Sun, Moon, and Earth - How it works



Formation of the Sun and Planets

Scientists today believe that the universe began between 10 and 20 billion years ago, with an event nicknamed the "big bang," The galaxies continued (and still continue) to move outward from that center, and among them was our Milky Way. Somewhere between the center and the rim of the Milky Way, a rotating cloud of cosmic gas formed about six billion years ago,

The center of the cloud, where the greatest amount of gases gathered, was naturally the densest and most massive portion as well as the hottest. There, hydrogen—the lightest of all elements—experienced extraordinary amounts of compression, owing to the density of the clouded gases around it, and underwent nuclear fusion, or the bonding of atomic nuclei. This hot center became the Sun about five billion years ago, but there remained a vast nebula of gas surrounding it. As the fringes of this nebula began to cool, the gases condensed, forming solids around which particles began to accumulate. These were the future planets.

THE PLANETS AND THE ELEMENTS.

Closest to the Sun, the planets and other satellites were formed of elements that could condense at high temperatures: iron, silicon, aluminum, calcium, and magnesium. These elements, along with oxygen-containing compounds, constituted the material foundation around which other particles accumulated to form the terrestrial planets: Mercury, Venus, Earth, and Mars. Further from the Sun, where temperatures were lower, gaseous compounds—including methane, ammonia, and even water—could condense. These compounds became the basis around which the five outer planets (the four Jovian planets and Pluto) formed.

The process of planetary formation involved additional steps and took place over a period of about 500 million years. Some of the factors that played a part in forming Earth are discussed in Planetary Science, but in the present context, let us consider the source of those elements mentioned here. Did they just magically form? Were they always there? In answer to the first question, there is nothing magical about the formation of "new" elements, though it almost seems so. As for the second question, the answer is yes and no.

EARTH WITH THE MOON IN ECLIPSE. (© Chris Bjornberg/Photo Researchers. Reproduced by permission.)
E ARTH WITH THE M OON IN ECLIPSE . (
© Chris Bjornberg/Photo Researchers
. Reproduced by permission. )

Elements and the Sun

The elements themselves were not always there in the universe or on Earth itself; however, the subatomic building blocks that make them up have indeed existed from the beginning of the universe. The basic atomic structure is as follows. There is a nucleus in which one or more protons (positively charged subatomic particles) may reside along with one or more neutrons, which have no charge. Spinning around the nucleus are one or more electrons, or negatively charged subatomic particles.

The number of protons and electrons in an atom is always the same, meaning that the atom has no electric charge. Atoms that have lost or gained electrons (in which case they would acquire a positive or negative charge, respectively) are called ions. Electrons, which move fast and possess very small mass compared with protons and neutrons, are very easy to dislodge from an atom; on the other hand, it takes an extraordinary event to change the number of protons in the nucleus.

This fact is significant, because it points toward the defining characteristic of an element: the number of protons in the nucleus. Atoms of a particular element always have the same number of protons, called their atomic number. The atomic number of an element can be determined by consulting the periodic table of elements: for instance, iron, with an atomic number of 26, must have 26 protons in its nucleus. If an atom has 25, it is manganese, and if it is has 27, it is cobalt.

FORMING NEW ELEMENTS.

The number of neutrons in the nucleus may vary for atoms within a given element. Atoms that have the same number of protons (and are thus of the same element) but differ in their number of neutrons are called isotopes. Most isotopes are stable, meaning that their chemical composition will remain as it is; however, some isotopes are radioactive, meaning that they experience the spontaneous emission of particles or energy over a given period of time.

Radioactive decay is one of two ways that one element can become another. When a radioactive isotope emits an alpha particle, for example, its nucleus expels a positively charged nucleus consisting of two protons and two neutrons, which is the same thing as a helium atom stripped of its electrons. This obviously changes the number of protons in the nucleus of the isotope and may result in its stabilization. The other means of forming a new element is by nuclear fusion, in which two atomic nuclei fuse or bond.

NUCLEAR FUSION.

Note that the first of these means by which elements are formed is subtractive; in other words, with radioactive decay, a different element is formed by the expulsion of protons. Nuclear fusion, on the other hand, is additive, resulting in the creation of different elements by the addition of protons to an atomic nucleus. Radioactive decay takes place inside Earth (among other places), while nuclear fusion is the source of the Sun's power.

Nuclear fusion involves the release of huge amounts of energy. On Earth, scientists have been able to bring about uncontrolled nuclear fusion in the form of the so-called hydrogen bomb, which is actually a "fusion bomb." They have yet to succeed in creating controlled nuclear fusion. If and when they do, it would provide a safe, clean source of almost limitless power and probably would constitute the greatest scientific or technological discovery since fire.

On the Sun, nuclear fusion has been taking place, and will continue to do so, for a long, long time. The 92 naturally occurring elements of the universe are the result of fusion reactions, meaning that all that we see around us was once part of a star. This represents a major break with the ancient belief that Earth is made of fundamentally different substances than are the bodies of space (see Earth, Science, and Nonscience). In fact, our world and everything in it—including our own bodies—is truly "the stuff of stars."

