Observe, for instance, Jupiter, on any clear night when the heavens can be well seen, and note his position with regard to the constellations in his neighbourhood--how he is to the right of this star, or to the left of that; directly between this pair, or directly pointed to by that. We then mark down the place of Jupiter on a celestial map, or we make a sketch of the stars in the neighbourhood showing the position of the planet. After a month or two, when the observations are repeated, the place of Jupiter is to be compared again with those stars by which it was defined. It will be found that, while the stars have preserved their relative positions, the place of Jupiter has changed. Hence this body is with propriety called a _planet_, or a wanderer, because it is incessantly moving from one part of the starry heavens to another. By similar comparisons it can be shown that the other bodies we have mentioned--Venus and Mercury, Saturn and Mars--are also wanderers, and belong to that group of heavenly bodies known as planets. Here, then, we have the simple criterion by which the earth's neighbours are readily to be discriminated from the stars. Each of the bodies near the earth is a planet, or a wanderer, and the mere fact that a body is a wanderer is alone sufficient to prove it to be one of the class which we are now studying. Provided with this test, we can at once make an addition to our list of neighbours. Amid the myriad orbs which the telescope reveals, we occasionally find one which is a wanderer. Two other mighty planets, known as Uranus and Neptune, must thus be added to the five already mentioned, making in all a group of seven great planets. A vastly greater number may also be reckoned when we admit to our view bodies which not only seem to be minute telescopic objects, but really are small globes when compared with the mighty bulk of our earth. These lesser planets, to the number of more than four hundred, are also among the earth's neighbours. We should remark that another class of heavenly bodies widely differing from the planets must also be included in our system. These are the comets, and, indeed, it may happen that one of these erratic bodies will sometimes draw nearer to the earth than even the closest approach ever made by a planet. These mysterious visitors will necessarily engage a good deal of our attention later on. For the present we confine our attention to those more substantial globes, whether large or small, which are always termed planets.