Earth, planets and orbits
1610
A telescope on Jupiter
Galileo Galilei
Does a heavy ball fall faster than a light one?
Galileo pointed a telescope at Jupiter and saw four moons circling it. Not everything goes around the Earth, after all. He also studied falling and rolling balls, and found that distance fallen grows with the square of time.
Positions come from published orbital elements and start at today’s date. Planet sizes are not to scale, and distances are squeezed unless you choose true scale.
How they figured it out
In 1609 Galileo heard about a Dutch spyglass and built his own, eventually magnifying about twenty times. In January 1610 he saw four little “stars” beside Jupiter that changed position night after night: moons, circling something other than the Earth. He published within weeks.
To study falling, he slowed gravity down by rolling balls down a gentle ramp and timing them with a water clock. In equal times the ball covered distances in the ratio 1, 3, 5, 7: the odd numbers. Add them up and you get the squares: distance grows with time squared. The famous drop from the Leaning Tower of Pisa is probably a legend.
The equation
Double the falling time and an object falls four times as far.
See it in real life
A hammer and a feather on the Moon
In 1971 Apollo 15 astronaut David Scott dropped a hammer and a falcon feather on the airless Moon. They landed together. On Earth, drop a phone and a tennis ball from the same height: apart from air, they hit the floor at the same moment.
Where engineers use it
Projectiles
sizes everything from drop tests to how long a dropped tool falls.
Optics
Telescope and camera design grew from the lenses Galileo improved.
Worked example: Dropped from a 20 m roof
- s, ignoring air.
Where it came from
- The Sun in the centre 1543
Led to
Jupiter’s moons showed that not everything circles the Earth, strong support for Copernicus.
What it led to
- Universal gravitation 1687
Built on
Galileo’s falling bodies and his idea of inertia became Newton’s laws of motion.