https://www.universetoday.com/143791/how-long-is-a-day-on-venus-astronomers-make-their-best-measurement-yet/In 1989, NASA launched the Magellan spacecraft. Magellan arrived at Venus in August 1990, and entered a three hour, near-polar elliptical orbit. After 487 days, and almost 1800 orbits, Magellan completed its mapping mission, and also measured Venus’ LOD at 243.0185 days, with an uncertainty of nine seconds.
Cornell University, the Jet Propulsion Observatory, and other institutions wanted to come up with a more accurate measurement. They analyzed 29 years of Earth-based radar data on Venus, from 1988 to 2017. Their paper is titled “The mean rotation rate of Venus from 29 years of Earth-based radar observations.” It’s published in the journal Icarus.
The radar imaging gave the team a look at surface features and their positions over 29 years. Rather than trying to nail down an exact, stable length of day (LOD) for Venus, they came up with a mean value of 243.0212 ± 0.0006d. This mean value is important for any future missions.
