The fact that comet orbits are parabolas explains their speedy disappearance.

The atmosphere of planet Mars is relatively deeper than that of the Earth, so that we, in observing the details of its surface, are looking down through an immense thickness of an obscuring medium.

The inclination of the axis of Mars is almost the same as that of the Earth.

Saturn is attended by a family of nine satellites, a larger number than belongs to any other planet, but with one exception they are exceedingly small and difficult to observe save with a very large telescope.

The most famous of all the markings in Jupiter is the Great Red Spot.