SMITHSONIAN CONTRIBUTIONS to ASTROPHYSICS VOLUME I SMITHSONIAN INSTITUTION WASHINGTON D. C. Contents of Volume 1 No. 1. New Horizons in Astronomy. Pages i-x, 1-181. Pub- lished December 19, 1956. No. 2. Papers on Reduction Methods jor Photographic Meteors. Pages 183-243. Published May 13, 1957. Smithsonian Contributions to Astrophysics VOLUME 1, NUMBER 1 NEW HORIZONS IN ASTRONOMY edited by FRED L. WHIPPLE DIRECTOR, ASTROPHYSICAL OBSERVATORY SMITHSONIAN INSTITUTION flfe SMITHSONIAN INSTITUTION Washington, D. C. 1956 UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1956 For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price $1.50 Smithsonian Contributions to Astrophysics The Smithsonian Institution's Astrophysical Observatory is passing through a period of reorganization. The scientific headquarters have been moved to Cambridge, Mass., where a close working association with the Harvard College Observatory is being developed. These changes, as well as the many advances in science and technology, force us to reevaluate the basic scientific policies and goals of the Astrophysical Observatory. With the present rapid progress in basic science and its application to technology, research in the fundamental astrophysical processes of the sun, earth, planets, and interplanetary medium is going forward at a rapid pace. Concurrently, our mushrooming technology has become more and more sensitive to these phenomena of the solar system, heretofore thought to be of only academic interest. In keeping with these developments, the Astrophysical Observatory must broaden the scope of its activities while continuing its long-estab- lished tradition of research—a tradition that began with the pioneering studies on aerodynamics by the observatory's founder, Samuel P. Langley, and continued during years of active investigations under Dr. C. G. Abbot in solar and terrestrial phenomena and their interrelationships. Thus, our future program will include not only radiation, its variations, and its effects on our planet, but also other phenomena that affect the earth and its atmosphere. We find, for example, that corpuscular radia- tion from the sun is associated with solar activity as well as with such geophysical phenomena as the aurorae, night air glow, terrestrial magne- tism, the ionosphere, radio transmission, and other observed phenomena. Meteoric bodies, dissipated from comets or chipped from asteroidal sources, not only produce meteors in our atmosphere and provide samples of cosmic material for our museums but also ionize the high atmosphere and contribute to its chemical composition, scatter sunlight in the zodiacal light, and provide us with a cosmic ballistics laboratory of ultravelocity projectiles. The Astrophysical Observatory, therefore, faces widening horizons. It will broaden the scope of its studies of solar phenomena and atmos- pheric effects to include related phenomena within the solar system such as meteorites, hypervelocity ballistics, and the zodiacal light. It will cooperate in exploiting new methods of research and technology such as nuclear processes and artificial satellites. To strengthen its effort the Astrophysical Observatory will not carry out its research in isolation; it will coordinate its work closely with that of other astrophysicists and geophysicists who are studying similar problems. Although research in astrophysics continues to expand, no correspond- ing growth has occurred in the avenues of publication. Observational results are often so compressed in print that they cannot be analyzed in detail by independent investigators; and advances in technical procedures or instrumentations are often presented with such brevity that their value to the potential user is small. In an effort to affect these trends and to provide a proper communi- cation for the results of research conducted at the Astrophysical Observatory, we are inaugurating this new publication, Smithsonian Con- tributions to Astrophysics. Its purpose is the "increase and diffusion of knowledge" in the field of astrophysics, with particular emphasis on problems of the sun, the earth, and the solar system. Its pages will be open to a limited number of papers b\r other investigators with whom we have common interests. We earnestly hope that Smithsonian Contributions to Astrophysics will serve to promote a greater understanding, appreciation, and enjoyment of that part of the universe in which we are privileged to live. FRED L. WHIPPLE, Director Astrophysical Observatory Smithsonian Institution Cambridge, Massachusetts November 14, 1956 A grant from the National Science Foundation toward the publication of New Horizons in Astronomy is gratefully acknowledged. New Horizons in Astronomy Preface The project of