What’s with Straight-line Motion?

To prove that gravity is proportional to and therefore a property of matter, Newton said that but for gravity, the moon would be traveling in a straight line.
The most obvious question, other than how does anyone know how the moon will be traveling without the force of gravity affecting its motion, is, what in the universe travels in a straight line?
Well, there’s one thing that does, a falling object so long as nothing interferes with its fall. But the moon is not a falling object, and even if it were removed from the Earth’s gravity field, it would not be a falling object, unless of course, it fell into the sun.
So there’s little chance the moon would be moving in a straight line. Where is there any object in space that is moving in a straight line? Nothing does, and to say nothing does because of gravity is sort of distorting the point. To move in a straight line, the moon would need a force moving it in a straight line, and the absence of a force moving it in other than a straight line.
Where is the force that is moving the moon in a straight line?
There is none!
Actually, Newton needed the moon to be moving in a straight line for one reason, and one reason, only. If the moon weren’t moving in a straight line but for the force of gravity, there’s no way he could compute the amount of gravity it would take to move the moon from its straight line motion, and thus, he wouldn’t have a second equation with which to balance his first equation.
His first equation dealt with the amount of gravity between the moon and the Earth and he assumed both bodies were made up of a uniform particle uniformly distributed in order to make the computation, a notably false assumption. Now in order to balance the first fallacious equation with a second equation, he had to make up another “fact,” he had to assume the moon would be traveling in a straight line but for the force of gravity.
Galileo had experimented with metal balls rolling down an inclined plane to come up with his own view on motion. As the metal ball gained momentum from falling down the inclined plane, it carried that momentum with it when it reached the end of the ramp and was let loose to fall in space. As gravity reclaimed its motion, the metal ball lost its momentum.
Galileo called this momentum inertia. Up until Galileo clouded the issue, the general consensus was that a moving object would eventually come to rest because of gravity. Galileo, in creating the concept of inertia, believed that if he rolled a ball down an inclined plane and up another of equal height, the only thing stopping the ball from reaching the same height was friction. If he could remove the friction, the ball would reach the same height from which it had started its roll.
From the first guy to measure the nature of gravity, this is pretty misguided. What Galileo is claiming is that gravity has no affect on the motion of the ball. Newton couldn’t really use Galileo’s inertia, which was only overcome by friction, because then his computation of gravity between the Earth and the moon wouldn't be capable of overcoming the straight line motion of the moon.
As a result of this little problem, Newton neither agreed with the general thinking that objects would eventually come to rest as a result of gravity, nor Galileo’s thinking, that the only thing restraining inertia was friction, Newton had to go to great difficulty in beginning the Principia with a restatement of the laws of motion. He needed a law that would justify his second equation, that the gravity of the Earth and the moon were overcoming the straight-line motion of the moon.
Newton therefore very carefully rephrased all prior points of view in a manner blatantly favorably to what he was attempting to do on a theoretical basis: Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.
The general thinking was, a moving object would come to rest as a result of gravity. Galileo’s was, inertia, once acquired, could only be overcome by friction. Newton turns both observations on their heads, and note I say observations simply because Galileo merely hypothesized a condition in which no friction existed, he didn’t say such a condition existed, by assuming motion, motion in objects that simply didn’t exist in reality.
And he did so quite cleverly. He created one law in which half the law dealt with observable reality and the other half dealt with a hypothetical reality, a reality based on no observable reality, and he made it sound so very reasonable. An object at rest will stay at rest and an object in motion would stay in motion unless another force intervened. Who could disagree with the statement? It was obviously a true statement. An object in motion would indeed continue in motion unless another force intervened. It’s only common sense.
The only problem with the dual statement is, there’s no equivalency between the two because the physical situation of each of its parts is not similar.
When an object is at rest, we know why it is at rest. It is at rest because of gravity. An object at rest with respect to gravity will continue at rest until some force overcomes the gravity.
However, an object in motion assumes that there is a force maintaining that motion. In our experience, or anyone’s experience, objects don’t move without a current force causing them to move.
Newton didn’t rely on Galileo’s inertia to keep his moon moving in a straight line but for the force of gravity because Galileo concluded that it would take friction, not gravity, to overcome inertia. Thus, Newton, a very religious man, simply said that God would keep the moon moving in a straight line but for gravity (for someone reputed to have uncovered how the solar system operated, Newton’s view of the planets was fairly parochial).
In fact, it wasn’t until the end of the 18th century, when science was shedding itself of religion, that it became necessary to replace Newton’s God with the ad hoc swirling mass of gas evoking, what else, but frictionless space, or Galileo’s inertia, ignorant as that era was of Newton’s carefully crafted law of motion to do away with the Galilean concept of inertia.
Today the planets move because of Galilean inertia, but their straight-line motion is overcome by Newtonian gravity.
Science unabashedly uses two conflicting concepts to explain something, while admitting the whole thing is a can of worms.
The fact that both sides of Newton’s equation were based on false assumptions pales in comparison to what science did when, toward the end of the 18th, and during the beginning of the 19th centuries, science discovered that Newton’s equations didn’t work anywhere in the solar system.