It's impossible to say exactly when scientists "learned about Venus" since our understanding of the planet has grown continuously over time. However, here's a breakdown of key discoveries:

* Ancient Times: Venus has been observed with the naked eye since prehistoric times. Its brightness and apparent movement across the sky made it a prominent object in many ancient cultures, who named it after various deities.

* Telescopic Observations: With the invention of the telescope in the 17th century, astronomers like Galileo Galilei were able to observe Venus' phases, similar to those of the Moon. This observation strongly supported the heliocentric model of the solar system.

* 18th and 19th Centuries: More detailed observations were made, revealing its thick atmosphere and slow retrograde rotation.

* Space Age: With the advent of space probes, starting with Mariner 2 in 1962, our understanding of Venus truly exploded. We gained knowledge of its incredibly hot surface, crushing atmospheric pressure, and volcanic activity.

* Continued Discovery: Even today, we continue to learn more about Venus through missions like the European Space Agency's Venus Express and NASA's upcoming VERITAS mission.

So, while we can pinpoint early observations, our knowledge of Venus has evolved over centuries. It's a constant process of refinement and discovery.