The Sun

The Sun

The Sun, a great ball of immensely hot gases about 865,000 miles in diameter, is at once the central body of the solar system, the sustainer of life on the Earth, and the nearest star. It contains over 99 per cent of all the material in the solar system, and if it were divided into a million equal parts, each part would be larger than the Earth. Even Jupiter, the largest and most massive of the planets, is a tiny dwarf compared to the Sun.

As a star the sun is relatively small, faint and cool. Although much larger than many stars, it is only a speck compared with supergiant stars like Antares and Betelgeuse. One of the nearest stars is 500,000 less luminous than the sun. On the other hand, some stars are thousands of times more brilliant. True, the sun appears to be much larger than other stars, but these appear like pin-points because they are so far away. The sun is distant 93 million miles, so its light takes only about eight minutes to reach us. Proxima Centauri, the nearest star, is so remote that its light takes just over four years.

Prominences of the Sun. Prominences of the edge of the sun.

Sunspots

The sun’s bright disk, called the lightsphere or photosphere, is often marked by dark spots. These invariably change in shape, size and number as they move across the disk. They are seldom found near the sun’s poles. Studies of their movements show that the sun rotates once in about 25 days at its equator, and then more slowly with increasing latitude to reach about 34 days near the poles. The photosphere cannot therefore be a solid surface. It merely represents the level through which, owing to the opacity of the solar gases, we cannot see.

The spots are really hot, bright regions. They appear dark because we see them in contrast to the hotter, brighter surrounding regions of the photosphere. The spots have temperatures of approximately 3,500 degrees centigrade, but the temperature of the photosphere is nearly 6,000 degrees centigrade. The number of spots varies from a minimum though to a maximum and then to a minimum again in an average period of 11 years. This period, known as the sunspot cycle, has been as short as six years and as long as 17. Large sport usually appear at sunspot maxima. The largest seen so far occurred in 1947. It grew from a few small spots to a group visible to the unaided eye, At its maximum development covered some 6,200 million square miles, or about one per cent of the sun’s disk.

Prominences

The hot gases above the level of the photosphere form a tenuous and fairly transparent “atmosphere”. This region consists largely of hydrogen and helium, but also contains a small percentage of other gases representing probably all 92 permanent elements from hydrogen to uranium. Here are found flame-like clouds of shining gases named prominences. Some form shapes like pyramids, trees and arches and keep fairly still for hours and even days. Others surge outwards with explosive violence to form spectacular steamers, loops and plumes or cascade sunwards like fountains of fire. Their observation used to be restricted to the times of total solar eclipses. But they now can be studied in full daylight with the coronograph, a telescope that artificially eclipses the sun, and also with telescopes equipped with special filters.

More violent than even the most active prominences are sudden explosive disturbances known as solar flares. Large ones can be seen directly with the telescope, The great majority of them are more readily detected when the sun is photographed in the red light of hydrogen. A large flare not only emits liberal doses of x-rays and ultraviolet rays. It also ejects streams of fast moving electrified particles (protons and electrons). If the flare occurs on the sun’s near side, both the rays and the particles have a good chance of reaching the earth. The rays make the journey in just over eight minutes, disturb the earth’s ionosphere, or electrified upper atmosphere, and upset radio communications. The particles take much longer. They eventually stream into the earth’s atmosphere to upset the directions of compass needles. They bring about vivid displays of northern and southern lights.

The Corona

During the total eclipse of the Sun, when the photosphere is completely hidden by the dark body of the moon, the sun’s atmosphere is seen to extend far into space. It then appears as a pearly-white aureole, often with a delicate structure of tufts and curved wisps of light. It consists of extremely thin gases, whose temperature, measured by the velocity of their atoms, is about one million degrees centigrade near the sun. Most of its light is sunlight scattered by atomic particles. It also continuously emits radio waves. These sometimes come in intense bursts usually associated with active sunspots or large solar flares.

The corona is no static medium, nor is it limited to the sun in the way that the earth’s atmosphere’s limited to the earth. Its form and appearance are definitely influenced by the sun’s activity. At sunspot’s minimum, long curved streamers of light reach to great distances from the sun’s equatorial regions, while relatively short brush-like streaks appear at the poles. At sunspot maximum, the long streamers disappear and the corona takes on a more regular and uniform appearance.

