Neptune is also so vast that the planet requires a period of 165 years in order to complete a single revolution. The changes of Neptunian seasons must, therefore, be extremely protracted. A man who was born at midwinter in Neptune would have reached extreme old age if he survived until the next ensuing mid-summer.
I cannot discuss the times and seasons of all the celestial bodies, so I have taken a few typical instances. Neptune was appropriate as being the most remote planet. Now let us speak of Jupiter, the greatest planet. The day and night on Jupiter are both extremely short, for together they do not quite amount to ten hours. Jupiter's year, however, is almost twelve of our years. Although a man on Jupiter would only receive one-twenty-fifth part of the heat of the sun that he would do on the earth, yet it does not seem likely that there would be reason to apprehend that Jupiter would be uninhabitable from cold. Quite the contrary is the case. Indeed, it seems not unlikely that the excessive heat of Jupiter would be found intolerable by beings with nerves like ours. This heat has, however, not come from the sun; it is the internal heat of the planet itself, which has not yet sufficiently cooled down from that original fiery condition characteristic of every body of our system in its initial stages.
Jupiter certainly has an atmosphere, but we do not know from what gases that atmosphere may have been blended. It might consist of materials noxious, if not actually poisonous; and in any case it is extremely unlikely that it should contain both the ingredients and the proportions suited to our organs of respiration. But there are independent grounds for knowing that Jupiter must be an impossible home for beings so constituted as we are. On
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