THE M00X.
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be further to the east of the star by rather more than its owndiameter; in two hours by more than twice its own diameter,and so on ; and this rate of motion takes it once round theheavens from west to east in about 27| days.
The Moon’s mean distance from the Earth being rather lessthan 240,000 miles, and the circumference of a circle of thatradius being about 1,500,000 miles, it follows that its averagespeed in journeying round the Earth must be at the rate ofrather under 55,000 miles per day, or of about 2,280 miles perhour. But the Earth itself is all the while revolving round theSun , with a speed of about 66,000 miles per hour. In order tokeep in close attendance upon it, the Moon must therefore, inaddition to its above-mentioned velocity round the Earth , alsopossess a velocity equal to that of the Earth round the Sun .
The Moon ’s motion is consequently compounded of twovelocities; one of about 66,000 miles per hour round the Sun ;the other of about 2,280 miles per hour round the Earth .
We are at once struck with the vastly greater magnitudeof the former. It shows that, so far as regards its positionin space at any given time, the Moon ’s motion round theSun is much more important than its motion round theEarth. We are consequently led to enquire whether the Sun or the Earth exerts the greater influence on the Moon . Anda little calculation will show, in opposition to what might atfirst sight have seemed probable, that the Sun has more thantwice as much effect as the Earth in attracting the Moon .
The weight of the Sun is about 330,000 times that of theEarth ; but its distance from the Earth being about 390 timesas great as that of the Moon , its attractive effect is therebyproportionally diminished, the diminution depending upon thesquare of the distance. If we square the number 390 theresult is 152,100. It consequently follows that instead of theSun’s power over the Moon being 330,000 times that of theEarth , we must, in order to get the true value, divide thisnumber by 152,100. The quotient is 2 and a fraction, whichproves that the Sun’s power over the Moon is after all morethan twice as great as that of the Earth .