last update: 31 July 2020

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.

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
  • 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 Days
These occur at the mid-way points between the Solstices and Equinoxes (they are sometimes called the "Mid-Quarter Days").

These days are associated with many familiar holidays whose astronomical roots have been largely forgotten.
Cross-Quarter Holidays:
  • First Cross-Quarter Day (Feb 4): Imbolc (Celtic: "in milk"), St. Brigit's Day, Candelmas, Groundhog Day, Setsubun (Japan)
  • Second Cross-Quarter Day (May 5): Beltane (Celtic: "fire of Bel", coming of summer), May Day, Walpurgisnacht, Feast of the Conception of Mary
  • Third Cross-Quarter Day (Aug 7): 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 7): 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).

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:
  • 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)
Names for Days of the Week

Dividing the Day
We divide the Day into 24 hours, with each day beginning at midnight.
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).
This division worked fine for sundials.

Equal Hours
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.
  • 1 hour was divided into 60 minutes.
  • 1 minute was divided into 60 seconds.
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
  • Person 15° east of you sees noon 1 hour earlier.
  • Person 15° west of you sees noon 1 hour later.

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])

  • 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.

Standard Time
The invention of rapid long-distance railroads and telegraph networks required a new way of standardized time keeping:
  • Coordination of interstate railroad schedules.
  • Telegraph lines linked many widely separated longitudes instantaneously, but needed to coordinate them.
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.
  • Divided the Earth into Time Zones by longitude from the Prime Meridian.
  • Basic time zones are 15° of longitude apart (360°/24h = 15°/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.
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.
  • 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.
This results in irregular time zone boundaries that cannot be easily computed a priori. One usually has to resort to consulting a map.


Wikipedia has a whole webpage dedicated to the Names of the Days of the Week, so just summarising:-
The seven-day week first dates from the
Romans, with an earliest reference 6 February, AD 60, and was widely used by the 4th C AD.

Monday, as an international standard, is considered the first day of the week. The word derives from the Old English Mōnandæg and the Middle English Monenday, originally translated from the Latin name diēs Lūnae 'Day of the Moon'.

Tuesday in Old English was Tīwesdæg and the Middle English Tewesday, meaning "Tiw's day". Tiw (Norse Týr) was a one-handed god associated with single combat and pledges in Norse mythology. The name of the day is also related to the Latin name diēs Mārtis, 'Day of Mars'.

Wednesday in Old English was Wōdnesdæg and the Middle English Wednesdei, meaning the day of the Germanic god Woden (known as Óðinn among the North Germanic peoples), and a prominent god of the Anglo-Saxons (and other Germanic peoples) in England until about the 7th C. It is also vaguely related to the Latin counterpart diēs Mercuriī, 'Day of Mercury'.

Thursday in Old English was Þūnresdæg and the Middle English Thuresday, meaning 'Þunor's day'. Þunor means thunder or its personification, the Norse god known in Modern English as Thor. Thor's day corresponds to Latin diēs Iovis, 'Day of Jupiter'.

Friday in Old English was Frīgedæg, meaning the day of the Anglo-Saxon goddess Fríge. It is based on the Latin diēs Veneris, 'Day of Venus'.

Saturday is named after the Roman god Saturn associated with the Titan Cronus, father of Zeus and many Olympians. Its original Anglo-Saxon rendering was Sæturnesdæg. In Latin, it was diēs Saturnī, 'Day of Saturn'.

Sunday in Old English was Sunnandæg, meaning 'Sun's Day'. This is a translation of the Latin phrase diēs Sōlis. English, like most of the Germanic languages, preserves the day's association with the Sun. Many other European languages, including all of the Romance languages, have changed its name to the equivalent of "the Lord's Day" (based on Ecclesiastical Latin diēs Dominica).

English - Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Italian - Lunedì Martedì Mercoledì Giovedì Venerdì Sabato Domenica
Spanish - Lunes Martes Miércoles Jueves Viernes Sábado Domingo
French - Lundi Mardi Mercredi Jeudi Vendredi Samedi Dimanche
German - Montag Dienstag Mittwoch Donnerstag Freitag Samstag Sonntag

Names of the Months