How long is the solar system's longest day? Venus has the answer Reuters | May 03, 2021 11:46 PM EDT Handout image courtesy of NASA shows the planet Venus at the start of its transit of the sun, June 5, 2012. Venus passed directly (Photo : REUTERS/NASA/AIA/Solar Dynamics Observatory/Handout) Data obtained by bouncing radio waves off Venus - treating it, as one scientist said, like a giant disco ball - is providing new insight into Earth's closest planetary neighbor, including a precise calculation of the duration of a Venusian day. The study also measured the tilt of the Venusian axis and size of the planet's core, allowing for a deeper understanding of an enigmatic world sometimes called Earth's 'evil twin.'