Mercury and Venus are called _inferior_ planets, because they have their orbits nearer to the sun than that of the earth; while all the others, being more distant from the sun than the earth, are called _superior_ planets. The planets present great diversities among themselves, in respect to distance from the sun, magnitude, time of revolution, and density. They differ, also, in regard to satellites, of which, as we have seen, three have respectively four, six, and seven, while more than half have none at all. It will aid the memory, and render our view of the planetary system more clear and comprehensive, if we classify, as far as possible, the various particulars comprehended under the foregoing heads. As you have had an opportunity, in preceding Letters, of learning something respecting the means which astronomers have of ascertaining the distances and magnitudes of these bodies, you will not doubt that they are really as great as they are represented; but when you attempt to conceive of spaces so vast, you will find the mind wholly inadequate to the task. It is indeed but a comparatively small space that we can fully comprehend at one grasp. Still, by continual and repeated efforts, we may, from time to time, somewhat enlarge the boundaries of our mental vision. Let us begin with some known and familiar space, as the distance between two places we are accustomed to traverse. Suppose this to be one hundred miles. Taking this as our measure, let us apply it to some greater distance, as that across the Atlantic Ocean,--say three thousand miles. From this step we may advance to some faint conception of the diameter of the earth; and taking that as a new measure, we may apply it to such greater spaces as the distance of the planets from the sun. I hope you will make trial of this method on the following comparative statements respecting the planets. _Distances from the Sun, in miles._ 1. Mercury, 37,000,000 2. Venus, 68,000,000 3. Earth, 95,000,000 4. Mars, 142,000,000 5. Vesta, 225,000,000 6. Juno, } 7. Ceres, } 261,000,000 8. Pallas, } 9. Jupiter, 485,000,000 10. Saturn, 890,000,000 11. Uranus, or Herschel, 1800,000,000 The _dimensions_ of the planetary system are seen from this table to be vast, comprehending a circular space thirty-six hundred millions of miles in diameter. A rail-way car, travelling constantly at the rate of twenty miles an hour, would require more than twenty thousand years to cross the orbit of Uranus. _Magnitudes._ Diam. in miles. 1. Mercury, 3140 2. Venus, 7700 3. Earth, 7912 4. Mars, 4200 5. Ceres, 160 6. Jupiter, 89,000 7. Saturn, 79,000 8. Uranus, 35,000