CHAP. XLIX.

ABOUT THE ORBITS OF THE PLANETS.

I MUST not omit to tell you that the orbits or paths in which the planets move, are not perfect circles, but on the contrary, are all elliptical; that is, they are a little longer one way than the other. The following are pictures of a circle and an ellipsis. The ellipsis shows the form of a planet's orbit.

Circle.

Oval, or Ellipsis.

The planetary orbits, as I have told you, are ovals, or ellipses; and it is in that elliptical figure alone, that either the primary planets round the sun, or the moon or other satellites round the earth, or round the other primary planets, do really move. The sun, or earth, or other planet, (s) is not really the centre (c) of their motion, or of their orbit, A D B E; but is always far away upon one side, (as s c), or else far away upon the other, as c s; and it is from this same figure of the planetary orbits, that the planets, primary and secondary, are always vary

ing, as they perform their revolutions, their distances from the sun, or from the planets round which they revolve.

You can easily discern, for example, that when a planet is at A or B, it is much nearer to the sun, or the earth or other planet, s or s, than when it is at D or E.

CHAP. L.

ABOUT COMETS.

I HAVE told you

of thirty heavenly bodies revolving round the sun; that is, of twelve primary planets, and eighteen moons, satellites, or secondary planets. But these are by no means all the bodies belonging to the Solar System. It is even probable, that there are still many planets, whose size is so small, or their distance from the earth so great, that they have not yet been perceived. We must certainly add, however, those that are among the most remarkable of the heavenly bodies, and which we call comets. These appear like feeble stars, with fiery or at least luminous appendages. Sometimes these appendages are faint in light, and short in length. Sometimes they are so long as to reach half

What are among the most remarkable of the heavenly bodies?

across the sky. The tail of a comet seen in the year 135 of the christian era, is said to have been as long as the Milky Way.

Sometimes comets appear very small, and like dim stars. Sometimes they are larger, and it is recorded that they have been seen as large as the moon, and even as the sun. Like the planets, they belong, as I have said, to the Solar System.

Comets appear only occasionally, or at certain intervals of time, embracing a greater or less number of years. I, however, have seen several. When I was a boy, I recollect that there was a very brilliant comet. Many people were very much alarmed, for fear the world was to be burned. I remember to have heard of an old woman, who drowned herself, because she was afraid of being burned to death by the comet!

Are the tails of comets long, or

How do comets appear? short? Are comets of different sizes?