Lecture 10: Telling Time
Timekeeping is tied to Astronomy
Divisions of the Year:
- Quarter & Cross-Quarter days
- Months & Weeks
Divisions of the Day:
- Hours, Minutes & Seconds
- Solar & Sidereal Time
Civil Timekeeping & Time Zones
Keeping track of Time
All of our time-keeping conventions are astronomically based:
- The Year is based on the time it takes Earth to orbit the Sun.
- The Month is based on the cycle of the Lunar Phases
("Month" comes from "Moon").
- The Day is based on the time it takes the Earth to rotate
once on its axis relative to the Sun.
Dividing the Year
Solstics & Equinoxes:
The first major division of the Year is into seasons marked by the
occurance of the Solstices and Equinoxes:
- Equinoxes and Solstices divide the year into quarters.
- Sometimes referred to as "Quarter Days" because they roughly
divide the year into 4 parts.
- Mid-way points between the Solstices and Equinoxes (sometimes
called "mid-quarter days").
- These days are associated with many familiar holidays whose
astronomical roots have largely been forgotten.
Solstice & Equinox Holidays
- Winter Solstice: Christmas, Yuletide, Saturnalia
- Vernal Equinox: Easter, Passover, Eoestre (Saxon)
- Summer Solstice: Midsummer (viz. A Midsummer
Night's Dream), St. John's Eve
- Autumnal Equinox: Mabon (Celtic/Welsh), Michaelmas (Feast of
St. Michael the Archangel)
- First Cross-Quarter Day (Feb 2-6): Imbolc (Celtic: "in milk"),
St. Brigit's Day, Candelmas, Groundhog Day,
- Second Cross-Quarter Day (May 4-7): Beltane (Celtic: "fire of
Bel", coming of summer), May Day, Walpurgisnacht, Feast of the
Conception of Mary
- Third Cross-Quarter Day (Aug 5-8): Lughnasa (Celtic: "games
of Lugh"), Lammas (loaf mass), Lughnasadh (Celtic: "games of Lugh"),
Feasts of St. Oswald and St. Justus of Lyon.
- Fourth Cross-Quarter Day (Nov 5-8): Samhain (Celtic: "summer's
end"), Halloween, Feast of All Saints, Feast of All Souls.
(Note: Halloween preceeds All Saints in the
same way Walpurgis Night preceeds May Day in the Spring).
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 in recent times,
whereas in the more distant past their coming was often marked by
actually observing (or at least estimating) the occurance 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" (such 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 Lunar 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).
Months & Weeks
The year is also divided into 12 months.
- Why 12? There are 12.4 lunar synodic periods (cycles of phases)
during a year.
- The word for Month derives from the word for the Moon.
Months are divided into Weeks:
Names for Days of the Week
- The week is traditionally divided into 7 days
- Seven for the 7 moving bodies ("planets")
visible to the naked eye (Sun, Moon, Mercury, Venus,
Mars, Jupiter, and Saturn)
Dividing the Day
We divide the Day into 24 hours, with each day beginning at
This wasn't always the case:
This division worked fine for sundials.
- The day usually began at dawn (sunrise).
- Equal division of Day and Night into 12 hours.
- The length of the hour was different for day and night
(except at the Equinoxes).
The invention of mechanical clocks in the 1300s led to a need
for equal hours:
- Ensured the clocks read true in the morning.
- Simplified clock design.
Medieval clocks were large and complex:
- Erected in towers in cities for everyone to see.
- Led to a standardization of time keeping:
- Personal timepieces became common centuries later.
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.
- 1 hour was divided into 60 minutes.
- 1 minute was divided into 60 seconds.
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:
- Person 15° east of you sees noon 1 hour earlier.
- Person 15° west of you sees noon 1 hour later.
Sidereal Time is measured relative to the stars.
As the Earth rotates through 1 day, it moves about 1º along
its orbit around the Sun.
(Click on the image to view at full scale [Size: 9Kb])
- As seen with respect to the stars, the Earth has move for
an extra 4 minutes in order for the Sun to return to the
observer's meridian (noon).
- Stars rise 4m earlier each night measured
against solar (civil) clocks.
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.
- Coordination of interstate railroad schedules.
- Telegraph lines linked many widely separated longitudes
instantaneously, but needed to coordinate them.
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
- Divided the Earth into Time Zones by longitude from the
- Basic time zones are 15º of longitude apart (360º/24h
- Each time zone keeps local solar time for a fixed
- All longitudes within that zone use "Zone Time"
instead of local solar time.
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.
- Cities, counties, and small countries want to
be on the same time system for ease of
- Some states refuse to have multiple time zones.
- Keep some island nations from being divided.
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.
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Updated: 2007 June 11
Copyright © Richard W. Pogge, All Rights