Shapiro I I
Science. 1967 Jul 28;157(3787):423-5. doi: 10.1126/science.157.3787.423.
Combination of two types of radar data shows the orbital plane and equator of Venus to be included by less than 2 degrees, and the sidereal rotation period to be 243.09 +/- 0.18 days (retrograde)-remarkably close to the 243.16-day period for which the spin would be in resonance with the relative orbital motions of Earth and Venus. In this resonance, Venus would make, on average, four axial rotations as seen by an Earth observer between successive close approaches of the two planets. Estimates of the instantaneous spin period, accurate within about 0.01 day, would provide important information on the difference of Venus's equatorial moments of inertia, on their orientation, and on the magnitude of the tidal torque exerted on Venus by the sun.
两种雷达数据的结合表明，金星的轨道平面与赤道的夹角小于2度，恒星自转周期为243.09 +/- 0.18天（逆行）——这与243.16天的周期非常接近，在该周期下金星的自转将与地球和金星的相对轨道运动产生共振。在这种共振中，从地球上的观察者来看，在两颗行星连续的近距离接近之间，金星平均会进行四次轴向旋转。对瞬时自转周期的估计精确到约0.01天以内，这将提供有关金星赤道转动惯量差异、其方向以及太阳对金星施加的潮汐扭矩大小的重要信息。