We now know that the corona, and therefore the sun, extends in a very real sense far beyond the earth. Hot hydrogen gas flows from the sun in all directions and rushes past the earth at about 900,000 miles an hour. It blows through the solar system like a swift wind, sweeping any fine particles, meteoric dust, and gases into interstellar space. The streams of particles blasted off by solar flares are like temporary gales in the outward flow.

How the Sun Shines

Sunspot, prominences, flares, and the corona are all outward visible signs of activity deep in the sun’s interior. The mainspring of the sun’s energy output is a process known as a thermonuclear reaction. Under the enormous pressures and temperatures of several million degrees which exist in the sun’s central part of core, hydrogen is transmitted into helium. Vast quantities of energy are released in the process. They pass through the photosphere and, in the form of light and heat, pour into space. Calculations show that the sun converts about four million tons of its mass into energy every second. But since the total mass of the sun is so enormous, the loss is only a few percent over several thousand million years.

Considerations based on theory indicate that the sun has been shining for some 6,000 to 8,000 million years. Also, that it has enough hydrogen in its core to keep it going without much change for at least another 2,000 to 3,000 million years. But when the available hydrogen has been used up, the sun will gradually swell into a reddish star several times larger than its present size. The change will probably take many millions of years, As the sun approaches giant status its immense heat could boil away our seas and oceans and bring about the end of life on earth.

Just as man is destined to destruction, so also is the sun. Its nuclear fire cannot burn forever. Modern theories of the evolution of the stars suggests that the sun will dwindle into a white dwarf. It will become a hot, small, and extremely dense star, the last stage towards final extinction. So if intelligent life on the earth escaped a hot death, it eventually would have to face death through darkness and extreme cold.

Moon

Moon

The Moon is a most remarkable object, even within the limitations of naked-eye observation. It appears to alter its shape in a regular monthly rhythm. At the same time, moves fairly swiftly against the background of stars. These changes, as the ancient Greeks realized, are because it shines by reflected sunlight. The moon also travels around the earth. It is our nearest neighbor in space, and earth’s only natural satellite.

Relative to the stars, the moon orbits the earth once in about 27 ½ days at an average distance of 238, 856 miles. Since its orbit is an ellipse, the distance varies between 221, 463 and 252, 710 miles. Its diameter, 2,160 miles, makes it one of the largest satellites in the solar system, only two-thirds smaller than the planet Mercury. Compared with the earth’s diameter of 7,926 miles, the moon is more like a planet than a satellite. Indeed, some astronomers think that it once traveled as an independent body about the Sun and then was captured by the Earth.

The Moon, photographed with the 36-inch refractor of the Lick Observatory

A Barren World

The telescope resolves the patchy appearance of the moon’s face into a scene of savage grandeur. The dark areas, once thought to be seas, are great plains. The bright areas stand out as a hodgepodge of mountains and circular formations usually, although loosely, called “craters”. Under a low sun the mountains and edges of the craters cast long, black shadows or exquisite sharpness. No twilight effects or half-lights soften the harshness of the scene. We see no clouds, rivers, seas, and no forests or grassy plains. The whole moonscape, bathed in brilliant sunshine, is apparently both airless and waterless. Nor can there be any sound, for sound waves cannot travel in a vacuum. The world of the moon lies silent, without air, water, or life. It is tortured by day by the grilling sun, and by night by the intense cold of interplanetary space.

As the moon orbits the earth it rotates once on its axis and therefore always shows the same face to the earth. Speculations about the nature of all the moon’s surface features ended when Soviet and American spacecraft took photographs of the far side of the Moon. They showed that the two sides are similar, although on the far side the plains or “maria” are remarkably few in number.

The Moon in Close-Up

Spacecraft, commencing with the U.S. Ranger 7, launched in 1964, also have taken great numbers of close-up pictures of the near side. They show that the moon’s surface is literally peppered with small depressions. Even a conspicuous crater like Copernicus has ramparts and mountain peaks which are remarkably smooth.

