THE EARTH IN SPACE

EARTH is one of nine planets revolving in nearly circular (elliptical) orbits around our star, the sun. Earth is the third planet out from the sun and the fifth largest planet in our solar system, having a diameter of about 7,918 miles. It completes one orbit around the sun in about 365¼ days, the length of time that gives us our unit of time called a year.

PLANETS vary in size, composition, and orbit. Mercury, with a diameter of 3,112 miles, is the planet nearest to the sun. It orbits the sun in just three earth months. Jupiter, about ten times the diameter of Earth (83,000 miles), is the largest planet and fifth in distance from the sun, taking about 11¾ earth years to orbit the sun. Pluto, the most distant planet, takes about 247¾ earth years to orbit around the sun. The inner planets have densities, and probably compositions similar to Earth’s; outer planets are gaseous, liquid, or frozen hydrogen and other gases.

THE SUN, an average-sized star, makes up about 99 percent of the mass of the solar system. Its size may be illustrated by visualizing it as a marble. At this scale, the earth would be the size of a small grain of sand one yard away. Pluto would be a rather smaller grain 40 yards away.

SATELLITES revolve around seven planets. Including the earth’s moon, there are 61 satellites altogether; Mars has 2, Jupiter 16, Saturn 18, Uranus 15, Neptune 8, and Pluto 1.

COMETS are among the oldest members of the solar system. They orbit the sun in extremely long, elliptical orbits. As comets approach the sun, their tails begin to glow from friction with the solar wind.

THE MOON, earth’s natural satellite, has about ¼ the diameter, the weight, and the density of our planet. The moon completes one orbit around the earth every 27⅓ days. It takes about the same length of time, 29½ days, to rotate on its own axis; hence, the same side, with an 18% variation, always faces us. The moon’s surface, cratered by meteorite impact, consists of dark areas (maria) which are separated by lighter mountainous areas (terrae). Terrae are part of the original crust, formed about 4.5 billion years ago; maria are basins, excavated by meteorite falls, filled by basaltic lavas formed from 3 to 4 billon years ago.

ASTEROIDS, the so-called minor planets, are rocky, airless, barren, irregularly shaped objects that range from less than a mile to about 480 miles in diameter. Most of the asteroids that have been charted travel in elongated orbits between Mars and Jupiter. The great width of this zone suggests that the asteroids may be remnants of a disintegrated planet formerly having occupied this space.

METEORS, loosely called shooting stars, are smaller than asteroids, some being the size of grains of dust. Millions daily race into the earth’s atmosphere, where friction heats them to incandescence. Most meteors disintegrate to dust, but fragments of larger meteors sometimes reach the earth’s surface as “meteorites.” About 30 elements, closely matching those of the earth, have been identified in meteorites.

THE EARTH’S MOTIONS determine the daily phenomenon of day and night and the yearly phenomenon of seasonal changes. The earth revolves around the sun in a slightly elliptical orbit and also rotates on its own axis. Since the earth’s axis is tilted about 23½° with respect to the plane of the orbit, each hemisphere receives more light and heat from the sun during one half of the year than during the other half. The season in which a hemisphere is most directly tilted toward the sun is summer. Where the tilt is away from the sun, the season is winter.

Tilted axis determines different positions of sun at sunrise, noon, and sunset at different seasons in middle north latitude.

RELATIVE MOTIONS OF THE EARTH

REVOLUTION is earth’s motion about the sun in a 600-million-mile orbit, as it completes one orbit about every 365¼ days traveling at 66,000 mph.

ROTATION is a whirling motion of the earth on its own axis once in about every 24 hours at a speed of about 1,000 mph at the equator.

NUTATION is a daily circular motion at each of the earth’s poles about 40 ft. in diameter.

PRECESSION is a motion at the poles describing one complete circle every 26,000 years due to axis tilt, caused by gravitational action of the sun and moon.

