The annual oscillations of the brightnesses observed at 12 and 25 μm by IRAS near the ecliptic poles are mainly due to the inclination of the symmetry plane (SP) of the interplanetary dust cloud upon the ecliptic, but also, secondarily, to the eccentricity of the earth's orbit.
Comparing the brightnesses at the poles and in the ecliptic (near 90° elongation) allows a retrieval of the inclination i and ascending node Ω SP/ecliptic through an inversion technique, with very little model-dependence. The results (i = 1.5°, Ω = 90°) conflict with some of those previously obtained from the same observations by more model-dependent approaches, but they agree with former optical determinations from D2A satellite and from Tenerife
ground-based data.