Samuel Chiltern answered
It's not easy to give a simple answer to this question, but there are two key factors that explain how the planets move:
On Earth, we are accustomed to seeing objects slow down and, eventually, stop. This is because whatever energy was initially given to the object in order to get it moving has been expended. Motion comes to an end as a consequence of the action of friction, for example from the ground or from the air.
Out in space there is very little friction, since there is very little matter (besides stars, planets and asteroids), so there isn't very much material to cause friction, and slow down planetary motion.
When the big bang occurred, the planets were given a huge amount of motion as a consequence, and the effects of friction haven't been compelling enough for them to lose that motion yet.
The Effect of Gravity
Pre-existing motion is not the full story, however. Sir Isaac Newton knew that an object with mass exerts a gravitational influence on other objects nearby. This phenomenon is hard to detect in small objects, but in massive objects - such as the sun and the planets - it can be observed easily.
The earth, for example, would carry on moving in a straight line, and eventually leave the solar system entirely, if it wasn't for the gravitational pull of the sun reigning it in to an elliptical path around the sun.
The hardest thing to understand about the effect of gravity within the solar system is why the planets don't just collapse into the sun, since it is acting upon them so strongly.
The easiest way to understand this is to think of each planet as a ball passing over the horizon of the sun, which is always curved. So, as the planet 'falls' towards the sun, the horizon 'falls' away too, leaving the planet roughly suspended in the same position as it was before.
- The motion that the planets were given when they were formed has not disappeared, and so is still acting upon them
- The effect of gravity influences the direction in which they move
On Earth, we are accustomed to seeing objects slow down and, eventually, stop. This is because whatever energy was initially given to the object in order to get it moving has been expended. Motion comes to an end as a consequence of the action of friction, for example from the ground or from the air.
Out in space there is very little friction, since there is very little matter (besides stars, planets and asteroids), so there isn't very much material to cause friction, and slow down planetary motion.
When the big bang occurred, the planets were given a huge amount of motion as a consequence, and the effects of friction haven't been compelling enough for them to lose that motion yet.
The Effect of Gravity
Pre-existing motion is not the full story, however. Sir Isaac Newton knew that an object with mass exerts a gravitational influence on other objects nearby. This phenomenon is hard to detect in small objects, but in massive objects - such as the sun and the planets - it can be observed easily.
The earth, for example, would carry on moving in a straight line, and eventually leave the solar system entirely, if it wasn't for the gravitational pull of the sun reigning it in to an elliptical path around the sun.
The hardest thing to understand about the effect of gravity within the solar system is why the planets don't just collapse into the sun, since it is acting upon them so strongly.
The easiest way to understand this is to think of each planet as a ball passing over the horizon of the sun, which is always curved. So, as the planet 'falls' towards the sun, the horizon 'falls' away too, leaving the planet roughly suspended in the same position as it was before.
