Direct observation with the help of the telescope enables us to discover the surface details of the planets and any changes which take place in them. Such changes in most cases imply the existence of an atmosphere. Observations of occultations of stars by the Moon or the planets lead to the same result. Theoretical astronomy tells us the distance between the planets and the Sun; physics tell us the quantity of light received by each planet from the Sun; and the period of revolution and the inclination of the planet’s axis tell us about the course of its seasons. Photometry gives us the amount of sunlight reflected by the surface of the planet, and thus furnishes indications concerning certain properties of the planetary surface, properties which permit us to decide with certainty, for instance, whether light is reflected by a solid surface such as the ground or whether the rays do not penetrate so far and are sent back in the upper parts of the atmosphere by lays of clouds. It is spectrum analysis which furnishes, as we know, the most important auxiliary information; it presents the heavenly bodies to the eye of the mind as the microscope unveils to the eye of the body the marvels of the infinitely small. The rays of light are messengers who, having passed through the spectroscope, bring to us news of the most distant worlds and tell us of the temperature of the fixed stars, of the metals volatilised in their atmospheres, of the incredibly low temperature of the nebulæ and of the gases which envelop the planets. We do not here wish to intone a hymn to spectrum analysis; we only wish to report briefly and simply what we know of the physical nature of the celestial bodies. But we must admit that the greatest amount of that knowledge is due to the spectroscope. THE PLANETS OF OUR SYSTEM (We continue the translation of the essay of the Potsdam astronomer.) In the light of contemporary astronomical knowledge let us make a rapid survey of the other worlds. _The Moon_