planets upon our earth, and by the earth upon the moon; and, lastly, by the fixed stars upon the sun and upon each other.
We begin with the moon because it is nearest to us, and, with the exception of the sun, is to us the most important of all the heavenly bodies.
If a half-dozen persons were asked how large the moon appears, they would give as many different replies: "The size of a cart-wheel"; "Twelve inches across"; "The size of a dining-plate"; "As big as a man's head," etc. Probably no one would mention a smaller measure, yet a cherry held at arm's length much more than covers its disk. It is difficult to believe that so small a body exerts any considerable influence on the earth which seems so immensely larger. It is easy enough to admit that the earth holds the moon in its orbit; but, that to do this, to bend its course into a nearly circular orbit, requires any great outlay of force, is not so clear. Our credulity would be taxed were we asked to believe that the moon in its efforts to move in a straight line would break away, although held by a bar of steel one foot square, for that means a force able to lift nearly 9,000 tons. An astronomer would grant it, making first a mental calculation to see if he were justified in doing so; but even he would hesitate, and perhaps would deny that it was possible the moon could pull asunder one of those great unit-bars one mile square, and equal to more than 27,000,000 bars each one foot square.
But he would have no hesitation in saying, "Impossible!" if told that, rather than change its course from a straight line to its present curve, our willful little satellite would snap like pack-thread not one, nor two, nor three of those unit-bars, but the united strength of 10,000—or, in other words, one gigantic bar whose section is 100 miles square. Yet, more than eight such bars, or, more precisely, 87,500 unit-bars, would but barely deflect the moon into its present path.[1]
You will say, "This is too much—no one will believe it!" Let us see. A few astronomical facts, with a very small amount of mathematics, will suffice to show that there is no exaggeration here. One need know only the weight of the earth and moon, and their distance apart, and the law that gravitation grows less as the square of the distance increases, and he has all the elements required for the calculation.
The weight of the earth is found by an experiment described in almost every school philosophy. It consists in comparing the attraction exerted by a ball of lead of known weight with that exerted by the earth. In this way the earth's weight has been ascertained to
- ↑ The non-astronomical reader may, perhaps, need to be reminded that the moon does not move easily and naturally in a circle—or ellipse—but that its path, if left to itself, would be a straight line—a tangent to its orbit. Consequently, the moon requires to be forced into a curve.
