1911 Encyclopædia Britannica/Zodiacal Light

by Simon Newcomb

ZODIACAL LIGHT, a faint illumination of the sky, surrounding the sun and elongated in the direction of the ecliptic on each side of the sun. It is lenticular in form, brightest near the sun, and shades off by imperceptible gradations, generally becoming invisible at a distance of 90° from the sun. Until a recent time it was never observed except in or near the zodiac; hence its designation. Its breadth varies with the time and place of observation, depending upon the position of the ecliptic with respect to the horizon. In the tropics, where the ecliptic is nearly perpendicular to the horizon, it may be seen after the end of twilight on every clear evening, and before twilight on every clear morning, unless blotted out by moonlight. It then presents a nearly vertical wedge-shaped form, the base of which extends 15° or 20° on each side of the point at which the ecliptic intersects the horizon. The point of the wedge is quite indefinite, the extremely diffuse light gradually fading into invisibility at a height which may range from 50° to 70° or even more, according to the keenness of the observer’s vision. The boundary everywhere is ill defined so that no exact statement of the extent of the light can be made. The brightness is at its maximum along its central line, called the axis of the light. Along this axis the brightness continually increases as the sun is approached. Owing to the softness of the outline, it is not possible to fix the position of the axis with precision; but, so far as observations have been made, it is found that it lies near the ecliptic, though deviating from it by a quite sensible amount.

Having this position, the conditions of visibility will be best when the ecliptic, and therefore the axis of the light, are nearly perpendicular to the horizon, and, as the angle between the ecliptic and horizon becomes acute, will deteriorate, slowly at first, more and more rapidly afterwards, owing to the increasing effect of atmospheric absorption. This effect is enhanced by the light being brighter as we approach the sun. More and more of the brighter regions of the light will then be near the horizon the more acute the angle. The result is that the light can be only indistinctly seen when the angle with the horizon is less than 45°, unless in a region where the atmosphere is unusually clear. From this statement of the conditions it will be seen that the tropical zone is the most favourable for observation, and that the most favourable hour of the day at which the light can be seen must always be the earliest after sunset and the last before sunrise. Practically, this is when twilight is first ended in the evening, and about to begin in the morning. At these hours the angle of the ecliptic with the horizon varies with the season. At the close of evening twilight the angle is greatest about three weeks before the vernal equinox. The months of February and March are therefore best for the evening observations in the northern hemisphere, but the light can generally be seen from January until April. Similar favourable conditions prevail in the morning from September to November.

It is clear that the light proceeds from a region surrounding the sun, and lenticular in form, the axis of the lens being nearly perpendicular to the ecliptic, while the circumference extends at least to the orbit of the earth. If it did not extend so far as this it could not be seen as frequently as it is at a distance of 90° from the sun. The accompanying figure shows the form of the outline, as it would appear to an observer on an outer planet were the light of the sun cut off. The hypothesis which best explains all the phenomena is that the light is that of the sun reflected from an extremely tenuous cloud of particles having the form and extent described, and becoming more and more tenuous as the earth’s orbit is approached until, immediately outside the orbit, it fades into complete invisibility. The fact that the light widens out toward the sun leads to the inference that it entirely surrounds the sun. It is therefore of interest to test this inference by observations at midnight in such a latitude that the distance of the sun below the horizon is no more than necessary to preclude the possibility of twilight. Such an opportunity is offered when the sun is near the summer solstice, in latitudes not differing much from 50° north. A transparent atmosphere and clear horizon are necessary, conditions which can best be secured on a mountain top. The visibility of a light corresponding to the inference was shown by Simon Newcomb, by observations at the top of the Brienzer Rothorn, in 1905. Previously to this, E. E. Barnard had observed the same phenomenon at Chicago. The only source of doubt as to the validity of the conclusion that this is really the zodiacal light arises from the possibility that, after the close of the ordinarily recognized twilight, there may be a faint illumination arising from the reflection of light by the very rare upper atmosphere, shown by the phenomena of meteors to extend some hundred miles or more above the earth’s surface. The problem of separating a possible effect produced in this way from the zodiacal light proper may seem to offer some difficulty. But the few observations made show that, after ordinary twilight has ended in the evening, the northern base of the zodiacal light extends more and more toward the north as the hours pass until, towards midnight, it merges into the light of the sky described by the two observers mentioned. Yet more conclusive are the observations of Maxwell Hall at Jamaica, who reached conclusions identical with those of Barnard and Newcomb, from observations of the base of the light at the close of twilight, which he estimated at 60° in the line through the sun.

