on others with which at first sight it could not possibly be supposed to have any thing to do.

(329.) As a rough consideration of the appearance of the earth points out the general roundness of its form, and more exact enquiry has led us first to the discovery of its elliptic figure, and, in the further progress of refinement, to the perception of minuter local deviations from that figure; so, in investigating the solar motions, the first notion we obtain is that of an orbit, generally speaking, round, and not far from a circle, which, on more careful and exact examination, proves to be an ellipse of small excentricity, and described in conformity with certain laws, as above stated. Still minuter enquiry, however, detects yet smaller deviations again from this form and from these laws, of which we have a specimen in the slow motion of the axis of the orbit spoken of in art. 318.; and which are generally comprehended under the name of perturbations and secular inequalities. Of these deviations, and their causes, we shall speak hereafter at length. It is the triumph of physical astronomy to have rendered a complete account of them all, and to have left nothing unexplained, either in the motions of the sun or in those of any other of the bodies of our system. But the nature of this explanation cannot be

These numbers, as well as all the other numerical data of our system, are taken from Mr. Baily's Astronomical Tables and Formulæ, unless the contrary is expressed.

understood till we have developed the law of gravitation, and carried it into its more direct consequences. This will be the object of our three following chapters; in which we shall take advantage of the proximity of the moon, and its immediate connection with and dependence on the earth, to render it, as it were, a stepping-stone to the general explanation of the planetary

movements.

(330.) We shall conclude this by describing what is known of the physical constitution of the sun.

When viewed through powerful telescopes, provided with coloured glasses, to take off the heat, which would otherwise injure our eyes, it is observed to have frequently large and perfectly black spots upon it, surrounded with a kind of border, less completely dark, called a penumbra.

Some of these are represented at a, b, c, plate iii. fig. 21, in the plate at the end of this volume. They are, however, not permanent. When watched from day to day, or even from hour to hour, they appear to enlarge or contract, to change their forms, and at length to disappear altogether, or to break out anew in parts of the surface where none were before. In such cases of disappearance, the central dark spot always contracts into a point, and vanishes before the border. Occasionally they break up, or divide into two or more, and in those offer every evidence of that extreme mobility which belongs only to the fluid state, and of that excessively violent agitation which seems only compatible with the atmospheric or gaseous state of matter. The scale on which their movements take place is immense. A single second of angular measure, as 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. Spots have been observed, however, whose linear diameter has been upwards of 45,000 miles* ; "Ingens macula in sole conspiciebatur,

* Mayer, Obs. Mar. 15. 1758. cujus diameter = diam. solis."

and even, if some records are to be trusted, of very much greater extent. That such a spot should close up in six weeks' time (for they hardly ever last longer), its borders must approach at the rate of more than 1000 miles a day.

Many other circumstances tend to corroborate this view of the subject. The part of the sun's disc not occupied by spots is far from uniformly bright. Its ground is finely mottled with an appearance of minute, dark dots, or pores, which, when attentively watched, are found to be in a constant state of change. There is nothing which represents so faithfully this appearance as the slow subsidence of some flocculent chemical precipitates in a transparent fluid, when viewed perpendicularly from above: so faithfully, indeed, that it is hardly possible not to be impressed with the idea of a luminous medium intermixed, but not confounded, with a transparent and non-luminous atmosphere, either floating as clouds in our air, or pervading it in vast sheets and columns like flame, or the streamers of our northern lights.

(331.) Lastly, in the neighbourhood of great spots, or extensive groups of them, large spaces of the surface are often observed to be covered with strongly marked curved, or branching streaks, more luminous than the rest, called faculæ, and among these, if not already existing, spots frequently break out. They may, perhaps, be regarded with most probability, as the ridges of immense waves in the luminous regions of the sun's atmosphere, indicative of violent agitation in their neighbourhood.

(332.) But what are the spots? Many fanciful notions have been broached on this subject, but only one seems to have any degree of physical probability, viz. that they are the dark, or at least comparatively dark, solid body of the sun itself, laid bare to our view by those immense fluctuations in the luminous regions of its atmosphere, to which it appears to be subject. Respecting the manner in which this disclosure takes place, different ideas again have been advocated.

Lalande (art. 3240.) suggests, that eminences in the nature of mountains are actually laid bare, and project above the luminous ocean, appearing black above it, while their shoaling declivities produce the penumbræ, where the luminous fluid is less deep. A fatal objection to this theory is the perfectly uniform shade of the pen umbra and its sharp termination, both inwards, where it joins the spot, and outwards, where it borders on the bright surface. A more probable view has been taken by Sir William Herschel*, who considers the luminous strata of the atmosphere to be sustained far above the level of the solid body by a transparent elastic medium, carrying on its upper surface (or rather, to avoid the former objection, at some considerably lower level within its depth,) a cloudy stratum which, being strongly illuminated from above, reflects a considerable portion of the light to our eyes, and forms a penumbra, while the solid body, shaded by the clouds, reflects none. The temporary removal of both the strata, but more of the upper than the lower, he supposes effected by powerful upward currents of the atmosphere, arising, perhaps, from spiracles in the body, or from local agitations. See fig. 1. d, Plate III.

(333.) The region of the spots is confined within about 30° of the sun's equator, and, from their motion on the surface, carefully measured with micrometers, is ascertained the position of the equator, which is a plane inclined 7° 20' to the ecliptic, and intersecting it in a line whose direction makes an angle of 80° 21′ with that of the equinoxes. It has been also noticed, (not, we think, without great need of further confirmation,) that extinct spots have again broken out, after long intervals of time, on the same identical points of the sun's globe. Our knowledge of the period of its rotation (which, according to Delambre's calculations, is 25d-01154, but, according to others, materially different,) can hardly be regarded as sufficiently precise to establish a point of so much nicety.

Phil, Trans. 1801.

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