The Moon and Earth

In comparison to the Sun, the Moon is altogether less remarkable. Below, we review some statistics about the sizes of each, but as every elementary-school student today knows, the Sun is much, much larger and exerts far more impact on the fate of the solar system—including Earth. The Moon does not even have its own energy sources: its light comes from the Sun, and the absence of an atmosphere, of volcanic activity, or even of a significant magnetic field makes it a very dull place indeed.

Yet the Moon has inspired at least as much fascination among humans over the ages as has the Sun. There is its physical beauty, though comparisons with the Sun are hardly fair, since we cannot look at the Sun without damage to our eyes. There is also its influence on earthly cycles ranging from the tides to the months themselves, though some claims of lunar influence have little basis in fact. During the Middle Ages, many believed that the Moon caused madness, a superstition still reflected in our word lunacy.

Still, humans have long associated a spirit of mystery with the Moon, in part because of its ever-changing appearance and in part because it has always showed just one side to Earth. (We discuss the reason why later.) Only in 1959, when the Soviet space probe Luna traveled to the "dark side of the Moon," did scientists gain a glimpse of it. Unmanned and later manned journeys, which culminated with the U.S. Moon landing in 1969, also changed astronomers' understanding of the Moon's origins.

THE "BIG WHACK."

One of the curious things about the Moon is its size in relation to Earth. Nowhere in the universe is there such a small size differential between a satellite and the planet around which it orbits, the only possible exception being Pluto and its moon, Charon. Because our Moon is so close in size to Earth, scientists once speculated that they might have shared origins, and this speculation informed several theories concerning the formation of the Moon.

According to the fission theory, the Moon was a piece of Earth that had been torn away, perhaps from the Pacific basin. The simultaneous creation theory likewise depicted Earth and the Moon as sharing origins, but in this case they literally had been formed together from the same materials. Finally, there was the capture theory, which, in contrast to the others, assumed quite different origins for the two bodies: the Moon had formed somewhere else in the solar system and had been captured by Earth's gravitational field after it wandered too close to the planet.

As it turned out, the capture theory was closest to the theory accepted today, though it was discarded along with the other two on the basis of data brought back from the Apollo Moon landings. According to the giant impact theory, sometimes called the Big Whack model or the ring ejection theory, at a young age Earth was sideswiped by a celestial object as large or larger than Mars. As a result of that collision, a ring of crustal matter was spewed into space, and over time the matter in this ring agglomerated to form the Moon.

Vital Statistics of the Sun and Moon

The Moon is about 240,000 mi. (385,000 km) from Earth, meaning that it takes about 1.25 seconds for its light to reach Earth. By contrast, the Sun's distance from Earth is so great that it takes eight minutes for sunlight to reach our planet, even though light travels through space at the speed of about 186,000 mi. (299,339 km) per second.

The distance between Earth and the Sun is the basis for the astronomical unit (AU), a figure used for measuring the distance between bodies in the solar system. Equal to the average distance from Earth's center to the center of the Sun, an AU is designated 1.49597870691 × 10 8 km, or approximately 92,955,807 mi. Usually we think of the solar system as the area encompassed by the orbit of the most remote planet, Pluto, but that is only 39.44 AU, a tiny figure compared with the diameter of the realm within the Sun's gravitational pull, which is a staggering 100,000 AU.

VOLUME AND MASS.

The Sun itself has a diameter of about 856,000 mi. (1,392,000 km), meaning that the distance across it is about 109 times that of Earth's diameter. Another way to consider that figure is this: if one were to draw a circle as big as the Sun around Earth, the edge of that circle would be about twice as far away as the Moon. The Sun's volume is so great that about 1.3 million Earths could fit inside it, and its mass is about 300,000 times that of Earth. In fact, it accounts for about 99.8% of the mass of the entire solar system.

By contrast, the Moon has a diameter of only about 2,160 mi. (3,475 km), a little less than the distance from New York to Los Angeles. Its mass is a little more than 1% of Earth's, and as a result, its gravitational pull is too small to retain the gases that make up the atmosphere. That small mass, combined with an imbalance in its distribution, explains why the Moon shows only one face to Earth. The side of the Moon facing Earth is of greater mass than the other side and is therefore more strongly attracted by Earth's gravitational force. The result is a phenomenon called gravitational locking, whereby the Moon rotates on its axis at exactly the same rate as it travels around Earth—once ever 29.5 days.

Clearly, the Moon is tiny in both volume and mass, and were it as far away as the Sun, we would hardly pay it notice. Yet it should be pointed that even the Sun itself, while remarkable in our own solar system, is far from a standout in the universe as a whole. It is a youngish star, of average size, and not all that different from billions of other stars throughout the cosmos. It is not even unique in being the only star with its own solar system. In 1999 astronomers discovered an entire solar system some 44 light-years from Earth, in which three large planets were found to be circling the star Upsilon Andromedae.



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