The results obtained by the US spacecraft indicate that the moon’s surface is similar in hardness and texture to well compacted soil on the earth. We know that certain areas are strewn with rocks and boulders. The surface in general is not rough and rugged. Some areas look particularly suitable for a manned landing and exploration of foot or in some special form of lunar vehicle. We also know that the surface as a whole reflects only about 7 percent of the light it receives. This means that it is not white or silvery but largely grayish-brown in colour. But the problem of how the craters and depressions were formed is still unresolved. According to one popular view, they are collision scars produced by impacting meteorites. Another view is that they are the result of volcanic activity or, more generally, of forces that acted outwards from within the moon.

The interior of the crater Copernicus, with mountains rising about 1,000 feet from the floor. A high resolution photograph, taken by Lunar Orbiter II

Extremes in Temperature

Since the moon rotates once as it orbits the earth, day and night on the moon each last about 14 days. At and near the equator, when the sun is high in the sky, the surface gets almost as hot as boiling water. Yet during the long lunar night the absorbed heat leaks into space so effectively that the temperature falls almost to that of liquid air. Studies of the moon using radio telescopes indicate that the great range of temperature is restricted to the surface layers. Several feet down the temperature seems to be fairly constant at about -40 degrees centigrade. This suggests that the moon is a comparatively cold body. That also suggests that its surface is an extremely poor conductor but good insulator of heat.

Since the moon has no appreciable atmosphere the lunar sky, like that in the planetarium, is of the deepest black. From the moon, the Milky Way and myriads of stars can be seen in daytime. But by far the most impressive sight in the earth, shining with the radiance of 50 full moons and appearing about four times as broad as the moon appears to as.

The Earth Seen From the Moon. Photograph of the Earth transmitted from the vicinity of the Moon by Lunar Orbiter 1. The Earth is shown in the upper part of the photograph, and the surface of the Moon is below

Earth in the Sky

Seen from the moon, the earth goes through a cycle of phases once every month. It first appears as a thin crescent in the lunar daytime. Then it waxes to reach full earth in the lunar night. Finally it wanes to become a crescent again. When it is full or nearly full, it lights up the dark part of the moon to produce the earthshine or the effect known as “the old moon in the new moon’s arms”. Occasionally it covers the sun to bring about a total solar eclipse. Its dark disk is then rimmed with a bright halo, emphasizing the red shades of sunset and providing striking evidence of the presence of the earth’s atmosphere.

Contrary to the dreams of astrologers, the moon has no effect whatever on the brains and minds of human beings. But it does raise tides both in the waters of the earth and within the earth itself. The waters directly beneath the moon heap up to form a slight bulge. Those on the opposite side, or side facing away from the moon, do the same. The two bulges stay in line with the moon as it revolves about the earth. Ad the earth rotates beneath them to produce a succession of alternate high and low tides. Without the tides, many estuaries and ports would be useless. Many of our shores would become permanent rubbish heaps.

The End of the Moon

The tides produced in the body of the earth are much smaller than those produced in the seas and oceans. They distort the earth’s crust, causing many places to fall and rise several inches a day. They also, along with the friction produced by tides in shallow seas, cause the earth’s rotation to slow down by a tiny fraction of a second a century. The day is therefore getting longer, and after several thousand million years from now the day will contain about 1,400 hours.

The earth exerts a similar braking action on the moon. The main effect of this action is to increase the moon’s distance and its period of revolution about the earth. When the moon has moved out to a distance of about 340,000 miles, its period of revolution also will equal about 1,400 hours. In other words, the length of the day will equal that of a lunar month. After that, for physical reasons, the moon will slowly draw closer to the earth. In the end it will probably break up to form rings of debris similar to the rings of Saturn.

The Sun photographed at sunspot maximum, December 21, 1957

Universe of Galaxies

The Universe of Galaxies

The discovery that our Galaxy is one among millions of other galaxies was one of the great scientific events in the first quarter of the 20th century. These other systems of stars extend in their number to the very limits of observation with the giant photographic telescope of modern times. Some 2,000 million are thought to be within reach of one of the world’s largest optical telescopes, the 200-inch reflector of the Mount Wilson and Palomar Observatories. One extremely distant galaxy, photographed with this telescope, has an estimated distance of 5,000 million light-years. If we represented the Galaxy by a dime, this remote object would be over two miles away.

