How long is a day on Venus?
Short answer: Either 116.75 or 243 Earth days, depending on how you define a "day".
Solar day: 116.75 Earth days
Sidereal day: 243.025 Earth days
Long answer: Measuring the length of a day on Venus is more complicated than Earth due to the unique way Venus rotates. Generally speaking, the word "day" means the time it takes a planet to spin around once on its own axis. Another way to measure a day is the time it takes the Sun to reach its highest point in the sky from one rotation to the next. On Earth these two measurements are almost the same but on Venus they are very different.
The first definition, the rotation period, is called a "sidereal day". On Venus this is 243.025 Earth days. This is actually longer than a year on Venus (224.7 Earth days).
The second definition, based on the Sun's position in the sky, is called a "solar day". On Venus this is 116.75 Earth days.
Note that Venus rotates "backwards" compared to most other planets, which is why there's a big difference between the sidereal and solar day.
If you were on the surface of Venus, the Sun would rise in the west and set in the east (the opposite of Earth).
Of all the planets in our Solar System, Venus rotates the slowest.
Author: Dave Owen