Earth wasn't "discovered" in the sense that a new planet was found. Earth has always existed. Humans have lived on Earth for millions of years, and have always known it as their home.

Perhaps you're asking about a specific event, like the first time:

* Humans realized Earth was a sphere: Ancient Greek philosophers like Pythagoras and Aristotle proposed this idea, but it wasn't definitively proven until Ferdinand Magellan's circumnavigation of the globe in the 16th century.

* We learned about Earth's place in the solar system: Early astronomers like Nicolaus Copernicus developed the heliocentric model, which placed the Sun at the center of the solar system, with Earth revolving around it. This was a major shift in understanding our place in the universe.

* We started exploring Earth's geography: Explorers like Christopher Columbus, Ferdinand Magellan, and James Cook charted the globe, revealing continents and oceans previously unknown to Europeans.

So, while Earth has always been here, our understanding of it and our place within it has evolved over time.