The New Zealand Reader
"Every schoolboy knows" that in England "the merry month of May" is the beginning of summer, and skating and snowballing are amusements for the Christmas holidays, while in New Zealand the month of May ushers in the winter, and at Christmas we may have a foretaste of harvest heat. We have our shortest days and longest nights when in Europe it is the time of longest days and shortest nights. It is well to know this, but better to know the reason of it.
This would be easy to explain if one page of a book would hold a diagram large enough to show the sun and the earth in their relative proportions, and at a distance proportionate to their bulk. But the fact is that, on a plan drawn to scale, if our globe were represented by a disc with a diameter of an inch the sun would be represented by a disc of about nine feet in diameter, and a circle drawn to represent the course of the earth in its annual orbit would have a diameter of about two thousand feet. It is necessary to have recourse to diagrams that ignore relative size or scale, and in which relative positions alone are accurately indicated.
Here is the elevation of a model which illustrates the relative positions of the sun and the earth at four different seasons of the year.
The circumscribing parallelogram is a section of a room, from the floor of which rises a stand supporting a lamp (S), partly hidden in the elevation by a shaded disc. This lamp represents the sun. A rod (RR') is so placed that its lower end (R) is exactly over the centre of the lamp, and this rod is held in position by a tube (T) in such a way that every part of the rod is equally distant from the wall behind. The tube is kept in position by an arm projecting from the back wall. In the elevation this arm, of course, is not seen. The rod has a wheel (W) fixed to it, which by means of a cord (C) led over a pulley on the side wall can be made to rotate, causing the rod also to rotate on its own axis. To page break page 273the fist rod another rod (RL) is fixed at right angles. From the outer end (L) of this second rod is suspended by a string a ball which represents the earth. The point at which the string is attached to the ball represents the north pole. A half-turn of the wheel brings the rod RL into the position indicated by the dotted line RL', and at the same time the ball is moved from the position D to the position J. The length of the string is such that the distance between the end (L) of the rod and the centre of the ball is equal to the distance from R to the centre of the lamp. In passing from D to J the ball goes behind the lamp. Another half-turn brings the ball again to the position D, and in the course of its motion it passes in front of the lamp, occupying for an instant the position indicated by the shaded disc halfway between J and D.
The position D represents the place of the earth about the 22nd of December. The light and shade represent day and night respectively. If the ball is made to spin by a slight twisting of the string, so that a spot at W comes round in front (from west to east) until it occupies the position E, and then goes round at the back till it reaches its original position, the model is thus made to illustrate the daily rotation of the earth. If the horizontal line We crosses the centre of the disc it represents the Equator, and a quarter of a turn (representing six hours) will bring the spot from W, its original position, to the line that divides the light from the shade. Another quarter of a turn will bring it to the position E; and, as it goes behind, a quarter of a turn will carry it out of the shade into the light, and the fourth quarter will restore it to its old position. During one-half of the rotation the moving spot will be in the light, and during the other half it will be in the shade. It is plain, then, that about the 22nd of December day and night at the Equator are equal. This, indeed, is the case all the year round.
But consider what occurs at the same time at the North Pole—represented by the point of attachment to the string. This point remains in the shade during the whole of the turn that carries a spot from W round in front to E, and then round behind to its first position. The point opposite to the North Pole is, on the contrary, in the light during the whole rotation.page 274
But when the system is turned round so far as to bring the ball into the position J, which represents the position of tha earth about the 21st of June, the model shows that the daily rotation cannot carry the North Pole into the darkness, and cannot bring the South Pole into the light.
The shaded disc, partly biding the lamp, represents the earth in the position it occupies about the 23rd of September. The shadow covers the whole disc, from the North Pole to the South Pole, and the light bathes the unseen half from north to south. The position for the 20th of March must be imagined as lying behind the lamp, and a moment's consideration is enough to satisfy us that, in that position also, one side of the ball from north to south must be illuminated, and the other side shrouded in darkness from north to south.
It will be observed that in the model the weight of the ball keeps the string always vertical. Any position occupied by the string as the model revolves is therefore parallel to every other position occupied by it; in other words, the axis of the ball maintains a constant direction. This is what really happens in the earth's annual revolution—one end of the axis always pointing to the north, and the other to the south. In most maps a north and south line is vertical, with the north end up. The model is adapted to this rule.
In the model, then, the axis of rotation has a constant direction, being always vertical. The axis of revolution, also has a constant direction, but its direction is not vertical, it is parallel to the direction of the rod RR' held in the tube. The line R'R is the axis of revolution of the point L, and a line parallel to R'R and passing through the centre of the lamp is the axis of revolution of the ball. Its direction is about 23½° from the vertical. These conditions of the model truly represent the conditions of the earth's daily rotation and annual revolution: (I) The axis of rotation has a constant direction; (2) the axis of revolution has a constant direction; (3) these two constant directions are not the same, but are mutually inclined at an angle of 23½°. This is, perhaps, when properly understood, as concise an explanation as can be given of the phenomena of the seasons.
