We identify an object in motion because it is transfering a certain mass from one position to the next.
Motion requires a mass, a force and a speed or excelleration.
It doesn't really matter what you take, but for something to move it must transfer its mass from one position to the other and teh only way to do so is to be affected by a certain force. It must also transfer its mass from one postition to another at a speed, any speed. However, the only objects that can create forces are particles, or to create a large enough gravitational or magnetic pull to actually affect anything considerably there must be a lot of them. Suspend a rubber sheet a cross something and then drop a large ball int he middle of it. This basicaly represents the Sun.