To obtain the specific number, as well as the tilt of its axis and core size, scientists repeatedly bounced radar off the planet's surface over a period of 15 years to reach their conclusions, which were published last week in the online journal Nature Astronomy. "Venus is our sister planet, and yet these fundamental properties have remained unknown," said Jean-Luc Margot, a UCLA professor of Earth, planetary, and space sciences who headed up the research. Credit: NASA/JPL These estimated radar measurements, averaged from data that could change up to 20 minutes per reading, indicate that one average day on Venus lasts 243.0226 Earth days, which is approximately two-thirds of an Earth year.