Locating events

The world described in terms of events

Noticing where something is (its position) and when it's there (what the time on the clock is) are both right-now recordings of data.

So where something is and when it is there are both at an instant.

A record of a journey is a set of these pairs (→d, clock time). These pairs are often referred to as events. A multiple exposure picture is a good depiction of such a sequence. Less valuable, but more accessible are successive frames on a movie clip of the motion.

You record a position and an instant to fix an event. A series of such recordings narrates the journey, as has become popular on websites where you're encouraged to share such stories by uploading your GPS tracks (which are just such a series of recordings of events).

Teacher: Let's look again at this motion on the movie clip. How long does each frame last?

Ed: A very short time – it depends on the video camera.

Teacher: What should we look for to get the best record?

Belinda: As short a time as possible.

Teacher: Good: as close to zero as possible.

Teacher: So when is the ball here [points]?

Carly: Just as the clock points to that time. Not before, not after.

Teacher: That's very clear. It's at that instant, and only that instant. At that clock time the position_{instantaneous} is x. And it's only there right then, because the ball keeps moving.