Many Skies: Alternative Histories of the Sun, Moon, Planets, and Stars

Chapter 7. The Rings of Earth

67 7 THE RINGS OF EARTH October always brings on the frigid days, the days when the Sun proceeds south to the point where it passes behind our great celestial ring system. The average daily temperature has dropped by 10 degrees just this week and we must get out our winter wear. Then in early November, the warmth returns, at least for a while. Like Saturn, we have this giant ring system girdling the Earth; by day the rings seem about as thick as feathery cirrus clouds, too thin to appear gray or any color other than brilliant white or even to cast shadows. But they do dim the sunshine behind just enough to bring a break in our climate with this prewinter cold spell. By night up there in the sunshine they blind our attempts to pick out the fainter stars. Where they pass into the Earth’s shadow, they are dark, even in the moonlight . If only we could get rid of them we could see the faint Milky Way all night every night, instead of only around midnight when the sunlit parts are at a minimum. On clear nights when observatories are open to the public for a look through a telescope, astronomers try to show the Moon with its very visible mountains and craters along with the visible shadows they cast if the Moon is not full. Evenings with the Moon near the first quarter phase are most frequently selected for public nights because it is then high in the sky and rich in shadowy relief and because in the event of thin clouds, it is the brightest sight and therefore most likely to shine right through them. After the Moon, the most popular sights are Jupiter and Saturn. Even at a modest magnifying power Jupiter shows its very oblate disk, wherein its equatorial diameter is fully 1 part in 15 larger than its polar diameter. For comparison, the oblateness of the Earth is but 1 part in 300; if we possessed the oblateness of Jupiter, our world would appear visibly flattened (see Fig. 7.1). Jupiter’s great equatorial bulge owes its existence in large part to the fact that it spins around on its axis once every 10 hours. With a 10-hour day, the massive Jupiter whirls so fast that such an ovate appearance is inevitable. One can see belts of one type of cloud against others of a different color across the equatorial flank of this giant planet. Along one of the belts not far from Jupiter’s equator lies the Great Red Spot, a feature known to exist since the invention of the telescope and maybe much longer. This giant vortex with circulation like a whirlpool or some kind of storm along the outer flanks of the planet is much larger than the entire Earth. This single hurricane, if it is one, would encompass all seven of our continents, the oceans, and much more. For 5 hours we can view the spot in even a modest telescope; then as it is carried by Jovian rotation around to the far side, it remains hidden for another 5 hours. But within a single night it can be seen at some hour. And finally there are the four big moons strung out along a line passing through the planet’s wide equator, which forever changed Galileo’s and our world when he first spotted them in 1610. The one other perennial winner at the telescope is Saturn. With those rings Saturn can dominate any viewing session. A moon or two or three are generally visible and the disk of the planet appears like a THE PLANETS 68 Fig. 7.1 The Earth as it would appear with the oblateness of Jupiter. Photo: NASA. smaller Jupiter (with an even greater degree of oblateness, 1 part in 10) with a less turbulent atmosphere, but it is the rings that amaze and entertain us. Mercury is a little thing lost in the twilight, and Venus and Mars are surprisingly almost featureless. Uranus and Neptune are visible as small greenish disks, so no other planet has a chance to attract attention as do Jupiter and Saturn, the two largest of our solar family. Star clusters, nebulae, and galaxies are faint, usually too faint to be seen against the light-polluted urban and suburban skies over most public observatories. That leaves the big three—the Moon, Jupiter...