Frazz Cartoon: What Is A Watch?

Frazz 300dpi - A Watch Is A Map


Sundial Time, Local Time, Standard Time: What’s the difference?

The mission of the TimeNavigators website is to introduce young people to the nature of time … how it feels as it goes by and how we mark it, measure it and tell each other what time it is. The vocabulary of time started in pre-history, but we moderns have managed to put our own spin on some of the concepts.

In the beginning, time reckoning was tightly connected to astronomy.

The motions of the sun & stars connected us with the passage of the hours during the day & night. The moon and seasons connected us to longer cycles of time, like weeks, months and years.

With TimeNavigators’s clock dials and calendars, our kids can begin to learn the vocabulary of time and begin to develop a sense for how time feels as it passes.

Before sundials and clocks came along, one way to reference time was to talk about where the sun was, is or will be in the sky. Examples of this are:

  • We left at sunrise.
  • It’s high noon now. Let’s eat.
  • We’ll stop the hunt at sundown.
  • Let’s meet at the fork in the north trail tomorrow afternoon, when the sun is three-hands above the horizon.

Pre-clock people lived in a world where they spent much more time outdoors than most modern people do. In those days, most were involved in agriculture, which is largely an outdoor activity. Because artificial lighting was expensive and poor in quality, even homemakers and craftspeople did as much of their work outdoors as possible. Pre-clock people became very good at tracking where the sun was (or should be on cloudy days). Just as you can look at your watch today to see if it’s getting late, they could watch the sky and get a feeling for how fast the day was passing.

So how were times referenced?

Early on, the pre-clock marking of time was fairly unstructured. One could use a hand gesture to indicate how high the sun should be at the beginning of some event. [See blog post “Drilling Wells and Telling Time in Niger” for a modern story about this.] Or sun positions could be quantified with a simple counting system like “two hands above the horizon,” or by using simple descriptors like “sunrise,” “sunset,” and “high noon.”

As time went on, people began to make use of the fact that the sun follows a predictable path through the sky each day. This path was divided into imaginary parts (hours) of equal length from sunrise to sunset. Although different ancient societies experimented with different numbering systems, today’s analog clocks are based on a system where daylight was divided into twelve equal parts. The following definitions applied to the daytime hours:

  • Sunrise, by definition, occurred at the “sixth hour” in the morning.
  • Noon was the “twelfth hour.” To be “noon,” two conditions had to obtain: (1) the sun had to be due south; (2) the sun had to be at the highest elevation above the horizon that it would be all day.
  • The next hour after twelve was the “first hour.”
  • Sunset happened at the “sixth hour” after high noon.

Why wasn’t there a “zeroth hour” in this counting system? The Roman numeral system on which the modern clock dial is based did not have a “zero.” It started with “one.” Adding “zero” to the list of counting numbers was the development of another society; today, the corresponding set of number symbols are called Indo-Arabic numerals.

The definition of the 6th, 12th, and 6th hours applied whether the daytime hours were short (as in winter) or long (as in summer). If the sun was on the horizon, it was the 6th hour of the morning (or evening), exactly. Before the invention of clocks, people understood and accepted that hours would vary in length throughout the year, and even from day to night. Summer hours were long and nights hours were short. And it was just the opposite in the winter.

Further comments on the meaning of an hour

While an hour can be thought of as a length  of time, it should be noted that the sun’s hour-angles in the sky, themselves, are better understood as points  in space  as well as points in time. Professional astronomers still use “hours” … in a measure similar to longitude that is called “right ascension” … to identify a star’s location east or west of a point on the celestial equator (which looks like the earth’s equator, only much bigger and appearing to float above it … out among the stars).

The following sentences use “hour” two different ways: “The sun is at the 10th hour. In 15 minutes, it will be a quarter past 10.”

In the first usage, note how the hour is a point in time … “The sun is at the 10th hour.” In the second usage, see how an hour is used as a length of time … “In 15 minutes, it will be a quarter of an hour past the hour of 10.”

Now what’s this about local time vs. standard time?

In the old world of sun-in-the-sky time and, later, with the early usage of shadow poles as sundials, all time designations were what we moderns would call “local time.”

Local noon in Houston occurs when the sun appears due south of Houston (and, not coincidentally, at the highest point above the horizon that it will be all day). Similarly, local noon in San Antonio is when the sun appears due south of San Antonio. Likewise, local noon in El Paso is when the sun appears due south of El Paso.

In the old days, when there was no instant communications between cities, it didn’t matter that local noon just doesn’t happen at the same time for Houston, San Antonio and El Paso. These three cities are located in places that are east and west of one another; the rotation of the earth causes the sun to cross the southern compass point at different times for each city. For San Antonio, local noon comes about 12 minutes later than for Houston, and El Paso’s local noon is about 44 minutes later than Houston’s.

When people walked or rode a horse across the country, these differences were not important. But when America and Canada started laying tracks and building stations for the great intercontinental railway system, it became impossible to print a table of train arrivals/departures for scores of cities if each town’s local noon variations had to be accounted for.

So standard time was invented.

The history of standard time in the United States and Canada began November 18, 1883 when United States and Canadian railroads instituted standard time in time zones. Before then, time of day was a local matter, and most cities and towns used some form of local solar time.

With Standard Time replacing local time, and with accurate clocks counting hours that do not vary in length from day to night or from season to season, modern people have broken the connection between sun location and the time indicated on a time piece. Nowadays, you have to read the newspaper … for your own city … to tell today’s times for sunrise and sunset. It will be different for places that are east/west of you or north/south of you. But that’s another story from Astronomy 101.