THE MOON’S MOTIONS.

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pare the sun’s mass with the earth’s. For preciselyas we have been able to show that under the influenceof terrestrial gravity the moon, at her distance, shouldfollow such a path as she actually traverses, so we candetermine how much a body should be deflected persecond at the earth’s distance from the sun, if hismass were equal to the earth’s; and by comparing thisamount with the actual deflection, we can compare thesun’s mass with the earth’s.

Or we may proceed in this way :—■

The earth, at a distance of 238,800 miles from themoon, has power to deflect the direction of the moon’smotion through four right angles in 27 - 322 days, themoon moving with a velocity which we may representby »“ * Now the sun at a distance from the earthequal to about 91,500,000 miles, has power to deflectthe direction of her motion through four right anglesm 365'256 days, the earth moving with a velocitywhich we may represent by No w, first, since

gravity varies inversely as the square of the distance,the sun would require (if other things were equal) tohave an attractive power exceeding the earth’s in thera ti° ( ^°y ) a to produce the same effect on her that sheproduces on the moonj and secondly, since the deflec-tion of a body’s line of motion is a work which will be

* We need not consider the velocity in miles per hour, or thelike; because, throughout the paragraph, relative and not absolutevelocities are in question. Hence we can represent the moon’svelocity by the radius of her orbit divided by her period, providedwe represent the earth’s velocity round the sun in like manner.