Measurements of the spin axis precession and length-of-day variations of Venus

We measured Venus’s spin axis orientation, spin precession rate, moment of inertia, and length-of-day variations with 21 Earth-based observations of radar speckles tied to the rotation of Venus obtained in 2006-2020. Venus’s obliquity at epoch J2000.0 is 2.6392 ± 0.0008 degrees (1 sigma). The spin axis precesses at a rate of 44.58 ± 3.3 arcseconds per year (1 sigma), which gives a normalized moment of inertia of 0.337 ± 0.024 and yields a rough estimate of the size of the core. The average sidereal day on Venus is currently 243.0226 ± 0.0013 Earth days (1 sigma). The spin period of the solid planet exhibits variations of 61 ppm (~20 minutes) with a possible diurnal or semidiurnal forcing. The length-of-day variations imply that changes in atmospheric angular momentum of at least ~4% are transferred to the solid planet.