Galileo invented a hydrostatic balance for accurately determining the specific gravity of objects, demonstrating his early aptitude for practical inventions.

Galileo designed and marketed a military compass, a proportional compass used for calculating and drawing geometric figures, as well as for aiming cannons more accurately. This invention was a source of income and further developed his understanding of applied mathematics.

Galileo conducted experiments on motion and mechanics, formulating principles such as the law of falling bodies (that objects fall at the same rate regardless of their mass, ignoring air resistance) and the principle of inertia. These findings challenged Aristotelian physics and paved the way for Newton's laws of motion.

Galileo significantly improved upon existing telescope designs, creating instruments with far greater magnification. He was among the first to use the telescope for systematic astronomical observations.

Galileo published "Sidereus Nuncius", a groundbreaking book detailing his telescopic observations, including the discovery of the four largest moons of Jupiter (now called the Galilean moons), the rough and uneven surface of the Moon, and the existence of countless stars invisible to the naked eye. These discoveries challenged the Aristotelian view of a perfect and unchanging heavens.

Galileo observed and documented sunspots, demonstrating that the Sun was not a perfect, unblemished sphere, further contradicting Aristotelian cosmology.

Galileo observed that Venus exhibits a full range of phases, similar to the Moon. This observation provided strong evidence in favor of the heliocentric model of the solar system, as it could not be easily explained by the geocentric model.