Other galaxies have a remarkable range in form, size, and structure. Some are much larger than our Galaxy, others are considerably smaller. Some, like our Galaxy, are disk-shaped systems, others are almost spheroidal. In the former we find a great deal of gas and dust, but the latter contain little, if any, interstellar material. Recently astronomers have discovered galaxies which are apparently interacting one with another; also galaxies whose nuclei or central parts are presumably exploding. All told, it is now reasonably clear that galaxies, like stars, evolve at different rates towards final dissolution. There is no permanence even among galaxies.

Universe and Galaxies : Deep Space Ships. The Man’s knowledge of the universe is very small, but the quest is endless. Illustration: Elena

The history of astronomy from the time of Newton to the present day is one of great interest, but the main threads of early enquiry become more difficult to trace as we approach the modern period. One present problem is the almost overwhelming rate at which new knowledge is being acquired. If the rate increases, which is more than likely, our picture of the universe in, say, one, two or three hundred years will be completely different from that at present. As different perhaps, as the present one is from the picture drawn by Aristotle.

Another sobering thought is that the more we discover, the more we find awaits discovery. The quest is endless. So, in the last analysis, Man’s knowledge of the universe will always be small compared with what he has still to discover.

The Sun and the Galaxy

The Sun and the Galaxy

Sun, a Moving Star

As the telescope grew in size and power, time-honoured beliefs in astronomy were swept aside. A whole new world of experience and discovery lay open to the astronomer who could construct telescopes superior to those used by Galileo Galilei. In 1655 Christian Huygens discovered Titan, the brightest member of Saturn’s large family of satellites. Four years later he announced that Saturn was accompanied by a “ring, thin, plane, nowhere attached, and inclined to the ecliptic”. In 1671 Jean D. Cassini came across Iapetus, another of Saturn`s satellites, and by 1684 had discovered three more. But the most exciting addition to the solar system came in 1761 when William Herschel, using a home-made telescope, discovered the seventh planet, Uranus.

Evidence that the old notion of fixed stars was incorrect was provided by Edmund Halley in 1718. A comparison of old and new catalogues of star positions revealed that three bright stars, Sirius, Arcturus, and Aldebaran, were moving in relation to their neighbours. Herschel and other astronomers made similar studies and concluded that the sun and its family of planets were moving through space. Copernicus had supposed that the sun was at the centre of the universe, but that centre turned out to be a moving one.

Herschel’s 40-foot-long reflecting telescope erected at Slough, England, in 1787. Engraving: Leisure Hour, November 2, 1867, page 729

The Milky Way System or Galaxy

With the aid of large reflecting telescopes which he made himself, Herschel found that the sun is one star in a system of many thousand million stars. At one time he thought that the system called the Milky Way System or Galaxy contained all the stars of the Universe. At another he suspected that there were other similar systems, but that they were so distant as to defy resolution into stars by even his largest telescopes.

We now know that the sun is a star in a vast rotating complex of stars, gas and dust, similar in shape to a pin-wheel. The complex is so enormous that the distance across it expressed in miles would give an absurdly great number. Astronomers therefore use a much larger unit, the light year. This is the distance the light travels in a year. Light travels at a speed of 186, 283 miles a second, so in a year it covers a distance of about 6,000, 000, 000, 000 miles. If it could travel across a diameter of the Galaxy, the journey would take about 100,000 years.

Hence we say that the diameter of the Galaxy is approximately 100,000 light-years. To obtain a mental picture of this, imagine that the entire solar system could be reduced to the size of a dime. The sun would then be a microscopic speck but the Galaxy would measure some 1,000 across.

Herschel thought that the sun was near the center of the Galaxy but we now know that it is about two-thirds of the way from the center to the edge. The sun and neighbouring stars. The sun and neighbouring stars travel round the centre at a speed of about 160 miles a second, but the distances involved in their flight are so immense that they take at least 200,000,000 years for one complete journey.

