INTRODUCTION.

THE object of the series of pictures and maps in the present work is to illustrate

more fully and satisfactorily than has hitherto been done, the cause and nature of the seasons, and to bring clearly before the student's mind the nature of the sun's path along the ecliptic, the central track of the great zodiacal zone. For this purpose I show first, in the frontispiece, the path of the earth around the sun, then in a series of twelve plates the varying presentation of the earth's surface towards the sun at four different hours of the day, in each of the twelve months of the year. These forty-eight views, each repeated three times, in order that the relations of the seasonal changes may be more clearly recognised, are followed by a series of views of the British Isles at midday in winter, spring or autumn, and midsummer, and at six morning and evening at midsummer. All these views may be called Sun Views of the Earth. They are followed by twenty-four zodiacal maps,-viz., twelve maps showing the zodiacal zone with the ecliptic, meridians, and declination parallels, and the sun's course from day to day all the year round, the sun's disc being shown of its true size and in its true place for each day in the year: the stars, meridians, &c., are shown in these maps dark on a white ground. Then follow twelve maps showing the zodiacal zone with stars only (meridianal and ecliptic divisions are marked in as minute crosses), the stars showing white on a dark ground.

Taking the frontispiece-Plate I.—first, we see in it the earth's path ABA ́B ́ around the sun at S, the point C being the centre of the path, AA' its longer, BCB' its shorter axis. The eccentricity SC is very obvious, on the scale of our diagram, but the ellipticity is not noticeable: in other words CA and CA' are not measurably different in length from CB and CB', or AA' from BB'. This many find inconsistent with the idea derived from the familiar statement that the earth's path is an ellipse; for the path seems to be a circle around C as centre. The path is really elliptical; but, on the scale of our picture, the ellipticity is not recognisable. Speaking with sufficient closeness to the facts for our purpose here, SC is about the sixtieth part of CA or CA', and SB is equal to either CA or CA'. Hence, since SCB is a right-angled triangle, the square on CB is less than the square on SB by the square on CS, or by only 1-3,600th part of the square on SB. Whence it follows (the student can readily satisfy himself of this by calculation) that CB is less than SB by only 1-7,200th part of SB. If AA' were 7 in. (it is really only about 62 in.), BB' would be less by only 1,000th part of an inch.

Placing ourselves in imagination at S, we should see the earth carried round

one day, and appearing to turn once round on its axis each day. I say appearing so to turn because as a matter of fact it is only to an observer at S that an exact rotation seems to be made in each day of our ordinary time. Evidently the earth has to turn a little more than once round to bring the same face towards S, when she has reached the end of each day's travel, as she turned the day before. In a quarter of her annual circuit he shas, in all, made one extra quarter rotation in this way; in half of her circuit, a half rotation; and in the whole year a full rotation more. Thus the earth turns on her axis not 365 but 366 times in a year, or 3651 solar days are equivalent to 3661 true rotations,-called sidereal days because they are the days we should have if we considered the earth's rotation as supposed to be seen from a star. Viewing the matter from the earth instead of the sun we may say that the sun appears to take a longer time in returning to the south, day after day, than a star does; the reason being that as viewed from the earth the sun is all the time travelling eastwards, or in a direction contrary to that of his apparent daily motion; and as he makes one eastward circuit in a year, he loses one apparent circuit of the heavens in making 3651. The mean solar day is thus longer than the sidereal day, or true rotation-period of the earth, by slightly less than 1-366th part of a day, or about four minutes.

Now let us suppose our observer at S, the sun, to watch the earth as she traverses the orbit ABA'B', considering first her apparent axial pose only. As her real axial pose remains unchanged during her entire circuit (for we may neglect that slow reeling movement which is accomplished in 25,900 years, as not producing any change in a single year,—at least none which need be considered here) it is manifest that the apparent axial pose will vary precisely as though the earth remained at rest, and our observer travelled round her. But also, we get just the same changes in the appearance of an object by travelling around it, as we would if the object were turned round on an axis square to our path of circuit. So we shall get all the changes of the earth's apparent axial pose, as viewed from the sun, by supposing her centre at rest, and her globe twirled around an axis square to the plane of the ecliptic:

K

E

B
FIG. 1.

Thus suppose PEP'E' a view of the earth on about March 20, when the polar axis is as shown, and the equator seen like a straight line across the centre of the disc, as EOE', the arrowed line KOL representing the earth's motion as watched from the sun-Then E will be brought into view and E' carried out of view as the

earth goes on; P, the north pole, being brought within the visible part of the earth, and P', the south pole, being carried to the invisible part. It is clear that we simplify our study of these changes, without any loss of fulness, by supposing the earth's globe to turn round the upright axis AB (square to KOL).

It is obvious that E will be carried along the line ECe parallel to KL, and E' along the corresponding line E'C'e' on the unseen half of the earth. The pole P will be carried along the line Pp on the visible half, and the pole P' along the corresponding line p'P' on the unseen half, of the earth, Pp and P'p' being parallel to KOL.

The actual changes in the earth's aspect from month to month are shown in the following figures:

Oct. 23.

Nov. 22.

Dec. 21.

Jan. 19.

Feb. 18.

March 20.

Sept. 23. FIGS. 2-15, ILLUSTRATING THE EARTH'S VARYING PRESENTATION TOWARDS THE SUN THROUGHOUT THE YEAR.

Now, the first twelve plates in this work show the earth in these twelve aspects, at four hours in each day, viz., at 6 A.M., noon, 6 P.M., and midnight, Greenwich mean solar time, each plate giving the aspects during three successive months, so that the progress of the change in the earth's aspect can be traced on from month to month, but always with the aspect for the preceding and following months visible for comparison.

Observe also that any face of the earth shown for any named date and hour represents the face of the earth turned from the sun at that hour six months later or earlier than the dates named on the maps; or the face of the earth turned directly towards the direction in which the earth is travelling, at that hour, three months later; or the face turned directly from the direction in which the earth is travelling,

To illustrate the effect of these changes in modifying the amount of sunlight received by any part of the earth's surface, Plate XIV. is added, in which we see the aspect of the British Isles, Denmark, and parts of France, Sweden, Germany, &c., at midday in midwinter, spring, or autumn, and midsummer, and also the aspect of the same regions at six on a midsummer morning, and at six on a midsummer evening. Again, viewing the earth from S, we see from the frontispiece, that she travels from right to left, or from west to east among the stars. The names of the signs outside the orbit indicate the directions in which the various signs lie as seen from S. On or about December 21 the earth as seen from the sun enters the sign Cancer, passing out of Cancer and into Leo on about January 19, into Virgo on about February 18, into Libra on about March 20, and so on.

The frontispiece would also represent the apparent path of the sun around the earth, if a very much smaller dot were set at S, and a larger series of dots round the orbit, to represent the larger sun circling around the smaller earth. But another change would have to be made. The names of the signs would have to be altered, each into the name now on the opposite side of the orbit. For it is clear that whereas between December 21 and January 19, the earth, seen from the sun appears to be passing across the zodiacal region Cancer, the sun, seen from the earth between those days, appears to be passing across the zodiacal region Capricornus.

The sun's path and his position from day to day, are shown in the twelve zodiacal maps forming Plates XV., XVI., XVII., XVIII., XIX., and XX., while the same zodiacal regions are shown, on the same scale, but without meridians and parallels, in the series of Plates XXI., XXII., XXIII., XXIV., XXV., and XXVI. It will be observed that each of the twenty-four zodiacal maps shows a sign of the zodiac, and on either side of the sign a strip five degrees wide belonging to the sign

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