The Universe and the Solar System

The Earth and the sun and the other seven planets are isolated in space. This set of eight spheres that circles the bright sun is poised in emptiness¹ and separated by unimaginable distances from everything else in the universe. Because the sun is its central figure, the family of bodies that accompanies it is called the solar system, which in its turn² is a part of a galaxy and eventually of the Universe.

Until the 17^th century the solar system was thought to consist of only five planets besides the earth and moon. In 1609, soon after having heard of the invention of the telescope in Holland, Galileo built one of his own and was able to add four new bodies to the system: the brighter of the moons (or satellites) that revolve around Jupiter. Since Galileo’s time telescopic improvements have made possible the discovery of many more members of the sun’s family.

It is common knowledge now that our neighborhood in space consists of our local star, the sun, and its family of eight planets, nearly 70 moons, millions of comets and countless asteroids. The mean diameter of the solar system is approximately 7 billion miles.

Dominating the entire solar system is the sun, which is nearly a thousand times more massive than all the planets put together. The energy the sun generates by nuclear fusion makes it luminousand provides the rest of the solar system with heat and light. Its gravity pulls the planets so that they move around it in almost circular orbits.

The list of planets now includes eight; in order from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. All planets are divided into two groups: four small rocky ones close to the sun and four big gassy ones farther out. All except Mercury and Venus have satellites.

Planets revolve aroundthe sun and rotate on their axes. Nearly all the revolutions and rotations are in the same direction, but the rotation rate is different, slow with some planets, rapid with others. Only the rotation of Venus and the revolutions of a few satellites are in the opposite direction. Uranus is an exception of a different kind since it rotates about an axis only 8⁰ from the plane of its orbit.

All the orbits except those of the comets lie nearly in the same plane. Planets stay on their orbits according to the law of universal gravity. All celestial bodies have an attraction of their own and pulleach other. This force decreases sharply the greater the distance. At the same time the force of their movements tends to pull them away from each other. This interaction was discovered by the great British scientist Isaak Newton in the 17^th century.

Astronomers have identified more than 4.000 pieces of rock, known as asteroids, orbiting the sun. However there are probably millions of these minor planets, some only a few metres across. Most are in the Asteroid Belts lying between Mars and Jupiter.

The farthest, Chiron, orbits the sun beyond Saturn.

The sight of a great comet hanging in the sky, looking like a ghostly dagger poised to strike³, is an impressive spectacle. Yet, comets are all snow and no substance – a “dirty snowball”, or lump of ice. The long, oval orbit of a comet carries it close to the sun and far away again. As it approaches the sun, the frozen surface starts to evaporate, forming a great head of gas, which the solar wind sweeps⁴ into a long tail. The comet’s moment of glory lasts a brief few weeks before it heads back to the icy outer reaches of the solar system.

Planets, asteroids and satellites are only visible by virtue⁵ of the sunlight they reflect. What we see of any of these objects at a particular time is limited to the half that faces the sun. Planets with orbits larger than that of the earth never come between us and the sun, so we can always see nearly the whole of their illuminated sides. Mercury and Venus, however, have orbits smaller than the earth’s and are between us and the sun for a good part⁶ of each revolution. In this position their dark sides are turned toward us, and we see them either not at all or as crescents.

Our local star

The sun is the glorious body that dominates the solar system and the origin and destiny of the earth as well as our daily lives are closely connected with solar phenomena. The astronomer has another reason for studying the sun closely, for it is in many ways a typical star. The properties of the sun that we can observe by virtue of its relative closeness, then, are interesting not only in themselves but also because they provide information about stars in general that would otherwise be inaccessible.

The sun is so large that 1,300,000 earths would fit into it. Like all other astronomical bodies, it is rotating, though with the peculiarity that its period of rotation is shorter near its equator than near its poles. Although conditions on the sun are very different from those on the earth, the basic matter of the two bodies appears to be the same¹. Even the relative amounts of different elements are similar, except for a vastly greater abundance of the lightest elements, hydrogen and helium, on the sun. At the low temperatures prevailing on the earth, most of the elements have combined to form compounds; in the hot sun the elements are usually present as individual atoms, most of them ionized.

The surface temperature of the sun is about 5,700 ⁰C. At this temperature all matter is gaseous, which means that the surface of the sun is a glowing gas envelope. Above the surface is a rapidly thinning atmosphere that consists principally of hydrogen, helium, and calcium. From this atmosphere great, flamelike prominences sometimes extend out into space, much like sheets of gas standing on their sides. During a total eclipse of the sun, when the moon obscures the sun’s disk completely, a wide halo of pearly light can be seen around the dark moon. This halo, or corona, may extend out as much as a solar diameter and seems to have² a great number of fine lines extending outward from the sun immersed in its general luminosity. The corona consists of ionized atoms and electrons in extremely rapid motion.

Although the corona that we can see is relatively near the sun, indirect evidence indicates that, in very diffuse form, it also pervades much of the region between earth and sun. Most authorities even regard the sun’s atmosphere as extending well beyond the earth’s – a radical change indeed from the older idea that interplanetary space is an all but total vacuum. The outward flow of ions and electrons in this atmosphere constitutes the solar wind which has been detected by rocket-borne instruments.