The light of the night sky: Astronomical, interplanetary and geophysical

SummaryWe bring together our general results in two figures. Figure 14 portrays the resolution of the light of the night sky into its three principal components based on a series of zenith observations extending over a year at the two stationse: Fritz Peak in Colorado, U.S.A., (latitude N 39°.9, longitude W 105°.5) and Haleakala in Hawaii, U.S.A. (latitude N 20°.7, longitude W 156°.3). The observations are from a current study by Roach and Smith (1964a) using photometers centered on wavelength 5300 Å. With respect to sidereal time the airglow continuum is a constant. Local time changes in the airglow may occur but such changes are averaged out with respect to sidereal time. The two Milky Way traverses are conspicuous features of the integrated starlight curves. The variation of the zodiacal light is the result of the variable ecliptic latitude of the zenith throughout the year. A refined analysis of the data, not shown in the plot, gives a further variation of the zodiacal light as a function of λ- λbd, the differential ecliptic longitude between the zenith and the sun. The zodiacal light is the brighter of the three components except when the Milky Way is in the zenith. The zodiacal light tends to be systematically brighter toward the horizon so that it is definitely the most prominent of the three for the sky as a whole.The interrelationships of the constituents of the light of the night sky are shown from a different point of view in Figure 15 where the ordinate is logarithm of the surface brightness and the abscissa is logarithm of the distance or extent. Moving downward in the plot the features of the night sky appear below the line corresponding to the end of twilight. The brightness of the nightglow, the zodiacal light and gegenschein, the integrated starlight and galactic light are comparable (on the logarithm scale) but one is impressed with the vastly different linear distances in connection with the several phenomena. The nightglow is a terrestrial phenomenon having a thickness of about one atmospheric scale height (log R ≈ 7). The zodiacal light is an interplanetary phenomenon with a characteristic dimension of one astronomical unit (log R ≈ 13). The integrated starlight from our galaxy has a characteristic maximum dimension of some 30 kpc (log R ≈ 23). Finally the extra galactic nebulae which collectively contribute much less than 1% of the light of the night sky are at distances as much as log R ≈ 28. They can be photographed individually in spite of the competition of the sky background and in spite of the hazard of extinction by intervening dust.In the preparation of this report the writer has been impressed with the confluence of several circumstances that make possible the observation of the universe in the visible part of the spectrum. Any one of several contingencies might have made such observations impossible.Let us consider the matter of contrast. The prime example here is the bright (but beautiful!) day sky which prohibits serious daytime study of the astronomical sky. There follows, during a diurnal terrestrial rotation the period of twilight which under the best of circumstances lasts a little less than 1 1/2 hours but which, during the local summer, in the vicinity of polar regions persists all night. The obliquity of the ecliptic is sufficient to make a stimulating annual sequence of seasons but small enough to keep the twilight period of reasonable duration over a good portion of the earth.A hazard narrowly averted is that due to the interplanetary dust cloud leading to the zodiacal light. The concentration of dust is very small indeed (Figure 10) so that an increase by a factor often would be trivial in terms of the constitution of the solar system. It is of interest to speculate that during the history of the solar system the interplanetary dust concentration may have been significantly greater than at present. But such an increase would result in a night sky so bright (average zodiacal light 2000 S10 (vis) instead of 200) that the Milky Way would be difficult to see and the airglow difficult to measure. The aesthetic gain in a rather spectacular zodiacal light pattern over the sky would hardly compensate for the loss from the absence of the details of our galactic universe. The effect of such an enhanced zodiacal light would correspond to that experienced in a planetarium when the operator adjusts the rheostats to bring on dawn and the celestial objects disappear.A permanent twilight that would have the same effect would be due to the hydroxyl nightglow if (a) it were concentrated in the visible part of the spectrum rather than in the near infra red or if (b) the human eyes were sensitive in the near infrared.The narrow escape from the cosmic ignorance that would have resulted from a situation in which the observer found himself in a less favorable environment is well illustrated by the “zone of avoidance” of extra galactic nebulae in the vicinity of the Milky Way plane. If our galaxy were not highly flattened so that its extent perpendicular to the plane is sufficiently small to permit an observational window outward we would not have been able to photograph the extra-galactic objects and we would have been content with a rather restricted concept of a universe consisting of a single galaxy. The same dire result would have occurred if the sun to which our planet is attached were more deeply embedded in the galactic dust near the galactic center. Thus we find compensation for our non-central location.There can be little doubt that human ingenuity would in time have overcome any or all of the above circumstances as the radio astronomers have done by changing the exploring frequency so as to avoid the difficulties. But this would have taken time, especially in the absence of the stimulation of the knowledge gained by visual and photographic observations. It is likely that the time lag would have been sufficient that the present review could not have been written by the present author. It may be conjectured whether other astronomers on other planets are as fortunate or whether, after all, this is “the best of all possible worlds”.