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73 8 NEXT DOOR TO A GIANT What a sight the Moon and nearby Jupiter make when they appear close to each other in the evening sky before and after sunset. The Moon subtends an angle of half a degree, 3 times that of Jupiter, if that huge planet were in the orbit occupied by Venus, or about 10 times the area in angular terms. Balanced against this is the fact that the big planet, perpetually enshrouded in a thick atmosphere, has 5 to 6 times the albedo or reflecting power of the airless Moon and, being only 70 percent of the distance of the Earth and Moon from the Sun, it receives twice as much sunlight per unit area of surface . Coupled together, these two factors, the greater sunlight plus the much larger albedo, offset the larger apparent area of the lunar disk. In the telescope we see two crescents, the larger Moon and the smaller, brighter Jupiter , the one revealing mountains and craters and the other, belts of clouds against a thick atmosphere, also of clouds but whiter. It is small wonder that, at moments like these, almost every observatory schedules an open house! Jupiter, when appearing alongside the crescent Moon, shows a thicker crescent since the angle between the Sun and the Earth would be larger and closer to 90 degrees. But unlike the crescent Moon, the crescent Jupiter would not show the whole of its disk in faint light. We see the entire Moon faintly, an effect called the old Moon or earthshine, and first explained by Leonardo da Vinci. The Earth, which appears 40 times as bright as seen from the Moon as the Moon does to us, is easily bright enough to illuminate the lunar night side for us to see it. Jupiter has no bright Earth nearby and thus its dark side would remain invisible. Being as luminous as the Moon, Jupiter is always easily visible in broad daylight unless it is right up alongside the glare of the Sun. If Jupiter happened to lie in the relatively close orbit of Venus, or even Mars, we would have one more object in our skies that would be seen as a globe with the naked eye. The four “terrestrial” or Earthlike planets—Mercury, Venus, Earth, and Mars—are similar to each other and not much else. They are all dense, some 4 to 5 times as dense as equivalent-sized blobs of water, whereas the Sun, the giant planets, and much of the rest of the solar system are about 1.5 times as dense as water . (Saturn is actually only about 0.8 times the density; the statement is frequently made that if there were an ocean large enough, Saturn could float on it.) The reason for this wide discrepancy in density between us and the Jovian worlds is very simple. The Sun and the rest of the universe are composed mostly of the lightest and simplest of all the elements, hydrogen. Hydrogen makes up over three-quarters of all the material of the universe. Every star, every nebula, every galaxy is composed mostly of hydrogen. It is by far the most abundant substance of all. Most of the rest is helium, the second lightest and simplest of the elements. Only about 3 percent of the universe, if that, is made up of the rest of the elements, the more than one hundred that spread from the third lightest, lithium, up through fermium and beyond. We must add here that some of the heavier elements may exist only in the laboratory . Astronomers have a very simple chemistry for most purposes; the amount of hydrogen is called X, that of helium is Y, and the rest combined are labeled Z such that X ⫹ Y ⫹ Z ⫽ unity, where, as mentioned above, X amounts to over three-quarters of the entire lot by mass, Y accounts for most of the rest, and Z comes to only 3 percent at best. Within the scrambled mess we call Z, the most abundant elements are carbon, nitrogen, oxygen, neon, and one or two more that account for the bulk of it, all of which are among the lightest elements after helium. A few in the next tier of elements by weight—silicon, sulfur, aluminum, magnesium, and iron—make up the majority of all the rest. Beyond iron the elements are found in trace quantities at best; thorium, gold, lead, uranium, and the rest of that heavy group just...

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