The succeeding diagrams are on a larger scale. The diagrams for June and December are amplified representa-page 275tions of the conditions illustrated by the ball in the positions J and D in the main diagram. In the diagram for June it is seen that the rays of light coming from the sun, in the direction indicated by the arrows, merely graze the earth's surface at A and B, while they fall perpendicularly on C. At A and B they can, therefore, produce no effect of heat; at C they produce their full effect. It is also plain that if a place is in the position A at midnight it "will in twelve hours move along the line A R, which is the Arctic Circle, and come to the position R, which is the position of the place at noon. It is plain, again, that about the 21st of June no place north of this line A R will at any time during the twenty-four hours lose the sunlight. It is equally plain that no place in the line BQ, which is the Antarctic* Circle, can at that time emerge from the shadow of night. When at this time of the year any place is at C not only will it enjoy the light of noonday, bub that light will fall perpendicularly on it from overhead, and its noonday will be the noon of a summer day. The line C N' represents the Tropic of Cancer, and every place on that line must come during the day under the perpendicular rays of sunlight. If a place is at N' at midnight it will arrive at the line NS at six in the morning; but long before that it will arrive at the line AB, and enter into the daylight.
The diagram for December shows that about the 22nd of that month no place north of the Acetic Circle can on that day have any daylight, and no place south of the Antarctic Circle can lose the daylight. It is plain, too, that the perpendicular rays fall on the point C' so that every place on the circle C'P'—the Tropic of Capricorn—comes once in the day directly under the sun. Since the perpendicular rays fall 23½° south of the Equator, and the grazing rays touch the point A, leaving the North Pole in darkness, and touch the point B, so that the South Pole is always in the light, this must be the time of northern winter and southern summer.
The regions north of the Arctic Circle and south of the Antarctic are the Frigid Zones; those that lie between the northern tropic (of Cancer—CN') and the southern tropic (of Capricorn—C'P') constitute the Torrid Zone; the page 276regions between the northern tropic and the Arctic Circle, and between the southern tropic and the Antarctic Circle, are the North and South Temperate Zones. England lies-in the North Temperate Zone; New Zealand in the South Temperate Zone.
The diagrams for March and September correspond to the appearance the ball would present to an observer standing near the pulley in the main diagram when the ball is between the lamp and the back wall (for March), and when it is in front of the lamp (for September). In these two positions the rod (RL) lies horizontal. The sun's rays, indicated by the arrows, graze the North and South Poles alike, and the direct (perpendicular) rays fall on the Equator at E. The north-and-south line is at these times the day-and-night line also, and the daily rotation brings all places from the midnight position, represented by the convex edge of the shaded half of the disc, to the day-and-night line in the interval between their midnight and six o'clock of their morning; and again in six hours it brings them to their noon position, represented by the convex edge of the bright limb. Day and night are of equal duration every where at the times to which these two diagrams relate, and therefore these times are called the equinoxes (nox is Latin for night). The times to which the June and December diagrams relate are called the solstices, because then the sun (sol) comes to a stand (sto, I stand). His direct rays fall on the Equator in March, but as the year wears on towards June the North Pole is tilted gradually towards the sun, and the places of the incidence of the direct rays are therefore north of the Equator, and day by day the direct rays fall further north, until at last, in June, they fall on the Tropic of Cancer. Then the process is arrested, and the reverse process begins. Day by day the direct rays fall further south, so that in September they strike the Equator again; and still further south they fall until December, when the southern progress comes to an end. The times at which these processes—of northward and southward progress—are arrested or stayed are the solstices, the slayings or standings of the sun.
The reasons for summer heat and winter cold are two, though they both depend on the motions that have been described. First, the sun's rays are more direct in summer page 277than in winter; secondly, the summer days are longer than the summer nights, and the winter nights are longer than the winter days. These phenomena have a great effect on local temperature. But climate does not depend entirely on the phenomena of summer and winter. Countries near the sea are more temperate than those that are remote from it; because water takes longer to warm and longer to cool than solid bodies. Where the air is dry the sun's rays have more power, and their absence is more keenly felt than in a damp climate. High lands are colder than low lands, because the lower air is packed closer than the upper air by the greater mass of air above it, and because (for reasons that cannot here be given) the less closely packed a gas is the cooler it is. And, besides, prevailing winds are hot or cold according as they come from tropical or frigid regions, and according as they blow across water, or heated land, or snowy heights. Further, sea-currents impinging on the land modify the temperature of the air according as they bring warmer water from the equatorial belt, or colder water from the neighbourhood of the poles. But, after all, climate depends principally on those relations of the earth and sun by which the seasons are determined, and by which the limits of the temperate zones are fixed.
Note—In Professor Bickerton's admirable model the plane of the Equator is represented as oblique, and the plane of the ecliptic as horizontal. That arrangement, as compared with that of the model here described, has the advantage of better representing the comparative stability of the plane of the ecliptic. There is a slow alteration in the direction of the earth's axis, in consequence of which the meaning of the word north as applied to the heavens is slowly changing in a cycle extending over about 26,000 years.
* Do not say a "Antartic": there is ac before the second t.