let. vi. Kepler and Galileo. gi

it is said to be in its perihelion ; and when at itsfurthest distance, or at B, it is said to be in itsaphelion, the mean distance being F C. In likemanner, when the earth is in its perihelium, thefun is said to be in its perigee; and when theearth is in its aphelium, the fun is said to be inits apogee.

But let us leave the further explanation ofthese terms for the present, and return again toKepler. This excellent astronomer, havingfirmly established the law before-mentioned, pro-ceeded to the consideration of another, of no lessimportance. He had happily conceived, thatthere might probably be some proportion be-tween the times of the revolution of the planets,and their distances from the fun ; and by prose-cuting the enquiry, which this idea suggested,his success was equal to his most sanguine ex-pectations. By calculations founded on a seriesof the most accurate observations, he discovered,that the squares of the times in which any twoplanets complete their revolutions in their orbits,are exactly proportional to the cubes of theirmean distances from the fun.

To illustrate this rule by an example: Venus,for instance, revolves round the fun in 224 days,and the earth in 365 ; and the mean distance ofthe earth from the fun is ninety-five millions ofmiles. Hence, according to Kepler, as the squareof 365 is to the square of 224, so is the cube of

ninety-