Planetariums Around the World

Planetariums Around the World

Twelve Zeiss planetariums were established in Germany and Austria between 1925 and 1928. Rome had one in 1928, and Moscow followed suit in 1929. The Adler Planetarium, Chicago, opened in May 1930, and had such great success that similar projects opened in Philadelphia (1933), Los Angeles and New York (1935), and Pittsburgh (1939). Zeiss universal instruments also were installed during the 1930’s in planetariums in Hamburg, Stockholm (moved to Chapel Hill, Carolina, in 1949), Milan, Brussels, Osaka, Paris, Tokyo.

During World War II most planetariums were either dismantled or destroyed. Soon after the war some of the staff from Carl Zeiss Jena established a rival factory at Oberkochen in West Germany, and since the 1950`s planetarium instruments have emanated from both sources. The instrument of the MacLaughlin planetarium in Toronto, Ontario, Canada was the 14th to leave Carl Zeiss Jena, and the second of its kind to arrive in Canada, the first being the Zeiss Oberkochen instrument of the Dow Planetarium in Montreal, Quebec, Canada.

Zeiss Telescope

Modern Zeiss planetarium instruments, although similar in overall appearance to prewar models, differ from them considerably in points of design. Both Zeiss firms have appreciated the importance of development and improvement in this field. The same is also true of Spitz Laboratories Inc., of Yorklyn, Delaware, a firm which has equipped several hundred small scale planetariums for high-school, colleges, and museums, mainly in the United States. The projector assemblies, compact, relatively inexpensive, and easy to service are designed to operate under domes of the order of 20 and 30 feet in diameter.

In Spitz instruments the star globe takes the form of a hollow dodecahedron or 12-sided figure in the center of which is mounted a single high-intensity point-source of light. The fainter stars are formed by pinholes of various appropriate sizes, while the brighter stars are reproduced by auxiliary lens systems.

The same principle of star formation is used in larger Spitz instruments. The latter, dumb-bell in shape, are installed in major planetariums at Montevideo, Colorado Springs, Colorado, Flint, Michigan while others of intermediate size, capable of additional rotation about a vertical axis are at East Lansing, Michigan; Houston, Texas; Trenton, New Jersey; Salt Lake City, Utah; Bradenton, Florida.

Large planetarium instruments are also made by Goto Optical Manufacturing Company of Tokyo, Japan. Several of these are in use in major planetariums in Japan; one is at St. Louis, Missouri, and another at Armagh Observatory, Northern Ireland.

Custom-made planetariums

This brief survey would not be complete without reference to two unusual but important planetarium instruments, both custom-made and having design features different from those found in the Zeiss models. The first, completed in 1952 for the Morrison Planetarium, San Francisco, took four years to construct and called for the combined skills and efforts of many craftsmen associated with the workshops of the California Academy of Sciences. The hemispheres containing the star field projectors are close to the center of rotation and the sun, moon and planet projectors are at the two ends. The starplates were formed in a most ingenious way. Grains of carborundum selected for size and shape were placed individually on a small glass plate in such a way as to represent in position and size the positions and brightnesses of the various stars in a particular area of the real sky. The plate was then sprayed with opaque black lacquer and when this was dry the tiny grains were removed one by one. The projected stars are therefore irregular in shape and look remarkably like real stars.

The other large custom-made planetarium instrument is found in the Charles Hayden Planetarium, Boston. It was designed and built in Springfield, Massachusetts, by two brothers, Frank and John Korkosz, and departs considerably from the dumb-bell shape. Individual projectors for the 88 brightest stars are distributed over two end hemispheres, each of which contains a 1000-watt light bulb. There are also four quarter-spheres, two for each hemisphere of the sky, in which are 500-watt light bulbs for the stars of the 3d and 4th magnitudes, and 250-watt bulbs for those of the 5th and 6th magnitudes. Altogether, more than 9,600 stars are projected. The instrument has movement in precision, but the moving planetary projectors have yet to be fitted.

(Text published in 1971).