Satellite Observations of Intensity Variations of the Zodiacal Light

Abstract

OBSERVATIONS of the intensity of the zodiacal light at 6530 Å have been made with a photometer on board the satellite D2A ‘Tournesol’. This satellite was launched on April 15, 1971, from Kourou (French Guiana) into an orbit of inclination 46° with a perigee of 456 km and an apogee of 700 km. The spin axis was oriented towards the Sun with an accuracy of ∼15 arc min and a spin period of 60 s (Fig. 1). The line of sight of the photometer is perpendicular to the spin axis and therefore scans the celestial sphere in a plane orthogonal to the ecliptic plane and to the direction of the Sun. Observations in a plane perpendicular to the Earth-Sun line (that is, at elongation ε=90° or ε=−90°) were carried out during the night phase, when the solar depression angle was >25° and the altitude of the lowest observed points was >135 km. Because of this, the signal contained no contribution from the night-glow, but was only due to zodiacal light and starlight. The data cover, without interruption, the period from April 19, 1971, to June 1973.