Page images

the luminary appearing to rise higher and higher in the heavens with a westerly motion, as the observer is carried forward easterly by the earth's diurnal rotation to A, where he has the sun in his

[merged small][graphic][merged small]

places on

earth, and he is

I to D, the

meridian, and it is consequently noon. The sun then begins to decline in the sky until the spectator arrives at B, where it sets, or is again in the horizon on the west side, and night begins. He moves on to C, which marks his position at midnight, the sun being then on the meridian of places on the opposite part of the earth, and he is then brought 'round again to D, the point of sunrise, when another day commences. (Hind.)

The unequal rate of diurnal motion.—Different points upon the surface of the earth revolve with different velocities. At the two poles the speed of rotation is nothing, while at the equator it is greatest, or over 1,000 miles per hour. At Quito, the circle of latitude is much longer than one at the mouth of the St. Lawrence, and the velocities vary in the same proportion. The former place moves at the rate of about 1,038 miles per hour; the latter, 450 miles. In our latitude (41°) the speed is about 780 miles per hour. We do not perceive this wonderful velocity with which we are flying through the air, because the air moves with us* Yet were the earth suddenly to stop its rotation, the terrible shock would, without donbt, destroy the entire race of man, and we, with houses, trees, rocks, and even the oceans, in one confused mass, would be hurled headlong into space. On the other hand, were the rate of rotation to increase, the length of the day would be proportionately shortened, and the weight of all bodies decreased by the centrifugal force thus produced. Indeed, if the rotary movement should become swift enough to reduce the day to 84 minutes, or about 1 its presont length, the force of gravity would be overcome, and, at the equator, all bodies would be without weight; if the speed were still further increased, loose bodies would fly off from the earth like water from a grindstone when swiftly turned, while we should be compelled constantly to "hold on” to avoid sharing the same fate. But against such a catastrophe we are assured by the immutability of God's laws. “He is the same yesterday, to-day, and forever.” The earth has not varied in its revolution to of a second in 2,000 years. (Note, p. 324.)

* An ingenious inventor once suggested that we should utilize the earth's rotation, as the most simple and economical, as well as rapid mode of locomotion that could be conceived. This was to be accomplished by rising in a balloon to a height inaccessible to aerial currents. The balloon, remaining immovable in this calm region, would simply await the moment when the earth, rotating underneath, would present the place of destination to the eyes of travellers, who would then descend. A wellregulated watch and an exact knowledge of longitudes would thus render travelling possible from east to west, all voyages north or south naturally being interdicted. This suggestion has only one fault; it supposes that the atmospheric strata do not revolve with the earth. Upon that hypothesis, since we rotate in our latitude with the velocity of 333 yards in a second, there would result a wind in the contrary direction ten times more violent than the most terrible hurricane. Is not the absence of such a state of things a convincing proof of the participation of the atmospheric envelope in the general movement ? (Guillemin.)

Fig. 29.

[merged small][ocr errors]

Z Unequal diurnal orbits of the stars.—Let O represent our position on the earth's surface, E Z B our meridian; EIB K our horizon ; P and P' the north

and south poles, Z the zenith, Z' the nadir; and GICK the celestial equator. Now PB, it will be seen, is the elevation of the north pole above the horizon, or the latitude of the place. Suppose we shonld see a star at A, on the meridian below the pole. The earth revolves in the direction GIC; the star will therefore move along A L to Z, when it is on the meridian above the pole. It continues its course along the dotted line around to A again, when it is on the meridian below the pole, baving made a complete circuit around the pole, but not having descended below our horizon. A star rising at B would just touch the horizon; one at I would move on the celestial equator, and would be above the horizon as long time as it is below—twelve hours in each case; a star rising at M, would just come above the horizon and set again at N.

Unequal diurnal velocities of the stars. The stars appear to us to be set in a concave shell which rotates daily about the earth. As different parts of the earth really revolve with varying velocities, so the stars appear to revolve at different rates of speed. Those near the pole, having a small orbit, revolve very slowly, while those near the celestial equator move at the greatest speed.

Appearance of the stars at different places on the earth.Were we placed at the north pole, Polaris would be directly overhead, and the stars would seem to pass around us in circles parallel to the horizon, and increasing in diameter from the upper to lower ones. Were we placed at the equator, the pole-star would be at the horizon, and the stars would move in circles exactly perpendicular to the horizon, and decreasing in diameter, north and south from those in the zenith, while we could see one half of the path of each star. Were we placed in the southern hemisphere, the circumpolar stars would rotate about the south pole, and the others in circlos resembling those in our sky, only the points of direction would be reversed to correspond with the pole. Were we placed at the south pole, the appearance would be the same as at the north pole, except that there is no star to mark the direction of the earth's axis.

MOTION OF THE EARTH IN SPACE ABOUT THE SUN.The earth revolves in an elliptical path about the sun at a mean distance of 913 million of miles. This path is called the ecliptic; its eccentricity, which is greater than that of the orbit of Venus, changes about 1000000 per century, so that in time the orbit would become circular, were it not that after the lapse of some thousands of years, the eccentricity will begin to increase again, and will thus vary through all ages within definite, although yet undetermined limits. Its entire circumference is nearly 600,000,000 miles, and the earth pursues this wonderful journey at the rate of 18 miles per second. This revolution of the earth about the sun gives rise to various phenomena, of which we shall now proceed to speak.

[ocr errors]

greater the the ecliptic.

« PreviousContinue »