In 1878 it was again thought that two bodies nearer to the sun than Mercury had been discovered during an eclipse. These observations have never been explained or confirmed; but it is thought that the objects seen were probably stars which were mistaken for planets by the observers. If a body so situated does exist, it is so near the sun that it probably can never be seen except during an eclipse, and the time of observation is then so short and mistakes are so easily made that it is difficult to verify the observation. The continued search for the cause of the perturbations of Mercury may finally lead to the discovery of something between it and the sun. But if it is a single body, this seems a much less promising task than the search for a planet, or planets, on the outer edge of the solar system. VI THE MOVEMENTS OF THE PLANETS In considering the movements of the planets, we have to regard their actual motion in space and that motion as it appears to us. They all have two principal motions in space. They revolve about the sun in their orbits, and they rotate on their axes. The manner in which they accomplish the rotation on their axes determines the length of their days and nights, or whether, indeed, they shall have any such grateful alternations of light and darkness. Those planets which, like the earth, turn on their axes in less time than they make their journey around the sun have one day and one night every time they make a complete rotation. Those that turn on their axes in the same time that they revolve around the sun, of which sort there seems to be at least one, face always toward the sun, and have no alternations of day and night. On one side it is always day; on the other it is always night. The number of days a planet has during each revolution around the sun depends upon how much time it requires to make a revolution, and how fast it spins on its axis. In one year here on the earth we have three hundred and sixty-five days and nights. Saturn, in its year, has more than twenty-three thousand days and nights.