Lecture 10: Telling Time

Key Ideas

Timekeeping is tied to Astronomy

Divisions of the Year:

Divisions of the Day:

Civil Timekeeping & Time Zones

For more information on time, see here. Cross-quarter days see here.

Keeping track of Time

All of our time-keeping conventions are astronomically based:

Dividing the Year

Solstics & Equinoxes

The first major division of the Year is into seasons marked by the occurance of the Solstices and Equinoxes
Quarter Days
[Click on image for a larger view] (Graphic by R. Pogge)
In many cultures, the Equinoxes and Solstices were marked by holidays, some of which we still keep (in altered form) to this day

Solstice & Equinox Holidays

Cross-Quarter Days

These occur at the mid-way points between the Solstices and Equinoxes (they are sometimes called the "Mid-Quarter Days").
Cross-Quarter Days
[Click on image for a larger view] (Graphic by R. Pogge)

These days are associated with many familiar holidays whose astronomical roots have been largely forgotten.

Cross-Quarter Holidays:

You will no doubt have noticed that our current dates of celebration of these holidays are not exactly on quarter or cross-quarter days. This is because these holidays were fixed in the calendar during the late-Middle Ages, whereas in the more distant past their coming was often marked by observing (or at least estimating) the occurrence of a particular station of the Sun along the ecliptic (i.e., observing the arrival of the Solstice or Equinox). Sites like Stonehenge in the UK are believed to be examples of such "observatories". The 4-fold divisions of the Solar ritual year on the cross-quarter days are visible in many ancient megalithic monuments, as well as being encoded in the Celtic Calendar of Coligny from the 2nd century AD. The Maya in Central America, and the Native Americans at the Cahokia mounds near St. Louis created similar "solar observatories" in their city/temple complexes, which were laid out on strict astronomical lines. The approximate dates we use today reflect half-remembered astronomical traditions that are older than our familiar calendars.

The Celtic Solar Calendar and traditional Japanese Luni-Solar Calendars used the cross-quarter days to mark the start of the various seasons, unlike the current tradition in the West where we say speak of the Equinoxes and Solstices proper as the first day of their respective season. The Celtic and traditional Japanese usages are actually more astronomically correct, at least for the latitudes of those societies. For example, in the traditional Japanese calendar the first day of spring (Risshun) is on the first cross-quarter day (Feb 3 or 4 - time of the traditional Setsubun festival which used to mark the beginning of the new year), Summer begins on May 6 (Rikka), autumn on August 8 (Risshuu), and winter on November 7 (Ritou).

In 2007, the approximate times of the cross-quarter days are as follows (all times are UTC):

   1st CQD  2007 Feb 4  05:18 UTC
   2nd CQD  2007 May 5  21:21 UTC
   3rd CQD  2007 Aug 7  21:31 UTC
   4th CQD  2007 Nov 7  19:24 UTC
These were computed using data provided by the JPL Horizons On-Line Ephemeris System to compute the ecliptic longitude of the Sun as seen from Earth (geocentric). Times are rounded to the nearest minute.

Months & Weeks

The year is also divided into 12 months.

Months are divided into Weeks:

Names for Days of the Week
Roman Anglo-Saxon English Spanish

Dividing the Day

We divide the Day into 24 hours, with each day beginning at midnight.

This wasn't always the case:

This division worked fine for sundials.

Equal Hours

The invention of mechanical clocks in the 1300s led to a need for equal hours:

Medieval clocks were large and complex:

Dividing the Hour

Until 1500s, clocks only kept time to the quarter hour.

Further division of the hours was needed as clocks became more complex.

Seconds didn't become common until the 1670s after the invention of the pendulum escapement: 39-inch pendulum clocks have a 1 second period to their swing.

Solar Time

The Day is measured using the Sun.
Local Solar Noon
Occurs when the Sun is on your meridian.

Mean Solar Day
The time between successive Noons.
When noon occurs depends on your longitude:

Sidereal Time

Sidereal Time is measured relative to the stars.

As the Earth rotates through 1 day, it moves a little less than 1° along its orbit around the Sun.
(Click on the image to view at full scale [Size: 9Kb])

Standard Time

The invention of rapid long-distance railroads and telegraph networks required a new way of standardized time keeping: Small differences in local solar time began to matter.

Time Zones

The idea of "Standard Time" arose in the 19th century, coinciding with the rise of railroads which connected great distances more quickly. Various local system arose in Britain, US, and Canada, with the international system of time zones being adopted in 1884. The creation of standard time was the work of many individuals, including William Wollaston (who developed the idea of a common time for all British railroads in the 1840s), Charles Dowd (who devised a multi-latitude system of time zones for US railroads in the 1870s), and Sir Sandford Fleming of Canada who devised the worldwide system we use today.

Added complications:

Actual timezone borders do not follow the meridians.

This results in irregular time zone boundaries that cannot be easily computed a priori. One usually has to resort to consulting a map.
Want to Know More?

In my opinion, the single best site on time and timesystems on the internet is the Time Service Department of the U.S. Naval Observatory. The Navy Dept. is the official timekeeper for the United States, and their website has a huge amount of information on timekeeping, clocks, time systems, sunrise/sunset/moon phase, times of solstices and equinoxes, and much more.

Updated: 2011 October 3, Todd A. Thompson
Copyright Richard W. Pogge, All Rights Reserved.