Olekcii Mach/Alamy Venus has a core that is approximately 7000 kilometres in diameter – about the same size as Earth’s – according to the first observation-based estimate. Jean-Luc Margot at the University of California, Los Angeles, and his colleagues examined Venus from 2006 to 2020, using the Goldstone Solar System Radar in California to hit the planet with radio waves. They then used both this and the Green Bank Telescope some 3000 kilometres away in West Virginia to track the echoes of the waves as they bounced back to Earth, a technique called radar speckle tracking. Advertisement This allowed them to measure very small changes in Venus’s spin and movement. They found the planet’s day, roughly equivalent to 243 Earth days, fluctuated by up to 21 minutes over the 15 years of observation. They also found that the axis of Venus wobbled very slightly in a pattern that their calculations suggest would repeat every 29,000 years.