Much of astronomy deals with the physical laws of motion and gravity, which Sir Isaac Newton described in the seventeenth century.
Newton’s laws of motion are:
1. Every body of matter remains at rest until it is moved by an external force.
2. When an external force compels an object to move, that object will move in the same direction as the force and at the same rate.
3. An object acted upon externally will react by exerting an equal force on the external force in the opposite direction.
You can easily test these laws by playing a game of marbles or croquet.
Newton’s laws of gravity state that every particle exerts a force of attraction on every other particle; the power of attraction is based on the mass of each particle and the distance between them.
This power of attraction is gravity, though we really don’t know how or why gravity works.
When an apple falls from a tree, the particles in the apple and in Earth attract each other. The bigger the object, the greater the force, so the apple falls to Earth. Theoretically, Earth responds to the apple’s gravitational pull by rising ever so slightly to meet it.
Motion and gravity always interact. How they do so depends on a given object’s power of motion and its size.
For example, the Sun’s gravitational pull is greater than Earth’s because the star is so much bigger than the planet. However, Earth does not crash into the Sun because it is in motion. It travels fast enough, and is far enough away, to counter the Sun’s pull, but not fast enough, or far enough away, to escape it.
Imagine Earth is a baseball and the Sun is the ground on which you stand. Throw the baseball. The ground’s gravity pulls the ball to the ground.
If you throw the ball faster, it travels farther before the ground’s gravity overpowers it. If the ball (Earth) traveled at 383 miles (612 km) per second, it would never hit the ground (the Sun), but go into orbit. Thrown faster than that, the ball (Earth) would be moving fast enough to escape the ground’s (Sun’s) gravity completely and sail off into space.
Earth’s average orbital speed is 18.5 miles (29.8 km) per second.