The foregoing are called the primary planets. Several of these have one or more attendants, or satellites, which revolve around them as they revolve around the sun. The Earth has one satellite, namely, the Moon; Jupiter has four; Saturn, seven; and Uranus, six. These bodies are also planets, but, in distinction from the others, they are called secondary planets. Hence, the whole number of planets are twenty-nine, namely, eleven primary, and eighteen secondary, planets.
You need never look for a planet either very far in the north or very far in the south, since they are always near the ecliptic. Mercury, which deviates furthest from that great circle, never is seen more than seven degrees from it; and you will hardly ever see one of the planets so far from it as this, but they all pursue nearly the same great route through the skies, in their revolutions around the sun. The new planets, however, make wider excursions from the plane of the ecliptic, amounting, in the case of Pallas, to thirty-four and a half degrees.
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 | 6. Juno, | } | |
| 2. Venus, | 68,000,000 | 7. Ceres, | } | 261,000,000 |
| 3. Earth, | 95,000,000 | 8. Pallas, | } | |
| 4. Mars, | 142,000,000 | 9. Jupiter, | 485,000,000 | |
| 5. Vesta, | 225,000,000 | 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. | Diam. in miles. | ||
| 1. Mercury, | 3140 | 5. Ceres, | 160 |
| 2. Venus, | 7700 | 6. Jupiter, | 89,000 |
| 3. Earth, | 7912 | 7. Saturn, | 79,000 |
| 4. Mars, | 4200 | 8. Uranus, | 35,000 |
We remark here a great diversity in regard to magnitude,—a diversity which does not appear to be subject to any definite law. While Venus, an inferior planet, is nine tenths as large as the earth, Mars, a superior planet, is only one seventh, while Jupiter is twelve hundred and eighty-one times as large. Although several of the planets, when nearest to us, appear brilliant and large, when compared with most of the fixed stars, yet the angle which they subtend is very small,—that of Venus, the greatest of all, never exceeding about one minute, which is less than one thirtieth the apparent diameter of the sun or moon. Jupiter, also, by his superior brightness, sometimes makes a striking figure among the stars; yet his greatest apparent diameter is less than one fortieth that of the sun.
Periodic Times.