These observations show that the outline on that portion of the light commonly seen in the morning or evening is concave instead of convex, as it would be were the cloud strictly lenticular. The actual outline of the cloud is that of which a section through the sun is shown in the figure. Since the tenuous edge of the lens extends beyond the earth's orbit it follows that there must be some zodiacal light, whether it can be seen or not, passing entirely across the sky, along or near the ecliptic. Observations of this zodiacal band are therefore of great interest. It has been seen to stretch across the sky at midnight by several observers, especially Barnard, to whom it appears 3° to 4° wide. He found it to be best seen during the months of October, November and May.

Intimately connected with this band and with the zodiacal light is the Gegenschein, or counter-glow, a faint illumination of the sky in the region opposite the sun, which may generally be seen by a trained eye when all the conditions are favourable. Unfavourable conditions are moonlight, nearness to the Milky Way, and elevation of the light above the horizon (and therefore a depression of the sun below the horizon) of less than 20°, and the presence in the region of any bright planet. The Milky Way renders the object invisible during the months of June, July, December and January. Its light is so faint and diffuse that it is impossible to assign dimensions to it, except to say it covers a region of several degrees in extent. Barnard, the most successful observer, assigns diameters of 5° or even 10° or more. From what has been said of its position it is evident that the zodiacal band, when seen across the sky, must include it. It may therefore be regarded as an intensification of this band, possibly produced by the increased intensity of the light when reflected nearly back toward the sun, and therefore toward the earth. From the description given of the zodiacal band and the Gegenschein, it is clear that these objects should be best seen at the highest elevation, especially within the tropics. But the only well-authenticated observations we have of this kind show anomalies which have never been cleared up. This is especially the case with those of Chaplain George Jones, who spent eight months at Quito, Peru, at an elevation of more than 9000 ft., for the express purpose of observing the phenomenon in question. He saw the zodiacal band at midnight as a complete arch spanning the sky, agreeing in this point with the observations of Barnard. One anomaly of his observations is his description of the arch as sometimes so bright as to resemble the Milky Way, a condition which would make it easily visible at ordinary altitudes. Another anomaly is that he never saw the Gegenschein, but describes the band as equally bright in all its parts, except near the horizon. We are therefore forced to the conclusion that either he must have been a quite untrustworthy observer, or that there are anomalies in the phenomena which are yet to be explained.

The latter possibility is also suggested by the curious fact that the visibility of the light does not seem to be proportional to the transparency of the atmosphere. Barnard reports it as sometimes best seen when the sky is slightly milky, while during the observations already mentioned from the Rothorn the Gegenschein was scarcely, if at all, visible, though the conditions were exceptionally favourable. It has even been said that observers at great elevations have failed to see the zodiacal light; but it is scarcely credible that this failure could arise from any other cause than not knowing what it was or where to look for it. Moreover, it has been well seen by Hansky from the observatory on the summit of Mont Blanc.

In studying the causes of the phenomenon we must clearly distinguish between the apparent form as seen from the earth, and the real form of the lenticular-shaped cloud. The former refers to the earth, which is continually changing the point of view of the observer as he is carried around the sun, while the latter relates to the invariable position of the matter which reflects the light. First in importance is the question of the position of the principal plane, passing through the sun, and containing the circumferential regions of the cloud. This plane must be near, but not coincident with, that of the ecliptic. It has therefore a node and a certain inclination to the ecliptic. The determination of these elements requires that, at some point within the tropics where the atmosphere is clear, observations of the position of the axis of the light among the stars should be made from time to time through an entire year. In view of the simplicity of the necessary appliances, and of the small amount of labour that would be required, we find a singular paucity of such observations. The most elaborate attempt in the required direction was made by the American chaplain, George Jones, during a voyage of the “Mississippi” in the Pacific Ocean, in 1852-54. Owing to the varying latitude of the ship, and the fact that the observer attempted to draw curves of equal brilliancy instead of the central line, the required conclusions cannot be drawn with certainty from these observations. More recently Maxwell Hall in Jamaica made a satisfactory determination during the months from January to March, July and October, and carefully discussed his results. But the observations do not extend continuously throughout the year, and do not include a sufficient length of the central line on each evening to enable us to distinguish certainly the heliocentric latitude of the central line, as distinct from its apparent geocentric position. Yet his observations are of the first importance as showing the smallness of the deviation of the central line from the ecliptic. When smoothed out, the maximum latitude is less than 3°, which seems to preclude the coincidence of the central plane of the light with that of the sun's equator. Hall also reaches the interesting conclusion that the plane in question seems to lie near the invariable plane of the solar system, a result which might be expected if the light proceeded from a swarm of independent meteoric particles moving around the sun. Chaplain Jones concluded, from his observations at Quito, that the central line of the arch made an angle of 3° 20′ with the ecliptic, the ascending node being in Taurus, near longitude 62°. This is about 40° from the ascending node of the invariable plane, so that there is a well-marked deviation of his results from those of Hall.

