Let us open this chapter with something of a thought experiment. Take a moment and think of everything you can about time. How do you conceive of time? My guess is that you think of time as something of a progression of units through the day and night, which move eventually through days and weeks and months and years. Those units are probably thought of as having stable duration (an ‘hour’ or a ‘minute’ or a ‘second’, or even an unspeciﬁed ‘moment’), with there being 60 minutes in an hour, 24 hours in a day, and so forth. The unit of time is constant. Every hour is like every other hour in length. We measure activities in terms of the number of hours they will take, or portions of hours. A ‘long’ movie might take over 3 hours. A short phone call might take only a quarter of an hour. The daylight in the summer has more hours in it than the daylight in winter and, hence, corresponds to ‘long’ days; conversely, the night time in summer has fewer hours than the night time in winter and, hence, corresponds to ‘short’ nights; and so forth.