Galileo Galilei

The father of observational astronomy, modern physics, the scientific method, and modern science.

Galileo Galilei portrait
Portrait of Galileo Galilei (1636), by Justus Sustermans

Time line of Galileo's life:

All truths are easy to understand once they are discovered; the point is to discover them.

-- Galileo Galilei

If you have time, you should read more about this incredible human being on his Wikipedia entry.