STAR-GAZIN G.*
ASTRONOMICAL phenomena possess an interest for all ; even the most uneducated and careless observer must be conscious of the facts that the days are longer in summer than in winter ; that the moon does not always exhibit the same appearance, and that sometimes stars are seen which are not visible at other periods. Consequently there are numerous grades of astronomical know- ledge, ranging from the uneducated (or perhaps, in these days, one should say the partially educated) ploughboy to the scientific astronomer. Leaving out of the question the mathematical consideration of astronomy, most persons know the causes of the variation of duration of daylight at different times of the year, of the phases of the moon, and of the relative positions of the principal planets. A knowledge of some of the constellations is also net uncommon, but the individual stars are known by comparatively few. This last is especially to be regretted at the present time, when much attention is being paid to the phenomena of luminous meteors ; these often come at unexpected times,—the best astronomer cannot predict the appearance of a large fire-ball, which any one may be fortunate enough to observe, and if the observer is also fortunate enough to know the stars, he will be able, by indicating the apparent path of the meteor amongst them, to materially assist in the determination of its actual position, direction, and velocity. Those desirous of learning the names of the stars will find Mr. Proctor's Star Atlas of great service. It contains all stars down to the sixth magnitude, inclusive, the celes- tial sphere being divided into twelve circular maps, overlapping one another, so that in many cases stars appear on two maps, which is very useful, when it is necessary to find the relative position between stars on different maps.
Those possessing telescopes are usually familiar with the motions of the satellites of Jupiter, with the rings of Saturn, with the phases of Venus, and the appearance of multiple stars and nebul, and necessarily with the apparent rapid motion of celestial objects due to the rotation of the earth. To them, and to many others, Mr. Norman Lockyer's Star-gazing will be of great interest. The work is founded on a course of lectures delivered at the Royal Institution in 1870, and brought into its present form with the assistance of Mr. G. M. Seabroke. It is divided into six books, in which the pretelescopic age, the telescope, time and space measurers, modern meridional observations, the equatorial telescope and astronomical physics are successfully treated. It also abounds with excellent illustrations. In the first book, we have an account of the astronomy of the ancients and of the earliest instruments, most of which are the foundation of the modern ones. The de- scription of Tycho Brahe's work is especially interesting ; with- out the use of the telescope, he succeeded in making accurate measurements to ten seconds of arc. In the book on the tele- scope, the laws of refraction of light are given and illustrated, and the functions of the eye are described, followed by dispersion of light, and the method of correcting lenses for chromatic aber- ration is explained in the chapter on the refractor. The modes of grinding specula and mounting them, and some of the results obtained by modern telescopes, and tests to be applied for deter- mining their value, are treated in the succeeding chapters.
Since the motion of the earth on its axis is regular, the best method for determining the positions of celestial bodies is to ascer- tain the exact moments at which they apparently pass a certain part of the sky. For this purpose, choice is made of the meridian,
New Star Atlas. By IL A. Proctor, B.A., F.R.A.S. Fourth Edition. London: Longmans: Green, and Co. 1877. mairaimr-gaourci/:. Itgit7i.nd Present. By J. Norman Lockyer,p.R.S. London: mac- whioh is a plane passing through the axis of rotation of the earth, and through the zenith of the observer's station. The meridian is chosen for various reasons. All fixed stars will pass the meridian daring one rotation of the earth on its axis, and the in- terval between two succeeding passages of the same star is called a sidereal day ; so that the apparent path of a star near the pole is a small circle, described once in a sidereal day, and a star op- posite the earth's equator will apparently pass through a great circle in the same interval of time. This would be true of all planes passing through the terrestrial axis, whether meridians or not, but the use of any other plane would be attended with several in- conveniences ; for example, it would be difficult to arrange an instrument which would always move exactly in the required plane; whereas, a telescope attached at right angles to an accurately horizontal axis must, when moving on this axis, always describe a plane passing through the zenith, and when the axis lies accu- rately east and west, the plane in which the telescope moves will also pass through the polar axis of the earth. The fixing of the proper position of such an instrument can only be effected by observing the exact moment