The various ways of measuring space and time are parallel.

Measuring space:

- A ruler measures length, that is, the distance between two points in space (A to B).
- An ruler turned upside-down measures length backwards (B to A).
- A tripmeter measures the travel distance of a vehicle trip.
- An odometer measures the cumulative travel distance.
- A measuring wheel measures the travel distance of a wheel being pushed.
- A road map measures travel distance of a standard vehicle. See
*Geodistance*.

Measuring time:

- A stopwatch measures time, that is, the duration between two points in time (A to B).
- A timer measures the time counting down from a set time, i.e., backwards (B to A).
- A GPS watch or time clock measures the duration of an activity, such as running or working.
- A GPS watch (or smartphone app) measures cumulative travel time (or flight time).
- A measuring wheel with a stopwatch measures the travel time of a wheel being pushed.
- A clock measures travel time synchronized with a standard motion.

Note that #2 shows time can be measured backwards. Space and time can both be counted up or counted down. There’s nothing magical about it.

*Related*