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Just to be clear, this answer to ‘which planet has the longest day’ is based on this criteria: a planets day is how long it takes it to complete one rotation on its axis. This is also referred to as its rotational period. So, Venus has the longest day of any planet in our solar system. It completes one rotation every 243 Earth days. Its day lasts longer than its orbit. It orbits the Sun every 224.65 Earth days, so a day is nearly 20 Earth days longer than its year.
Length Of A Day On The Planets In Our Solar System
Mercury: 58 days and 15 hours
Venus: 243 days
Mars: 24 hours, 39 minutes and 35 seconds
Jupiter: 9.9 hours
Saturn: 10 hours 45 minutes 45 seconds, but can only be approximated because of atmospheric density.
Uranus: 17 hours, 14 minutes and 24 seconds
Neptune: 16 hours, 6 minutes and 36 seconds, but it is a bit more complicated than that. The equator and poles rotate at different speeds. You would have to do more research on the planet to fully understand the varying day on Neptune.
Now, back to why the Venusian day is longer than its year. Venus is closer to the Sun; therefore, its orbit takes a shorter period of time than its rotation upon its axis. The planet also orbits in retrograde. That means it orbits in the opposite direction than the Earth. If you were standing on Venus, you could see the Sun rise in the West and set in the East.
A manned Venus flyby mission was proposed in the late 1960s. The mission was planned to launch in late October or early November 1973, and would have used a Saturn V rocket to send three men. The flight would have lasted approximately one year. The spacecraft would have passed approximately 5,000 km from the surface about four months into the flight. There have been several unmanned probes and flybys of the planet, including MESSENGER and the Venus Express. Future proposed missions include the BepiColombo, Venus InSitu Explorer, and the Venera-D.