Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30

Lecture 10: Telling Time

Key Ideas

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

• 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

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. [Picture]
• Sometimes referred to as "Quarter Days" because they roughly divide the year into 4 parts.

Cross-Quarter Days:

• Mid-way points between the Solstices and Equinoxes (sometimes called "mid-quarter days"). [Picture]
• 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)

Cross-Quarter Holidays:

• First Cross-Quarter Day (Feb 2-6): Imbolc (Celtic: "in milk"), St. Brigit's Day, Candelmas, Groundhog Day, Setsubun (Japan)
• 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

• 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:

• 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

This wasn't always the case:

• 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).

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

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

• 1 hour was divided into 60 minutes.
• 1 minute was divided into 60 seconds.

Solar Time

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

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 4^m earlier each night measured against solar (civil) clocks.

Standard Time

• Coordination of interstate railroad schedules.
• Telegraph lines linked many widely separated longitudes instantaneously, but needed to coordinate them.

Time Zones

• Divided the Earth into Time Zones by longitude from the Prime Meridian.
• Basic time zones are 15º of longitude apart (360º/24^h = 15º/hour)
• Each time zone keeps local solar time for a fixed reference longitude.
• All longitudes within that zone use "Zone Time" instead of local solar time.

Added complications:

Actual timezone borders do not follow the meridians.

• Cities, counties, and small countries want to be on the same time system for ease of governance.
• Some states refuse to have multiple time zones.
• Keep some island nations from being divided.

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.