Galileo Galilei was an Italian astronomer, physicist and engineer, sometimes described as a polymath, from Pisa. Galileo has been called the "father of observational astronomy", the "father of modern physics", the "father of the scientific method", and the "father of modern science".

Here are some of the key things he studied:

Astronomy:

* The phases of Venus: He observed that Venus goes through phases just like the moon, which supported the heliocentric model (sun-centered) of the solar system.

* Moons of Jupiter: He discovered four of Jupiter's largest moons, which further strengthened the heliocentric model by showing that not everything revolved around Earth.

* Sunspots: He observed sunspots, proving that the Sun wasn't perfect, as was previously believed.

* The Milky Way: He observed the Milky Way and concluded it was made up of countless stars.

Physics:

* Motion: He conducted experiments on falling objects and inclined planes, which helped establish the foundations of classical mechanics.

* Inertia: He developed the concept of inertia, which states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an external force.

* Pendulums: He studied the properties of pendulums, which later led to the development of accurate clocks.

* Telescopes: He improved the telescope, making it a powerful tool for astronomical observations.

Other Areas:

* Mathematics: He was highly skilled in mathematics and used it extensively in his scientific research.

* Engineering: He designed and built various instruments, including telescopes, compasses, and military fortifications.

Galileo's work was groundbreaking and revolutionary. He challenged the established scientific views of his time and paved the way for the scientific revolution. He is considered one of the most influential scientists of all time.