OUR SOLAR SYSTEM revolves around the center of our Milky Way Galaxy. Our portion of the Milky Way makes one revolution each 200 million years.

GALAXIES seem to be receding from the earth at speeds proportional to their distances.

THE SUN is the source of almost all energy on earth. Solar heat creates most wind and also causes evaporation from the oceans and other bodies of water, resulting in precipitation. Rain fills rivers and reservoirs, and makes hydroelectric power possible. Coal and petroleum are fossil remains of plants and animals that, when living, required sunlight. In one hour the earth receives solar energy equivalent to the energy contained in more than 20 billion tons of coal, and this is only half of one billionth of the sun’s total radiation.

Just a star of average size, the sun is yet so vast that it could contain over a million earths. Its diameter, 864,000 miles, is over 100 times that of the earth. It is a gaseous mass with such high temperatures (11,000° F at the surface, perhaps 325,000,000°F at the center) that the gases are incandescent. As a huge nuclear furnace, the sun converts hydrogen to helium, simultaneously changing four million tons of matter into energy each second.

Solar prominences compared with the size of the earth

THE MILKY WAY, like many other galaxies, is a whirling spiral with a central lens-shaped disc that stretches into spiral arms. Most of its 100 billion stars are located in the disc. The Milky Way’s diameter is about 80,000 light years; its thickness, about 6,500 light years. (A light year is the distance light travels in one year at a velocity of 186,000 mi. per sec., or a total of about 6 trillion miles.)

GALAXIES are huge concentrations of stars. Within the universe, there are innumerable galaxies, many resembling our own Milky Way. Sometimes called extragalactic nebulae or island universes, these star systems are mostly visible only by telescope. Only the great spiral nebula Andromeda and the two irregular nebulae known as the Magellanic Clouds can be seen with the naked eye. Telescopic inspection reveals galaxies at the furthermost limits of the observable universe. All of these gigantic spiral systems seem to be of comparable size and rotating rapidly. Nearly 50 percent appear to be isolated in space, but many galaxies belong to multiple systems containing two or more extragalactic nebulae. Our galaxy is a member of the Local Group, which contains about a dozen other galaxies. Some are elliptical in shape, others irregular. Galaxies may contain up to hundreds of billions of stars and have diameters of up to 160,000 light years. Galaxies are separated from one another by great spaces, usually of about 3 million light years.

Many galaxies rotate on their own axes, but all galaxies move bodily through space at speeds of up to 100 miles a second. In addition to this, the whole universe seems to be expanding, moving away from us at great speeds. Our nearest galaxy, in Andromeda, is 2.2 million light years away.

About 100 million galaxies are known, each containing many billions of stars. Others undoubtedly lie beyond the reach of our telescopes. It seems very probable that many of the stars the galaxies contain have planetary systems similar to our own. It has been estimated that there may be as many as 10¹⁹ of these. Chances of life occuring on other planets would, therefore, seem very high, although it may not bear an exact resemblance to life on earth.

WHIRLPOOL NEBULA in Canes Venatici, showing the relatively close packing of stars in the central part

GREAT SPIRAL NEBULA M31 in Andromeda is similar in form but twice the size of our own galaxy, the Milky Way.

THE CHEMICAL ELEMENTS are the simplest components of the universe and cannot be broken down by chemical means. Ninety-two occur naturally on earth, 70 in the sun. They develop from thermonuclear fusion within the stars, in which the elementary particles of the lightest elements (hydrogen and helium) are transformed into heavier elements.

THE ORIGIN OF THE UNIVERSE is unknown, but all the bodies in the universe seem to be retreating from a common point, their speeds becoming greater as they get farther away. This gave rise to the expanding-universe theory, which holds that all matter was once concentrated in a very small area. Only neutrons could exist in such a compact core. According to this theory, at some moment in time—at least 5 billion years ago—expansion began, the chemical elements were formed, and turbulent cells of hot gases probably originated. The latter separated into galaxies, within which other turbulent clouds formed, and these ultimately condensed to give stars. Proponents refer to this as the “Big Bang” theory, a term descriptive of the initial event, perhaps as long as 10-15 billion years ago.

