Even in a small backyard telescope, Jupiter is quite a sight. In the eyepiece, it is a disk noticeably flattened because of its high spin rate of two revolutions per day. With parallel equatorial bands and whitish color, its appearance is totally distinct from what Carl Sagan called our "pale blue dot" Earth, a blue planet shrouded with wispy clouds. The most remarkable property of Jupiter visible with a backyard telescope is the presence of four moons, pinpricks of light that can be seen to move over a few hours' time. The rhythmic mathematical dance of the four largest moons, first observed by Galileo in 1612, was a stunning scientific finding, because it resembled a miniature Coperni-can solar system where orbital motion could be directly observed.
What cannot be seen by telescope, of course, is the exotic interior of this planet. Jupiter is a giant gas ball that gets hotter and denser with depth, but like the other giant planets in the solar system, it has no surface. Jupiter is mostly hydrogen and helium, and deep in its interior, the pressure is so high that electrons are not bound to individual hydrogen atoms but instead move freely from atom to atom, as in a metal. At pressures of a million atmospheres in the interior of Jupiter, hydrogen is actually in a metallic state.
It is enchanting in a small telescope, it has fascinating properties and a rich history, but observed with the unaided eye, this giant planet is just a spot of light—another "star" in the sky. From a distance of half a billion miles, it is difficult to imagine (at least for an astrology agnostic) that this distant planet with its frigid upper atmosphere could have any effect on Earthlings. Remarkably, however, Jupiter's existence and its time and place of formation have profoundly influenced our Earth's ability to provide and maintain a stable environment for life.
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