Time goes by. The future comes towards us—or, to some, we move towards it. The future arrives in the present and then both settle into the past, gone but not forgotten.

Or so it seems. Here’s another possibility: there is no past or future of this sort at all, no flow of something called time that moves from future to past at a steady rate.

Of course, events did happen in the past. The evidence is all around us: rock, oceans, old trees, pyramids, Mr. Vernon, battlefields, cemeteries. We may think such evidence strengthens our everyday notion of time going by in an orderly fashion. But time­­, and especially our experience of it, is not so simple. Time is a human thing, built from memories, from the apparent movement of the sun and stars, from measuring tools, and from traditions about the value of the past and the knowability of the future.

And yet It seems sensible to me to think of us as living only in the present.* Being alive is a present state, a current condition; no organism can literally “live in the past.” But the past is certainly with us every step of the way. We are born with codes from our parents’ DNA, we store memories in our bodies, and we hang on to useful recollections in detail. We draw from this storage continuously to manage the endless present that we live in. We use it also to imagine, wish for, plan on, and worry about the future. Time is the name we give this flow, but it is we, not time, that are flowing.

So to make time useful; we measure it and mark it. “It’s a two-hour movie.”  The label classifies the movie but is hardly precise. Say the movie, the feature itself, runs for two hours, three minutes and sixteen seconds. We would still call it a “two-hour movie” as a convenient description, but the movie runs for as long as it runs. The time is not the duration itself. Clocks and calendars help our lives tick by, but they are not time itself.

A related simplification is the belief that time moves in one direction only, from future to past (or from past to future). But as we well know, time also moves in cycles. Units of time come round again and again even though the present moves on. “I meant next week, not this week.” In fact, we label any particular moment in time using clock-and calendar units that repeat themselves. Like the rotating dials on a bicycle lock, the numbers and words  converge on a single combination. Right now it’s 9:10 am Eastern Standard Time on Tuesday, February 8, 2020.

Three of these units connect much more closely to the cycles of nature than others: these are day, month, and year. The early ancestors of the word day referred to just to daylight, in contrast to the dark of night, until the Babylonians and Romans applied it to the 24-hour cycle of day-and-night that we know as a day. The word month comes from the duration of the cycle of the waxing and waning moon. And year is anciently related to early versions of words like season and summer, probably with the sense of “that which makes a full cycle.”

So organisms both get “older” and at the same time partially renew themselves in sync with the three repetitions: the rotations of the earth, the orbiting of the moon around the earth, and the orbiting of the earth around the sun.

These cycles have embedded themselves in all bodies:

• The daily (circadian) rhythm of darkness and daylight, cold and warmth, triggers sleeping and waking, rest and exertion.
• The moon pulls the tides twice a day. And as it waxes and wanes each month, it lightens and darkens the night. Under a bright moon, lions hunt and snakes hide. Many species have adapted to these moon-and-tide patterns to survive.**
• Earth’s yearly orbit around the sun brings the four seasons, prompting flowering, mating, ripeness, hibernation.

These rhythms are built into our genes. “Clock-proteins” switch genes on when conditions are good and become built-in timers that operate even when the conditions are absent. Birds living for years in a controlled environment, for example, with 10 hours of light and 14 hours in the dark will molt (lose their feathers) at the same time of year that they would have if they were living in the wild. This field of study is chronobiology.

Because time is everywhere and nowhere, it strains language and can boggle clarity. So in closing, here are the two points that I find I can hold on to. One is that time, as in a week, is short-hand for a number of days and their events, but a measurement is not the thing itself, not even the duration itself. The second point is that our experience of time, how it happens, how it affects organisms, is neither a straight line nor a cycle but a strange and exquisite melding of both.

Time is our yardstick, essential for organizing what we call our past and our future and for agreeing on the duration of events. But what actually goes by are lives, through another day, another season, to the rhythms of the earth, moon, and sun.

*To be precise, we live a split second behind the actual moment of “now.” Visual data comes through our eyes and along the optic nerve to be interpreted by the brain. The process takes one fifth of a second. To compensate, as Bill Bryson writes, the brain “continuously forecasts what the world will look like a fifth of a second from now, and that is what it gives us as the present” (The Body, p. 55). Even the present moment, it seems, is an educated guess.

**Both the moon’s cycle and a woman’s menstrual cycle average about 28 days and many women feel synched to the phases of the moon. So it’s tempting to conclude that the moon was an evolutionary source for the duration of women’s cycles. But the evidence doesn’t point that way. See Wikipedia on Menstruation/evolution.