Yet more divergent are the conclusions of Francis J. Bayldon, R.N.R., who made many observations while on voyages through the Pacific Ocean between Australia and the west coast of North America. He places the ascending node at the vernal equinox, and assigns an inclination of 4°. He found that as the observer moved to the north or south the axis of the light appeared to be displaced in the direction of the motion, which is the opposite of the effect due to parallax, but in the same sense as the effect of the greater atmospheric absorption of the light on the side nearest the horizon. He also describes the moon as adding to the zodiacal light during her first and last quarters, a result so difficult to explain that it needs confirmation. It is noteworthy that he could see the zodiacal band across the entire sky during the whole of every very clear moonless night in tropical regions.

If we accept the general conclusion already drawn as to the form and boundary of the region from which the light emanates, the next question is that of the matter sending it forth. The most plausible view is that we have to do with sunlight reflected from meteoric particles moving round the sun within the region of the lens. The polariscope and the spectroscope are the only instruments by the aid of which the nature of the matter can be inferred. The evidence afforded by these instruments is not, however, altogether accordant. In 1867, Ångström, observing at Upsala in March, obtained the bright auroral line (W.L. 5567), and concluded that in the zodiacal light there was the same material as is found in the aurora and in the solar corona, and probably through all space. Upsala, however, is a place where the auroral spectrum can often be observed in the sky, even when no aurora is visible, and it has generally been believed that what Ångström really saw was an auroral and not a zodiacal spectrum.

Professor A. W. Wright, of New Haven, also made careful observations leading to the conclusion that the spectrum differs from sunlight only in intensity. Some evidence has also been found by the same observer of polarization, showing that a considerable portion of the light must be reflected sunlight. The observations of Maxwell Hall also embraced some made with the spectroscope. He was unable to see any marked deviation of the spectrum from that of the sun; but it does not appear that either he or any other of the observers distinctly saw the dark lines of the solar spectrum. Direct proof that we have to do with reflected sunlight is therefore still incomplete.

The question whether the Gegenschein can be accounted for by the reflection of light from the same matter as the zodiacal band is still unsettled. Taking the general consensus of the observations it would seem that its light must be so much brighter than that of the band as to imply the action of some different cause. In this connexion may be mentioned the ingenious suggestion of S. Arrhehius, that the phenomenon is due to corpuscles sent off by the earth and repelled by the sun in the same way that they are sent off from a comet and form its tail. In other words, the light may be an exceedingly tenuous cometary tail to the earth, visible only because seen through its very great length. The view that no cause intervenes additional to that producing the zodiacal band is strengthened, though not proved, by a theorem due to F. R. Moulton of Chicago. He shows that, supposing the cloud of particles to move around the sun in nearly circular orbits immediately outside the earth, the perturbations by the earth in the motion of the particles will result in their retardation in that part of the orbit nearest the earth, and therefore in their always being more numerous in a given space in this part of the orbit than in any other. This view certainly accounts for some intensification of the light, to which may be added the intensification produced by the vertical reflection of the sunlight.

A new interest was given to the subject by the investigations of H. H. Seeliger, published in 1906, who showed that the observed excess of motion of the perihelion of Mercury may be accounted for by the action of that portion of the matter reflecting the zodiacal light which lies nearest to the sun. Plausible though his result is, the subject still requires investigation. It seems not unlikely that the final conclusion will be that instead of the reflecting matter being composed of solid particles it is an exceedingly tenuous gaseous envelope surrounding the sun and revolving on an axis the mean position of which is between that of the sun’s equator and that of the invariable plane of the solar system.

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