Final answer:
Venus' significant external effects on its rotation include spinning in a backward direction and a longer day than year. Its rotation period is 243 days and was determined through radar observations. The slow rotation of Venus affects its calendar as well.
Step-by-step explanation:
Venus has significant external effects on its rotation. One significant effect is that Venus spins in a backward or retrograde direction, from east to west. This is different from the forward rotation of most planets in our solar system.
Another effect is that the rotation period of Venus is 243 days. This was determined by bouncing radar signals off the planet's clouds and identifying topographical surface features that showed up in the reflected radar signals. The slow rotation of Venus also affects its calendar. Venus takes 225 Earth days to orbit the Sun, but it takes 243 Earth days to complete one rotation on its axis. As a result, the day on Venus is longer than the year.