Natural Time

Natural time is an ongoing attempt to re-frame the way I think about time, and to better connect the passage of time with the motion of the Sun, Moon, and stars, from which it was originally derived.

Background

Prior to the advent of accurate timepieces, the time of day was determined according to the position of the Sun. Midday is defined as the point when the Sun reaches the highest point of its arc from East to West.^[1] From there, the 360° of rotation is divided into twenty-four 15° increments corresponding with the hours of the day; 11:00 am is the time at which the Sun is 15° East of its position at midday. In this way, the time of day is determined by the position of the Sun, rather than the time of day existing unto itself and the motion of the Sun being projected onto that scale. Under a solar time system, it is redundant to ask ‘Where will the Sun be at 11:00 am?’—or at any other time—becuase the time of 11:00 am is defined by the Sun reaching the position assigned to 11:00 am.

Atomic measurements

Today, the second is an SI unit defined according to the observed oscillation frequency of a Cesium atom.^[2] From these observations we derive International Atomic Time (TAI), a coordinated time standard denoting the observed passage of time on Earth.^[3] Over time TAI drifts from observed solar time,^[4] so there is a different coordinated time standard which underpins modern life, Coordinated Universal Time (UTC). UTC is derived by taking TAI and offsetting it by an integer number of leap seconds to keep it from drifting too far from observed solar noon at 0° latitude (the prime meridian).

You would be forgiven for thinking that UTC, given the precision with which it is maintained, always provides an accurate representation of the time of day. For reasons we will get to, this is actually quite far from the truth! But first, a further digression on how we have historically thought about time.

Historical Time Systems

As the Earth moves around the Sun, the amount of daylight varies throughout the year due to the tilt of Earth’s axis. The rate of change of daylight hours is non-uniform over the year,^[5] changing the most from day-to-day at the Spring and Fall equinoxes and the least at the Summer and Winter solstices.^[6]

Civil time, which is the time system that we are familiar with, uses units of fixed length—an hour is the same amount of time in Summer as in Winter—but this was not universal among historical time systems. Often, the day and night would each be allocated a fixed number of hours, the length of which would vary depending on the amount of daylight at that time of year. In addition, a ‘day’ was generally figured from one midday to the next, rather than our current system of midnight to midnight.

You may notice that a trend is emerging of things being defined in terms of solar noon. Why might that be? Why is noon more special than any of the other 23 hours of the day? And here we arrive at the first hint at the lie that is civil time: a day is not uniformly 24 hours long. While we think of a day as ‘one revolution of the Earth’, the Earth is moving through its orbit at the same time it is rotating. So, when it has rotated a full 360°, it has also advanced a bit and the observed Sun will be a bit East of where it was at the same ‘time’ yesterday; the Earth needs to rotate 360° and then a bit more in order for the Sun to return to its zenith and reach solar noon.

In fact, the length of time from one observed solar noon to the next varies about 24 hours^[7] by as much as 30 seconds longer to 20 seconds shorter over the course of the year. You may be wondering, how on Earth^[8] did we sweep that under the rug of our civil time system to the point that the average person has no clue about it?

Mechanization and the Mean Sun

The deeper you dig into the celestial movements that actually create our experience of time, the messier things get. Our neat, tidy civil time system is a tablecloth thrown over the vagaries of the physical world, and the lynchpin of the illusion is the Mean Sun. Rather than attempt to model the irregular motion of the Sun,^[9] civil time models an abstraction which is smipler and more predictable. The Mean Sun is a virtual Sun taken to have the same orbital period as the apparent Sun,^[10] but with an orbital velocity equal to the yearly average velocity of the apparent Sun.^[11] Therefore, the daily Eastward motion of the Mean Sun does not vary, and the length of a mean solar day is a constant 24 hours all through the year.

The discrepancy between a varying and constant orbital velocity causes the apparent Sun to lag and lead the Mean Sun in turns over the course of a year, and civil noon may differ from solar noon by ~15 minutes in either direction at various points of the year. To account for this we turn to the Equation of Time, which is a periodic function describing this difference with respect to the day of the year.

This may seem strange and convoluted—and it is!—but there is a good reason why our time system relies on an idealized abstraction of the Sun. To wit, accurately representing apparent solar time using eighteenth-century technology required additional components and increased manufacturing complexity.^[12] The relative simplicity of modeling mean solar time gave it an edge as precision clock and watches became widely adopted, leading to the ubiquity of our current system.

What time is it?

So, you may be asking, what’s the point of all this? Why does it matter that we use mean time instead of apparent time? What difference does it make that we model each day as being the same length?

Have you ever said ‘Good morning!’ to someone, only to think ‘Actually I guess it’s not morning anymore, it’s afternoon.’ But it just really didn’t feel like afternoon? Well, that’s because it probably wasn’t. Take the Equation of Time, sprinkle in Daylight Savings and longitudinal deviation from the standard time of your Time Zone, and apparent solar noon can be as late as 1:30 PM. We’ve been using exclusively mean civil time since we learned to read clocks, and yet we are so attuned to the motion of the Sun that we get mixed up when civil time disagrees with apparent solar time.

We are wired to connect with the motion of the Sun, Moon, and stars, as we have been doing for millenia, but our modern time system severs this connection. We are used to thinking in civil time, and imposing it on celestial motion. But celestial motion is time. The Mean Sun approximation is a relic of the technological path we took to get here, but we have surpassed that model and can define elapsed time precisely using only atomic observation.

My aim with this project is to re-examine how we think about time at the human level, and whether we can devise a time system that maintains all of the benefits we have realized in precision and coordination while getting us more in touch with the physical reality around us.