Newton's theories were eventually backed up by astronomical observations by the astronomer Flamsteed and we have used them ever since.

Knowing about Galileo's work on projectiles, Newton had the idea or "thought experiment" of a very big cannon on a very high mountain. The more powerful the force of the gunpowder the further the cannonball would travel until it eventually travelled into orbit around the Earth.

See the Java Applet from Virginia.edu here.

Earth's gravity keeps you on Earth's surface and you need a big extra nudge to get you out to the Moon.

It took 300 years for someone to come along and make rocket flight a reality. Even then people didn't believe him - it just wasn't common sense. Sound familiar?