The Sun's Distance from the Galactic Plane

Abstract

Use is made of 93,106 parallaxes from the Hipparcos catalog, with a mixture of spectrum-luminosity classes, to derive the position of the Galactic plane. The reduction technique, mixed total least squares-least squares, takes into account the errors in the parallaxes, and the condition that the direction cosines of the Galactic pole have unit Euclidean norm is rigorously enforced. To obtain an acceptable solution it is necessary to eliminate the stars of classes O and B that belong to the Gould belt. The Sun is found to lie 34.56±0.56 pc above the plane. The coordinates of the Galactic pole, l _g , b _g, are found to be: l _g =0.°004±0.°039; b _g =89.°427±0.°035.This agrees well with what radio observations find and demonstrates that the IAU's recommendation in 1960 to use only radio observations to determine the Galactic pole, although correct at the time because of the paucity of optical observations, can no longer be justified given the plethora of observations contained in the Hipparcos catalog and an adequate reduction technique, unavailable in 1960. The reduction technique is also demonstrably superior to others because it involves fewer assumptions and calculates smaller mean errors.