« 上一页继续 »
Many of the smaller islands are lovely to the eye, beyond all other lands, "made," says the bishop, “to rest in peace, like the light of stars reflected upon the surface of this tranquil ocean." But there is no true peace there; the inhabitants are heathens, and even cannibals, as the New Zealanders were forty years ago. The bishop has already made several voyages to some of these islands, and their people, won by the dignity and kindness of his demeanour, have intrusted to him some of their children, to be trained up under his direction, that they may return again to be the teachers of their countrymen.
SIMPLE DEFINITIONS IN ASTRONOMY.
The reader must attend carefully to the definitions given here, and in the lessons on Geometry, p. 235. The words in this and in the following Lesson printed in Italics are defined either here or in Lessons 75, 76.
ASTRONOMY is the science which gives an account of the heavenly bodies.
A sphere is a globe, such that every point on its surface is equally distant from a point within it. This point is called the centre of the sphere. A round ball is a sphere.
The axis of a sphere is a straight line, supposed to be drawn through the centre, to meet the surface each way.
The poles of a sphere are the extremities of the axis.
The earth is a sphere, or very nearly so.
The supposed line round which the earth turns, is called the axis of the earth.
The extremities of the axis of the earth are called the North and South Poles.
If a sphere be cut through by a plane in any direction, it is divided into two parts, the plane surfaces of which are circles.
If the cutting plane pass through the centre of the sphere, it divides it into equal parts, called hemispheres.
A circle may be drawn upon a sphere where the cutting plane would pass.
If the cutting plane passes through the centre, the circle is called a great circle.
If the cutting plane does not pass through the centre, the circle is smaller than a great circle, and is called a small circle. ́
The great circle, described as a cutting plane at right angles to the axis of the earth, is called the Equator.
The equator is equally distant from the North and South Poles.
The equator divides the earth into the Northern and Southern Hemispheres.
Circles of latitude are small circles parallel to the equator.
A meridian is a great circle passing through the poles.
The cutting plane which would form a meridian would pass through the axis from pole to pole. Meridians are also called circles of longitude.
A vertical line is a line drawn from any point on the earth's surface to its centre.
A plumb-line hangs vertically, because the earth draws all bodies to its centre.
A horizontal plane is a plane at right angles to a vertical line.
The surface of still water is horizontal.
As we stand upon the earth, we stand vertically, our feet towards the centre of the earth.
The point in the heavens, immediately above our heads is called the zenith of the place where
If we suppose a plane at our feet to extend to the vault of the heavens, it will seem to be bounded by a circle with a vast hemisphere above it. This boundary is called the horizon.
HYMN FOR THE EPIPHANY.
BRIGHTEST and best of the sons of the morning!
Star of the East, the horizon adorning,
Guide where our infant Redeemer is laid!
Cold on His cradle the dew-drops are shining,
Low lies His head with the beasts of the stall
Angels adore him in slumber reclining,
Maker and Monarch and Saviour of all!
Say, shall we yield him, in costly devotion,
Myrrh from the forest or gold from the mine?
Vainly we offer each ample oblation;
Vainly with gifts would His favour secure :
Dearer to God are the prayers of the poor.
Brightest and best of the sons of the morning!
Guide where our infant Redeemer is laid!
THE ANNUAL MOTION OF THE EARTH.* THE earth turns round its axis once in twenty-four hours.* It also moves round the sun once in a year.
Imagine a spherical humming-top spinning, but, instead of resting in one spot, suppose it to move in a circle round a lamp set upon the floor. This will give some idea of the two motions of the earth. The spinning round the axis represents the diurnal, or daily motion; the circular movement round the lamp, the annual or yearly motion, the lamp being the sun. The path in which the earth moves corresponding to the circle round the lamp is called the orbit of the earth.
Now, if the axis of the top be vertical, one-half of the top will always be light, and the other dark, and the light will be divided from the dark by a great circle passing through the poles. As the top turns round its
* See 'Third Reading Book' (New Series), Lesson 88, p. 279.
axis every part will in turn be light and dark. But if the axis at one part of the orbit were inclined a little towards the lamp, then as the top spun round the upper pole, and a small part of the top round it, would continue light, and the lower pole and the parts round it dark, so long as the top remained at that point in the orbit. The parts also nearer the upper pole would be light for a longer time than they would be dark, and those nearer the lower pole would be longer in the dark.
If the axis were always inclined in the same direction, when the top came to the point in its orbit exactly opposite to that we have just considered, the upper pole would now turn away from the lamp, and would be always dark, and the lower pole always light, and the parts which before were longer in the light would now be longer in the dark, and vice versâ.* But when the top is half-way between these two points in its orbit, the axis will not incline either to or from the lamp, and the circle which separates light from dark will pass through the poles, and every part of the top will be light and dark for the same time while the top spins round at these points.
A top moving in this way will give a more exact idea of the motion of the earth.†
* Vice versa, two Latin words meaning by a change of turns. The phrase is used when two things change places by turns. Thus "what was light is dark, and vice versâ” means what was light is dark and what was dark is light." The light and dark have changed places.
That this illustration may not give a wrong notion of the distance of the earth from the sun, it is proper to add that if the earth be represented by a common-sized orange, the distance from the earth to the sun, or the radius of the earth's orbit (in the same proportion), will be 1000 yards, or more than half a mile.