collecting a series of papers oriented toward new horizons in astronomy began in the 1954-55 Panel of Astronomy to the National Science Foundation. The initial impetus came largely from the enthusiasm of Dr. Otto Struve, who has presented some of his thoughts in a paper entitled "The General Needs of Astronomy" (Publ. Astron. Soc. Pacific, vol. 67, p. 214, 1955). The resulting deliberations led to the establishment of an ad hoc committee on the "Needs of Astronomy," of which I was made chairman. I felt that some of the goals of this com- mittee could be attained by publishing a collection of papers by leaders in the various fields of astronomy who would present their concepts of research projects most likely to advance the science of astronomy during the next decade. The purpose is at least fourfold: To provide material that might assist the National Science Foundation, in its allocation of funds, to support the astronomical research most likely to be productive. To help younger astronomers choose the most fertile fields for active research. To encourage established astronomers to pause for a moment and reflect on the most advantageous planning in their own fields of research, and to supply them with a broader insight into the activities in other fields of astronomy, possibly related to their own. To encourage research in new or neglected fields of astronomy, par- ticularly those that border on other areas of physical science such as geophysics, electronics, chemistry, fluid dynamics, and the like. The publication of New Horizons in Astronomy has been supported jointly by the Smithsonian Institution and the National Science Founda- tion. The Astrophysical Observatory shares with the National Science Foundation the hope that these papers may promote intensified efforts and increasingly significant results in astronomical research. The hearty cooperation of the many busy scientists who took time from their other duties to write these papers should establish a debt of gratitude in the minds of all those who will profit by these contributions. I am deeply indebted to Mr. Paul H. Oehser, Mr. Ernest Biebighauser, and Mr. John S. Lea, of the Editorial and Publications Division of the Smithsonian Institution, and to Mrs. Lyle Boyd, of the Harvard College Observatory, for their great interest and effort in this project; otherwise these papers would not now rest in your hands. FRED H. WHIPPLE Contents Page Techniques and Instrumentation Optics: I. S. Bovven 1 Solar Instrumentation: J. W. Evans and R. B. Dunn 5 Astronomical Seeing and Scintillation: Geoffrey Keller 9 Spectroscopy: Charlotte E. Moore 13 Radial Velocities: R. M. Petrie 15 Astrometry: K. Aa. Strand 21 Photoelectric Astronomy: A. E. Whitford 25 Interferometers in Radio Astronomy: B. F. Burke and M. A. Tuve . 31 Related Sciences Rocketry: R. Tousey 39 Computing Machines in Astronomy: Paul Herget 45 Turbulence: S. Chandrasekhar 47 Astroballistics: Richard N. Thomas 49 Electrodynamics of Fluids and Plasmas: Max Krook 53 Hydromagnetic and Plasma Problems: Eugene N. Parker 59 Solar System Earth's Magnetism: Walter M. Elsasser 67 Time: William Markowitz 73 Geographical Positions: John A. O'Keefe 75 Celestial Mechanics: G. M. Clemence 79 Meteorites: John S. Rinehart 81 Meteors: Fred L. Whipple 83 Minor Planets: Paul Herget 87 Planets, Satellites, and Comets: Gerard P. Kuiper 89 Airglow and Aurora: A. B. Meinel 95 Solar-Terrestrial Relationships: Weather and Communications: Walter Or Roberts 99 Solar Physics: Leo Goldberg and Donald H. Menzel 103 Stars and the Galaxy Spectral Classification: W. W. Morgan . 115 Stellar Atmospheres: Marshal H. Wrubel 119 Aerodynamic Problems in Stellar Atmospheres: Richard N. Thomas. 123 Emission Nebula*: Lawrence H. Aller 127 381014—56 2 CONTENTS Page Stars and the Galaxy—Continued Double and Multiple Stars: Otto Struve 131 Stellar Dynamics: U. Van Wijk 135 Variable Stars: C. Payne-Gaposchkin 137 Interstellar Matter: Lyman Spitzer, Jr 139 21-Centimeter Research: Bart J. Bok 141 Some Observational Problems in Galactic Structure: Harold Weaver. 149 Galactic Clusters and Associations: A. Blaauw 159 Globular Clusters Observed through a Crystal Ball: W. A. Baum. 165 Stellar Structure and Evolution: M. Schwarzschild 177 Radio Sources: John D. Kraus 179 Techniques and Instrumentation Optics By I. S. Bowenl Geometrical optics is one of the oldest of critical definition for telescopes of very large sciences. Most of the basic laws have been aperture. Schmidt telescopes have been suc- known for centuries, yet in the past two or cessfully made with apertures up to 48 inches three decades advances in optical designs have with a focal length of 120 inches. However, in revolutionized astronomical techniques. An order to avoid chromatic