Fig. 5.
the pull of the band， was to make the path of the weight lie between the two. In this case the forces at every instant almost balanced each other， and the path was nearly a circle. It is the force of gravitation acting against the inertia， or the centrifugal force， of the heavenly bodies which holds them in their orbits.
4.Relation of the Earth to Sun and Moon. -Not only do all the planets revolve around the sun， but certain of these them- selves have other smaller bodies revolving around them. We call such small bodies satellites or moons. The earth has one of these satellites and Saturn has the greatest number of all， ten， one hav- ing been discovered as late as 1905. Our own moon has a diam- eter of about 2000 miles and a weight of about 1/80 that of the earth. Its average distance from the earth is about 240，000 miles. Compared with the distance of the other heavenly bodies it is in- deed very near.
The sun， although a near neighbor as compared with the rest of the stellar community， is at an average distance of about 93，000，000 miles. It is so big that if it were hollow and the earth were placed at its center with the moon as far away as it now is， there would be almost as great a distance between the moon and the sun"s surface as there is between the moon and the earth. A good way to get an idea of the relative size of these bodies is to let a pencil dot represent the moon， a circle an eighth of an inch in diameter the earth， and a circle with a diameter of a little more than thirteen and one half inches the sun.
SURFACE OF THE MOON.
Showing the great crater-like depressions.
Both the sun and the moon are of the greatest interest to us， as they have much to do with our existence. If it were not for the sun， we should have almost no heat or light on the earth， and life could not exist. If the sun were much nearer， it would be too hot for life as we know it， and if much farther away， too cold. If it were not for the moon， the beauty and variety of our nights would be largelylacking， and we should have no tides strong enough to sweep clean our bays， removing the sewage， and to help vessels over the bars into some of our harbors. If the distance of the moon were changed， the height of the tides would be changed， and this would greatly affect our coast towns.
Although we see the moon as a very bright object at night for a partof every month， yet it has no light of itself， and all the light it gives us is reflected from the sun. It has a rough， barren， rocky surface， full of great crater-like depressions. As far as known， it has no air or water upon it. As the earth goes around the sun， and the moon around the earth， the position of these three in relation to each other is constantly changing， and it is these changes which give us the varying heights of the tides and the different phasesHIGH TIDE IN Nova SCOTIA.
LOW TIDE AT THE SAME PLACE.
Showing the clean swept sea floor.
of the moon. It is profitable to try to picture to oneself the changing phases of the moon. A good way to do this is to carry a ball around a bright light and observe what part of the surface is illuminated in the different positions.
THE PHASES OF THE MOON.
Showing roughly the positions of the sun， moon， and earth.
Summary. -We have seen that the earth is one of eight planets which revolve about the sun. The sun and the bodies revolving around it comprise the solar system. The fixed stars， which we see only at night because of the great brilliancy of the sun in the daytime， are suns andmay-like our sun-be the centers of separate solar systems.
Everything in the universe is composed of matter， which has certain definite properties， like extension， inertia， gravitation， and so on. The action of some of these properties maintains the relation of the different heavenly bodies-keeps the earth revolving around the sun， and the moon around the earth.
The sun and the moon have more influence upon the earth than do any other bodies. The sun gives us energy in the form of light and heat and so maintains all the life upon the earth. The moon， though shining only by light reflected from the sun， exerts a great attraction upon the earth， causing the tides， which help to keep the waters of our harbors clean.
QUESTIONS
Why do we see no stars in the daytime？
How is the appearance of the stars explained？ What is the difference between stars and planets？ What starlike bodies make up the solar system？
Name and illustrate three universal properties of matter.
What daily experiences of yours are explained by these properties？ What forces keep the moon moving around the earth in its orbit？ Draw circles illustrative of the size of the earth， moon， and sun. Why are the sun and moon particularly interesting to us？
How long would it take an express train running thirty miles an hour and stopping neither day nor night to go over the distance from the earth to the moon？ From the earth to the sun？
【中文阅读】
1．傍晚的天空--到了傍晚，夜幕开始降临，太阳的光芒便在大地上渐 渐消褪，我们看见星星一颗一颗地眨着眼睛出现了，最后整个天空布满了繁 星。这时我们发现越亮的星星出现得越早，而到了早上，整个过程则正好相 反。星星开始按部就班地逐渐隐没于黎明的天空，最亮的星星也在最后消失于 天际。
我们都有这样的经验，一根火柴或蜡烛的光亮在黑暗的房间里会显得很 亮，而在电灯泡面前，它们就黯然无光了。看起来，太阳的万丈光芒对星星的 光亮很可能有同样的效应，这个看法也可以从日食的时候星星也像傍晚那样出 现得到印证。天文学家们也一定会同意一个结论：要是没有太阳耀眼的光芒， 我们便会随时看见满天的星星。
在我们地球的北半球，除了在北极我们看见星星在天空似乎以划圈的方式 运动以外，其他任何地方我们都看见星星像太阳一样，东升西落，永不停息。 如果注意观察这些从我们的东方、东南方、东北方升起的满天星斗，我们还会 发现它们是在不同的时间在地平线上升起的。
我们的先民们也注意到了这一点，他们富有创意地解释说，地球处于一个 空心天球的中心，星星就布满在这个天球的内表面上，这个天球还围绕着地球 不停地旋转，并轻微改变着它与地球的相对位置。而今天我们已经知道，地球 是在沿着一条假想的轴线自转，并不断地改变着与星星的相对位置。而且如果 这根自转轴延伸得足够长，它便正好可以穿过在北边天空中心的一颗星星，这 就是北极星。地球表面上与这跟轴线的交点就是我们熟悉的南北极。
