. Compendium of meteorology. Meteorology. NIGHT-SKY RADIATIONS FROM THE UPPER ATMOSPHERE By E. O. HULBURT Naval Research Laboratory, Washington, D. C. Introduction In places remote from artificial illumination at night, with no moon, the luminosity of the sky is due to several sources of light, all of which are at a considerable dis- tance. The sources are (1) radiations from the gases of the upper atmosphere, (2) polar aurorae, (3) zodiacal light, (4) comets and possibly scattered sunlight in interplanetary space (if there is something there to scatter the sunlight), and (5) stars and nebulae
. Compendium of meteorology. Meteorology. NIGHT-SKY RADIATIONS FROM THE UPPER ATMOSPHERE By E. O. HULBURT Naval Research Laboratory, Washington, D. C. Introduction In places remote from artificial illumination at night, with no moon, the luminosity of the sky is due to several sources of light, all of which are at a considerable dis- tance. The sources are (1) radiations from the gases of the upper atmosphere, (2) polar aurorae, (3) zodiacal light, (4) comets and possibly scattered sunlight in interplanetary space (if there is something there to scatter the sunlight), and (5) stars and nebulae in interstellar space. If we omit polar aurorae from consideration, the radiations from the high atmospheric gases are the strongest source of night illumination, being four or five times as intense as all of the other sources combined. In more detail, the intensity of the night-sky light averaged over the sky is divided as follows: For the photographic spectrum [5] region from 3500 to 4500 A about }4 is due to the stars, f-Q to the high atmos- pheric emissions, and possibly Je to emissions from interplanetary space (the amount attributed to sources in interplanetary space has decreased as the measure- ments have improved; it is now down to about }â £ of the whole and may eventually become a few hundredths or even less); for the visible spectrum [17] seen with the dark-adapted eye, about }i is due to the stars and % due to the high atmosphere; for the entire spectrum from about 3000 to 10440 A, because of its strong infrared nitrogen emissions, the high atmosphere con- tributes more than %o of the nocturnal radiation and the stars less than 3^o- These fractions are average values; for areas in the sky such as the Milky Way where there are many stars the fractions of the sky luminosity due to the stars are greater than the average values, and for areas in the sky where there are few stars the fractions are less. Spectrum The night-sky light is feeble, and even with spectro-
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