Radial Velocity Measurements on the Zodiacal Light Spectrum

Abstract

THERE has been speculation for some time about the possibility of detecting the motion of zodiacal dust particles by accurately measuring the wavelength position of a solar Fraunhofer absorption line scattered in the cloud. Ingham¹ derived the Hβ absorption line profile that would result from solar radiation scattered by dust particles in circular Keplerian orbits about the Sun. He showed that, in the ecliptic, at an elongation of 30° from the Sun, the expected wavelength shift is 0.3 Å to the blue for dust particles co-rotating with the Earth, and 0.8 Å to the red for contra-rotating dust. Ring et al.² made measurements on the Hβ absorption line from Mount Chacaltaya, in the Bolivian Andes, and found a blue wavelength shift in the evening zodiacal light, a possible interpretation of which could be in terms of a zodiacal dust cloud co-rotating with the Earth at twice the expected circular orbital velocity. During a programme of observations to map the night sky Hβ emission line³, many spectra were recorded in the region of the evening zodiacal light between elongations of 35° and 180° from the Sun. The observations were made with a gas pressure scanned Fabry–Perot interferometer⁴, working at a resolving power of 8,000, from an 11,500 foot observatory on Testa Grigia in the Italian Alps (lat. 45° N.). The instrument scanned, in steps of 0.14 Å, a range of 2 Å centred on the 4861.33 Å laboratory Hβ wavelength, thus providing about 14 closely spaced points across the spectral feature. Wavelength calibrations were made before and after each spectrum was recorded.