Eclipse seasons occur, on the average, at intervals of a little less than six months. The gradual advance of the dates of eclipses is due to a slow twisting of the plane of the moon's orbit in a direction contrary to that of her motion, which brings the line of nodes into a line joining the earth with the sun at an earlier date each year. The law governing this motion, and the dates of eclipses of the sun and moon for twenty years, were shown in an article by the writer in the SCIENTIFIC AMERICAN for May 25, 1907. When an eclipse season occurs near the beginning of the year, there may be three eclipse seasons in that year. The year 1908 will be an example of this kind. The eclipses will be as follows: January 3, a total eclipse of the sun; June 28, an annular eclipse of the sun; December 7, a lunar appulse; December 22, a central eclipse of the sun. Fig. 1 is a plot of the moon's orbit for January, 1908. The position of the moon is shown for each day of the month at Greenwich noon. If this page is placed in a horizontal position, it may be regarded as the plane of the ecliptic. The part of the orbit shown by the full line is above that plane, and that part represented by the dotted line is below. An edge view of these planes, looking in the direction of the arrow B, is shown by the intersecting lines. (Fig. 2.) The line of intersection of these planes, which is the line of nodes represented by the point N in Fig. 2, is the line NW in Fig. 1. The direction of the moon's motion is indicated in both figures by the arrows. In Fig. 1 the arrow a indicates the approach to the ascending node N, where the moon passes from the space below to that above the plane of the ecliptic. The arrow a! shows the approach to the descending node W. The direction of the apparent revolution of the sun around the earth is indicated by the arrow A. The positions of the sun and moon are here shown by their longitudes; the formed at intervals of five days, and also on the 3d and 18th, the dates of new and full moon. The arrows radiating from the earth indicate the direction in which the sun is seen at the dates attached. The conditions necessary to an eclipse of the sun or moon are as follows: The moon must be at or very near one of the nodes at the time of new or full moon. The position of the moon, when the eclipse of the sun will occur on January 3, is shown between the positions marked 3 and 4, when the sun and moon will have the same longitude. The actual time of the eclipse is January 3, 9h. 45m.; the day beginning at Greenwich noon. The moon will be approaching the descending node N', and her center will be very near the plane of the ecliptic. (Fig. 2.) The moon will also be very near perigee, which she will reach on the 4th, at which time she subtends a maximum angle. Notwithstanding the fact that the earth will be very near perihelion, when the apparent diameter of the sun reaches a maximum, the moon will subtend the larger angle; and along a narrow path on the earth's surface, there will be a total eclipse of the sun. In Fig. 3 the earth is projected on a plane parallel to its axis, and perpendicular to the ecliptic. The point 0, which is on the meridian, a portion of which is represented by the line c O, gives the position of an observer to whom the central eclipse will be visible at noon. This point will be nearly 12 deg. south of the equator. A line extending from this point to the moon's center, and produced to the sun's center, will form a very small angle with the plane of the ecliptic, because the moon's distance from the earth is about sixty times the earth's radius, and the sun's distance is nearly four hundred times that of the moon. In the figure the line of vision (in the direction of the arrow) is therefore very nearly parallel to the plane of the ecliptic. The path along which the total eclipse may be observed is shown by the heavy line a Ob, which crosses the equator twice. (Fig. 4.) In this illustration the portion of the earth's surface from which the eclipse will be visible is limited by the dotted line, and is represented in Mercator projection. Beyond the limits of the path of totality, which is wholly in midocean, this eclipse is only partial, and within a limited land area. An eclipse season may include two or even three eclipses. Two weeks after the occurrence of a solar eclipse, an eclipse of the moon may be looked for. During this interval of time, the moon moves through half her orbit, from the position of new to that of full moon. In the latter position, if the moon is sufficiently near to a node, she will come within the earth's shadow. But in January, 1908, the date of full moon is the 18th, and the moon is at the ascending node on the 17th. (Fig. 1.) In the drawing, the sun, the earth, and the moon will not be in the same straight line until the 18th. The arrow ~b shows the direction of the line from the sun to the earth produced, indicating that the position of the full moon will be a short distance beyond that given in the plot; the actual time of full moon being 18d. lh. 37m., when the moon is too far above the plane of the ecliptic to come within the shadow of the earth. (Fig. 3.) How to Protect Our Eyes Against Ultra-violet Rays. In an interesting lecture delivered before the recent Congress of German Naturalists and Physicists, Dr. Schanz and Dr. Stockhausen discussed the action of ultraviolet rays on the eye. Dr. Stockhausen, while working on electric arcs, had been attacked by a severe "electric" opthalmia caused by the ultra-violet rays of this source of light. Such rays, while invisible to the human eye, are readily detected by means of photography. A glass plate inserted between the eye and the illuminant had so far been considered a sufficient safeguard, but Dr. Stockhausen, though protected by eyeglasses, was made seriously ill. The two authors therefore undertook an investigation as to how far glasses will absorb ultra-violet rys; only those of very short wave length, shorter than 300 w, were found to be absorbed by ordinary lamp and spectacle glasses. Now, the rays most readily absorbed will penetrate into the human organism to the smallest depth only. The most efficient, and accordingly most dangerous ultra-violet rays are those intermediary between 300 and 400 which traverse ordinary lamp and spectacle glasses. Ordinary protective spectacles will allow the blue ultra-violet rays to pass with ease. Smoke-gray spectacles, while reducing the intensity of the rays, as they do those of the visible spectrum, will not extinguish them entirely. Further experiments were made on the percentage of ultraviolet rays in artificial illuminants, decomposing their light by the aid of Watt's spectrographs. The results of those investigations, extending from pine torches and Roman oil lamps to the most recent electric lamps, show the percentage of ultra-violet rays to have increased greatly with the increasing intensity and temperature of our artfficial illuminants. No attempt has so far been made to keep away from the eyes those rays, which, being invisible, are of no aid in the act of seeing. Common experience shows that when a given amount of work can just be performed without any appreciable fatigue of the eyes in natural daylight, the eyes will become tired much more rapidly by performing the same amount of work in artificial light. This phenomenon, which is especially noticeable in the case of catarrhal affections of the eye, proves that sunlight is not very rich in invisible rays, the latter being absorbed rapidly by our atmosphere, while a considerable portion of them is lost by multiple reflection before getting to our eyes. In the eye itself, the lens will protect the retina against the effects of ultra-violet rays. By submitting the lens to the action of these rays, a strong fluorescence is obtained, showing the conversion of the invisible into visible rays. Now the question arises whether or not in the course of time this conversion of energy will result in appreciable alterations of the transforming organ. Some experimenters have indeed observed an alteration of the lens when subjected to an intense ultra-violet radiation, and it is suggested that the cataract observed in old age is due to such a cause. The authors finally point out the desirability of designing efficient safeguards against the action of ultra-violet rays, which are the more imperative, as apart from the possibility of accelerating the occurrence of old-age cataract, these rays doubtless irritate the front part of the eye. They further mention a special glass made by themselves for absorbing ultra-violet rays at a higher rate than is done by ordinary glasses. According to a contemporary, a brick which contains 60 per cent or more of silica expands on heating, but a brick containing 53 per cent or less of silica contracts at temperatures above 3,000 deg. F. On this account, if a high silica brick is used in lining a cupola, there should be a layer of sand between the shell and the brick in order to make an elastic lining.
This article was originally published with the title "The Total Eclipse of the Sun in January, 1908" in Scientific American 97, 26, 474 (December 1907)