Lecture 10: Telling Time

Key Ideas

Timekeeping is tied to Astronomy

Divisions of the Year:

Divisions of the Day:

Civil Timekeeping & Time Zones

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]
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]

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

Cross-Quarter Holidays:

Extra Notes on Cross Quarter Days

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.

This particular lecture attracts a great deal of attention from outside my students, and I wish to thank everyone over the years who has sent comments, questions, and suggestions for improving the information on it.

Return to [ Unit 2 Index | Astronomy 161 Main Page ]
Updated: 2007 September 29
Copyright Richard W. Pogge, All Rights Reserved.