Arc

The camera curves around the subject, so the background rolls past behind them while they hold the middle of the frame. That sweep is the proof that the scene is a space and not a picture.

It also changes what is behind somebody without moving them. A character can stay exactly where they are and end the shot against a different wall, which is a quiet way to change the meaning of a line.

The camera travels along part of a circle with the subject at the centre. The radius stays roughly constant, so the subject does not grow or shrink while the background sweeps.

Speed is read from the background, so the further away the background is, the slower the same arc appears. A tight interior arcs fast. The same move in a field looks almost still.

Give it a start and an end. An arc that begins on a profile and finishes on a three quarter view has a shape. An arc that just revolves has none.

Use it around a person making a decision, around two people facing each other, and to open up a space that a static frame has flattened.

Avoid it when the subject is already moving. An arc around a walking figure fights the walk, and both moves lose.

A 360 degree orbit goes all the way round and returns to where it started. An arc is a part turn and stops somewhere new, which is why it can finish on a composed frame.

The test is whether the move closes. If the background comes back to the frame it started on, that is an orbit. If it lands somewhere else, it is an arc.

The model delivers a straight sideways track, so the subject slides across the frame instead of holding the centre. Say the path is curved and the subject stays centred throughout.

The other failure is a full revolution when you wanted part of one. Name the finishing frame, such as ending on a profile, and the model stops there instead of going round.