THE ORIGIN OF THE SOLAR SYSTEM is not fully understood, but the similar ages of its components (Moon, meteorites, Earth at about 4.5-4.6 billion years) and the similar orbits, rotation, and direction of movement around the sun, all suggest a single origin. The theory currently most popular suggests that it formed from a cloud of cold gas, ice, and a little dust, which began slowly to rotate and contract. Continuing rotation and contraction of this disc-shaped cloud led to condensation and thermonuclear fusion—perhaps triggered by a nearby supernova, from which stars such as the sun were formed. Collision of scattered materials in the disc gradually led to the formation of bodies—planetismals—which became protoplanets. The growing heat of the sun probably evaporated off the light elements from the inner planets (now represented by the dense, rocky “terrestrial” planets—Mercury, Venus, Earth, Mars, and the Moon). The outer planets, because of their greater distance from the sun, were less affected and retained their lighter hydrogen, helium, and water composition. Perhaps they formed from mini-solar-planet systems within the larger disc. This composition may well reflect that of the parent gas cloud.

Each planet seems to have had a distinct “geologic” history. Some, like Earth and Io, a moon of Jupiter, are still active. Others, like Mercury, Mars, and our moon, had an earlier active history, but are now “dead.”

This theory, in an earlier version, has a long history, going back to Immanuel Kant, the philosopher, in 1755, and the French mathematician Pierre-Simon de Laplace (1796).

Artist’s interpretation of the dust-cloud theory

THE EARTH’S ATMOSPHERE is a gaseous envelope surrounding the earth to a height of 500 miles and is held in place by the earth’s gravity. Denser gases lie within three miles of the earth’s surface. Here the atmosphere provides the gases essential to life: oxygen, carbon dioxide, water vapor, and nitrogen.

Differences in atmospheric moisture, temperature, and pressure combined with the earth’s rotation and geographic features produce varying movements of the atmosphere across the face of the planet, and conditions we experience as weather. Climatic conditions (rain, ice, wind, etc.) are important in rock weathering; the atmosphere also influences chemical weathering.

Gases in the atmosphere act not only as a gigantic insulator for the earth by filtering out most of the ultraviolet and cosmic radiation but also burn up millions of meteors before they reach the earth.

The atmosphere insulates the earth against large temperature changes and makes long-distance radio communications possible by reflecting radio waves from the earth. It also probably reflects much interstellar “noise” into space, which would make radio and television as we know them impossible.

THE RATIO OF GASES in the atmosphere is shown in the chart at left. Clouds form in the troposphere; the overlying stratosphere, extending 50 miles above the earth, is clear. The ionosphere (50-200 miles) contains layers of charged particles (ions) that reflect radio waves, permitting messages to be transmitted over long distances. Faint traces of atmosphere exist in the exosphere to about 500 miles from the earth’s surface.

OUR PRESENT ATMOSPHERE and oceans were probably derived by degassing of the semi-molten earth and continuing later additions from volcanoes and hot springs. These gases—such as hydrogen, nitrogen, hydrogen chlorides, carbon monoxide, carbon dioxide, and water vapor—probably formed the atmosphere of earlier geologic times. The lighter gases, such as hydrogen, probably escaped. The later development of living organisms capable of photosynthesis slowly added oxygen to the atmosphere, ultimately allowing the colonization of the land by providing free oxygen for respiration and also forming the ozone layer, which shields the earth from ultraviolet radiation of the sun.

Some evidence for this sequence in the development of the atmosphere is contained in the sequence of Precambrian rocks and fossils, which suggests a transition from a non-oxygen to free-oxygen environment.

Atmospheric circulation involves the continuous recirculation of various substances.