Some of Galileo’s inventions were created because Galileo needed money to support his family and help provide dowry for his sisters. He created a simple thermometer that could register variations in temperature but did not make any money off of it. In 1596, he built a compass used for aiming cannonballs. It was also adapted for civilian use in land surveying. He made quite a bit of money off of this new invention, and he sold some of the compasses to his students many of which were wealthy members of aristocracy. Many of Galileo’s inventions were built in an effort to earn more money to support his family.
Galileo also built a telescope. Although he had heard that a Dutch scientist was working on one, he had never seen it. With some vague ideas regarding how to build the device and his own innovation, he created a telescope with 3x magnification. He eventually built a telescope with 30x magnification. The telescope was also known as a spyglass and used on land and sea. Though Galileo is not credited with inventing the telescope himself, he was able to create one almost without any input and made a good sum off of it.
Another invention that Galileo worked on was a pendulum clock. Although Christiaan Huygens was credited with creating the pendulum clock, Galileo discovered isochronism – that the time that it takes for the pendulum to swing is not linked to the arc of the pendulum. He also created a design for a pendulum clock in 1641, but never completed one before his death. These were not his only inventions. In 1954, he patented a pump that raised water with the use of a horse to power it.
Most of Galileo’s inventions were not actually instruments he created. Instead, he made many discoveries. When Galileo studied the Moon with his telescope, he discovered that its surface was rough and uneven, covered with mountains and craters. Many believed that the Moon was completely smooth, so Galileo’s discovery was not well received at the time. With his invention of the telescope, Galileo discovered the first four moons around Jupiter. Although he first named them after a powerful Italian who would be his patron, the moons were renamed after himself. Galileo studied the tides on Earth and comets as well as other celestial objects. He is also famous for being a proponent of heliocentrism.
Astronomy Cast has an episode on choosing and using a telescope.