at which the stars pass through the centre of the field of the telescope, and this cannot be done with- out a clock. The clock is, therefore, one of the most important instruments connected with astronomy ; and in the fourth book is given a history of clocks, from the time of the earliest, set up in Old Palace Yard, in 1288, to that of the standard clock at Green- wich, made in 1871. Meridional observations of the kind above mentioned were made by Tycho Brahe, by watching the passage -of a star through a hole in a wall, and of course without a tele- scope, and the methods of observation of these trausits of stars have been undergoing improvement until the present day. Besides knowing the moments of passage of stars across the meridian, it is requisite to know their distances from some fixed point or plane. For this purpose, their polar distances or their declination (angular distance from the equator) must be de- termined. This cannot be effected by the clock, and con- sequently the angles must be measured by means of graduated circles ; again, the diameter of planet; the distances between the components of multiple stars, must be measured ; and for this the micrometer is used, an instrument enabling measurements to be made to the extraordinary accuracy of one-fifth of a second of arc. For measurements of this latter kind, it is obvious that the object must remain in the field of the telescope as immovably as possible. To do this, the telescope must be turned in such a manner as to counteract the effect of the rotation of the earth. Au observer at one of the poles of the earth would see the stars moving at a con- stant distance from the horizon. They would neither rise nor set, so that he would be enabled to keep a star in view, if his telescope were mounted on a vertical axis which was turned round by clockwork once in a sidereal day. On the other hand, an observer at the equator would require his telescope mounted on a hori- zontal axis lying north and south, in order to follow the star. In intermediate positions, the axis to which the telescope is fixed must be inclined to the horizon, and must, in fact, be parallel to the earth's axis. A telescope placed on such an axis is said to be equatorially mounted, and if the clock driving the axis be pro- perly regulated, the star will remain apparently immovable in the field, so that the necessary measurements can be applied to it. The largest telescopes mounted on this principle are Mr. Newall's refractor, with an object-glass nearly 25 inches in diameter ; the great equatorial at Washington, with an aperture of 26 inches ; Mr. Lassell's 4 -ft. reflector ; the Melbourne equatorial, and that at the Paris Observatory, with reflectors of the same size.
As science progresses and accuracy increases, there is a ten- dency to eliminate, as far as possible, the physiological element. In the transit instrument, for example, the eye-piece has five or seven wires, and the observer has to note the time that the image of the star crosses the wires ; for this purpose, he listens to the licks of the clock while watching the star, and if it crosses the wire between two ticks, he has to estimate the fraction of the second at which it passed. It is found in this, as in all eye observations, some observers obtain different results from others, some anticipating the phenomenon, and others recording it a little too late. The necessity for estimating the fractions of seconds is obviated by means of the instrument called the chrono- graph, which consists of a drum, rotating at a tolerably uniform speed by clockwork ; the sidereal clock causes an electric current to pass through an electro-magnet at every second, making a series of dots on the drum, the interval between each pair repre- senting a second of time ; the observer at the transit instrument is able by means of a key to send a current through another electro- magnet of the chronograph, which makes similar dots by the side of the second marks on the drum. At each passage of the star across a wire, he depresses the key, and in this way the time is much more accurately measured, but still the personal equation interferes with the precision. It is hoped that means may be devised of photographing the position of the star at known periods, which would get rid of all personal equation. Photography is already employed in many astronomical observations, such as eclipses, transits of planets over the sun, sun-spots, and for making accu- rate records f the positions of the bright and dark lines in the spectra of the sun, stars, and of terrestrial elements.
Of recent years much attention has been paid to astronomical physics, that is, the investigation of the quantity and kind of light and heat reaching the earth from celestial bodies. In this way, by means of the thermo-electric pile, the spectroscope, and the polariscope, many important results have been obtained, not only with regard to the constitution of these bodies, but also of their proper motion. It would take us too far to follow these investigations, which are fully treated in Mr. Lockyer's book.
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