Heaven and Earth

© 1998 by Robert C. Moler

Heavenly bodies, though wonderous, are governed by the
same laws of physics that work on the earth

In ancient times Greek philosophers thought of the earth as a place of change and decay, where imperfection ruled. The heavens, however, were thought to be perfect and eternal. That was a long time ago. But today most of the population has equally wacky ideas of the differences between the heavens and the earth.

It is part of the reason why many are dazzled by some celestial events, still believe in astrology after the earth was shown not to be in the center of the universe, and the movements of the planets were not governed by gods willing to spell out the future, but a universal force called gravitation which is affected only by mass and distance. And perhaps why some of the same people believe in UFOs while also believing humans never walked on the moon. Or why some seemly normal folks commmited suicide in order to hitch a ride on a UFO supposedly following Comet Hale-Bopp.

Let's face it. We live in a unusual part of the universe, pinned by gravitation to the surface of a celestial body. These celestial bodies can move freely in response to gravitation and the forces that set them in motion. Most of the universe is reasonably empty space and not planet surfaces where we live.

In the common sense world of the Greek philosopher, who disdained the idea of actually trying an experiment to check their 'common sense' notion that any body placed in motion would soon stop. Or that heavier objects fell faster than lighter ones. No one actually checked up on these 'common sense' notions until Galileo did, and it became obvious that there was a reason for the former observations, and it wasn't in the nature of the object itself but it was friction. Friction is things rubbing on other things that transfer and transform energy to slow the object down. In the vacuum of space between the planets and stars there is very little friction, so the stars and planets seem to wheel eternally by.

Galileo founded physics as we know it. His pioneering experiments and observations lead to Isaac Newton's discovery of the three laws of motion.

First Law: A body will stay at rest or at a constant velocity unless acted upon by a force. This flew in the face the Greek Philosopher's assumptions.

Second Law: A body, when acted upon by a force will be accelerated along the line of the force. The acceleration is proportional to the magnitude of the force and inversely proportional to the mass. This law quantifies the first law and introduces the concept of inertia, the more massive the body the harder it is to accelerate. That's why football linemen are so big.

Third Law: This is colloquially stated: For every action there is an equal and opposite reaction. If a body applies force on another body this second body applies the same force back to the first. That's how rockets work. The escaping exhaust gasses are pushed away from a rocket but the gasses also push back against the rocket to provide thrust. It also explains why one must be careful when stepping from a small boat to the dock.

Newton capped this with his masterpiece, the Law of Universal Gravitation. It explains why we are plastered here on the face of planet earth, instead of floating around in the vacuum of space. Every mass attracts every other mass in the universe with a force that is proportional to the sum of the masses and inversely proportional to the square of their distance apart.

That the inertial mass of Newton's second law and the gravitational mass of his law of universal gravitation have the same value has profound implications. Einstein's General Theory of Relativity is a theory of gravity in which the observer cannot tell if he or she in being accelerated by some force or in a gravitational field without looking outside to see which it is.

This month's meeting will present Hands-On Astronomy. It will hopefully bring heavenly concepts down to earth.


Questions? Comments? Send Email to me at bob@bjmoler.org

Updated: 12/30/99