The brightness of the zenith sky during twilight
- 16 Downloads
A visual photometer is described for measuring the brightness of the zenith sky during twilight. The results of measurements made with it at Bombay on a large number of clear moonless evenings during the period December 1937 to March 1938 are discussed. A red filter RG1 (Schott and Gen.) and a green filter VG1 (Schott and Gen.) were used. With the latter, it was possible to continue observations even when twilight conditions had completely ceased. The variations in brightness agree with a relation proposed by Link, namely,dI/I=β sin θ sec2 θdθ, where I is the brightness of the zenith sky, θ the angular depression of the sun below the horizon and β a constant. If the brightness is due only to primary scattering from the atmosphere illuminated by sunlight, it is possible to connect β with the temperature of the atmospheric layer responsible for the scattering. The difficulty however is to ascertain to what extent secondary scattering is important.
KeywordsColour Filter Green Filter Standard Candle Electric Lamp Secondary Scattering
Unable to display preview. Download preview PDF.
- 2.P. GrunerHelv. Phys. Acta., 1932,5, 31–58, 145–60, 351–61; 1936,9, 595–601. P. GrunerGerland’s Beitr. z. Geophys., 1935,46, 1–2, 202–07. Also see the article by P. Gruner which contains a list of all the other references on the subject, inErgebnisse der Kosmischen Physik, Band III, 1938, 113–54.MATHGoogle Scholar
- 3.K. R. RamanathanMem. Ind. Met. Dept., 1927,25, Pt. I.Google Scholar
- 4.B. FessenkoffAstrophys., Observatory Publications, 1923,2, 113. (Resume in French at the end of the paper.)Google Scholar
- 7.F. Link The values are taken from the table in Gruner’s article “Neueste Dammerungsforschungen”, inErgebnisse der Kosmischen Physik, Band III, 1938, 139.Google Scholar
- 8.H. N. Russel, R. S. Dugan and J. Q. StewartAstronomy, Pt. II, 612.Google Scholar
- 9.Jean DufayBulletin de l’observatoire de Lyons, Tome XI, no, 2, 45–50, 1929 Feb.Google Scholar
- 10.Bauer, Danjon and LangevinComptes Rendus, 1924,172, 2115. The values are quoted from Gruner’s article referred to above.Google Scholar
- 12.G. H. Godfrey and W. L. Price The problem of secondary scattering of sunlight in a plane-stratified atmosphere has been treated by A. Hammad and S. Chapman inPhil. Mag., 1939,28, 99.Google Scholar