You can measure distance by time. How far away is it? Oh about 20 minutes. But it doesn’t work the other way. When do you get off work? Around 3 miles.

– *Jerry Seinfeld*

Actually, you can measure time by distance, as was done above but expressing *the* time by distance requires that time be seen as changes of place. The sun and stars would represent different places throughout the day and night, rather than different times. A circular clock with hands would be seen as pointing to different places on the rim, and measured by the circumferential distance. Or time could be correlated with the distance traveled by a space probe leaving the solar system.

It’s easy to think of space and time as the *inverse* of each other because that is what it would take to turn an equation with one as the independent variable into the other. For example, an equation with time as the independent variable, *r*(*t*), if invertible, would become *t*(*r*). That is correct as far as it goes but that does not reflect the full symmetry of space and time in the context of a characteristic (modal) speed, a conversion factor between space and time such as the speed of light.

Space and time are symmetric because space can be measured by any asynchronous variable and time can be measured by any synchronous variable, and there is a symmetry between synchronous and asynchronous motions. Why? Because motion is motion, whether it is utilized for synchronous or asynchronous purposes.

One can measure a distance by a travel time because a distance represents a distance traveled, which is always associated with some travel time. Similarly, one can measure a time by a distance traveled because a time represents a travel time, which is always associated with some distance traveled.

Every speed corresponds to a pace with the change in spatial position measured by a travel time and the change in time measured by a distance traveled. And so on with velocity, acceleration, and every measure of motion.

For example, multiplying time by the speed of light converts it to distance, as is often done in relativity. The corresponding light travel time or distance traveled can measure both space and time. The synchronous and asynchronous nature, respectively, of space and time do not change. But the type of units can be either linear or circular.