Dreams of an astronomer

by Camille Flammarion

EVERYBODY notices, year by year, in a southerly direction, a bright star which half the spectators believe to be the evening star, but which so far from being Venus, is its very antithesis.

To mistake Jupiter for Venus is a sufficiently gross error. But even this error is preferable to nothing. It is better to be mistaken than not to see anything. A large portion of the human race sees nothing at all, thinks nothing, and passes its whole life in the stupidity of plants and slugs.

To notice the star in the sky, even to give it a wrong name, is something, at all events. It shows that one does not go about with eyes cast on the ground or occupied with worldly affairs and with the spirit absorbed by material interests.

Venus, the Shepherds’ Star, the morning and evening star, is never far from the sun. It is never in the south, neither in the evening nor at midnight. Its orbit round the Sun lies within our own. Jupiter, on the contrary, revolves round the same Sun along an orbit outside our own, five times as far as the central luminary, at 485 million miles instead of 92 million miles. The distance between us and Jupiter is therefore always greater than 393 million miles except when the Earth reaches the extremity of its elliptical orbit (“aphelion”) at a time when Jupiter is in “opposition” to the Sun. We often see it at a medium distance of 450 million miles. If the atmosphere extended over all that distance, an aeroplane flying without a stop at 62 miles an hour, and therefore covering 1,500 miles a day, would take no less than 300,000 days or 812 years to complete the voyage. It would be wise to take some provisions with us!

But on what sort of a world should we arrive?

A giant world, an immense world, a strange world. It is only a thousand times smaller than the Sun—that is to say, 1,000 times larger than the Earth, or even more. Jupiter is eleven times larger in diameter than our globe, i.e. 1,300 times larger in volume. Gravitation is enormous at its surface, 2½ times what it is here. A man weighing 10 stone here would weigh 25 stone there. But its density is very low, being one-quarter of that of the earth. It is a world of water and more or less dense gas.

Astronomers observe it with great interest since they see it furrowed with various currents and enveloped in clouds and vapours. The currents at the surface revolve with different speeds, so that it is difficult to know what is the rotation of the planet itself. At the equator, this rotation is accomplished in 9 hours 50 minutes 30 seconds; not far from the equator, in the subtropical regions, it takes 9 hours 55 minutes 41 seconds. Since Jupiter’s year is twelve times as long as ours, there are more than 10,000 days per annum.

Here is a curious circumstance: on account of the difference of rotation, the Jovian year comprises 87 more (Jovian) days at the equator than it does in the subtropical zone. If it were the same on our globe, the inhabitants of the Congo, Colombia, Borneo, and Sumatra would have a day more in their year (and even five in four years) than those of Senegal, the Antilles, Siam, or India, a difference of 125 days per century! It would be difficult to keep their calendars in agreement.

There are seven or eight kinds of currents between the equator and the poles, so that one can say of Jupiter, as of the Sun, that it does not turn in one piece.

And those speeds themselves vary with the years. In the southern tropical zone there is a curious spot which has been followed with some interest for fifty years. Although it is much larger than the Earth, it seems to float in the current. Constant observation shows its period of rotation to have been 9 hours 55 minutes 41 seconds in 1900 and 9 hours 55 minutes 39 seconds in 1906, and its period returned in 1913 to the figure of 1900. What is the nature of this floating spot? In 1890 its colour was red. It then gradually got paler, and then pink once more. Its shape was that of a long oval, measuring 26,000 miles in length, or more than three times the diameter of our globe. The current in which it floats has not the same period of rotation as the spot itself. The spot is pushed from west to east, and it has shifted 57 degrees in two years. Now, a degree on the globe of Jupiter in that latitude represents 720 miles. This Jovian formation has, therefore, been displaced 41,000 miles, or a distance more than five times the diameter of our planet. It is as if Australia were to detach itself from the bottom of the sea and float about on the surface of the Pacific Ocean! Does this oscillation, which we only see in plan, and not in elevation, indicate the formation of a satellite trying to disengage itself from its parent planet and not succeeding? It looks as if there were on this giant world of Jupiter no surface at all, but irregular aerial layers one over the other, and full of clouds. The temperature must be very high, and enormous masses of vapour are formed, to wrestle in prodigious storms. Though mythology is of no importance here, it is evident that Jupiter is indeed the god of thunder-storms. Jupiter is a world in the making, a sun which has lost its light, but not its heat. Its density, nearly equal to that of the Sun, is barely greater than that of water. A globe of vapour varied by mountains of clouds, impalpable Himalayas, aerial Alps in convulsions, fluid and constantly agitated Pyrenees! Its colossal bulk has prevented its cooling as fast as our Earth, and it will no doubt take tens of hundreds of millions of years to arrive at a temperature fit for inhabitants. In all probability there is nobody yet on Jupiter, neither men nor animals nor plants. This immense world is in its primordial state and prepares itself for the future.