Since its orbital revolution occupies nearly twelve years, Jupiter comes back into opposition with the Sun every 399 days, _i.e._, 1 year, 34 days, that is with one month and four days' delay each year. At these periods it is located at the extremity of a straight line which, passing by the Earth, is prolonged to the Sun. These are the epochs to be selected for observation. It shines then, all night, like some dazzling star of the first magnitude, of excessive whiteness: nor can it be confounded either with Venus, more luminous still (for she is never visible at midnight, in the full South, but is South-west in the evening, or South-east in the morning), nor with Mars, whose fires are ruddy. In the telescope, the immense planet presents a superb disk that an enlargement of forty times shows us to be the same size to all appearance as that of the Moon seen with the unaided eye. Its shape is not absolutely spherical, but spheroid--that is, flattened at the poles. The flattening is 1/17. We know that the Earth's axis dips a certain quantity on the plane of her orbit, and that it is this inclination that produces the seasons. Now it is not the same for Jupiter. His axis of rotation remains almost vertical throughout the course of his year, and results in the complete absence of climates and seasons. There is neither glacial zone, nor tropic zone; the position of Jupiter is eternally that of the Earth at the season of the equinox, and the vast world enjoys, as it were, perpetual spring. It knows neither the hoar-frost nor the snows of winter. The heat received from the Sun diminishes gradually from the equator to the poles without abrupt transitions, and the duration of day and night is equal there throughout the entire year, under every latitude. A privileged world, indeed! It is surrounded by a very dense, thick atmosphere, which undergoes more extensive variations than could be produced by the Sun at such a distance. Spectral analysis detects a large amount of water-vapor, showing that this planet still possesses a very considerable quantity of intrinsic heat.