Weekly Seven-Day Cycles

The idea of dividing the cycle of the moon into four seven-day weeks may have begun in Babylon. In its familiar modern form, it probably derives from a Jewish model, echoing the story of Creation as told in Genesis, in which God, having made the world in six days, rested on the seventh – and ordered humanity and their animals to do likewise. As a result, every week connects us to the beginning of time itself, as the days plot the round of our work and leisure, the recurrent rhythm of our existence.

Our Language and Beliefs

The weekday names depend on our language and our beliefs. The names that we give the weekdays in English are an inherited meditation on the cycles of time, as we observe the pattern of the sun, the moon and the planets circling above us – though the story they tell us is for English-speakers only, since nobody else’s week is quite the same as ours.

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Days Named After Gods

Sunday, Monday – the week begins with the sun and the moon, whose separate movements mark the months and years. After them, come the days of the easily visible planets. In Romance languages, this is Mars, Mercury, Jupiter, and Venus – the sequence that the Romans followed and left behind.

In England, around the seventh century, the planets tethered to the gods of Rome were renamed for the equivalent northern gods, and it is their Anglo-Saxon names – Tiw, Woden, Thor, and Frige – that distinguish the days for English-speakers on Tuesday, Wednesday, Thursday, and Friday. On Saturday, the Anglo-Saxon gods are joined by Saturn, which retained its Latin name, making our week, like our language itself, a peculiar German-Latin hybrid.

Cosmological History

Encompassing the different cycles of sun, moon and the five planets, every week thus implies not just a long span of many years, but also the company of gods and the vastness of space itself. In the names of our days is the entire solar system – the time-space continuum as it was known in the ancient Mediterranean world and transmitted to the north of Europe. The turn of the week is – in English – a concise cosmological history, in which we still live every day with the gods of our ancestors inhabiting an ancient, but stable structure of time.

Living With the Gods: On Beliefs and Peoples
Neil MacGregor

Background on Neil MacGregor

Oldest Calendars

The oldest ‘calendars‘ are vast archaeological sites that aligned posts or megaliths (giant stones) with the rising of the Sun or Moon on significant dates, such as the summer or winter solstice. The earliest structures built as calendars seem to be designed to help calculate the solar and lunar months. Priest-astronomers used these. 

Prehistoric astronomers left no user manuals for their monuments – their uses had to be rediscovered by archaeoastronomers – archaeologists with knowledge of astronomy. 

Warren Field, Scotland, found in 2004, is the earliest site found so far. It tracks events some 10,000 years ago.

Development of Accurate Calendars

Calendar development was often driven by the need to fix religious festivals and observance, an impulse that continued with the formation of new religions such as Christianity and Islam. Both put astronomy to use in this way.

Arab astronomers and engineers were zealous in their pursuit of improved methods for keeping time so that daily prayers could be received by the devout at the right time.

Time-keeping on a larger scale was essential in scheduling religious festivals.

Lunar Calendars 

Time is naturally divided astronomically by the Earth’s orbit around the Sun (a year), the Earth’s rotation (a day) and the phases of the Moon. A lunar month (a full cycle from one new or full moon to the next is approximately 29 1/2 days. A year is 365 1/4 days.

Inconveniently, a year is 12.37 lunar months long. For early societies, a lunar month was a useful and countable period of time, one that could be easily observed and checked just by looking up at the night sky.

But if you use 12  lunar months as the basis of your year, the calendar will drift out of sync quite quickly. It will be a month out after only 3 years, and 6 months out after 18 years. To avoid this, an extra (intercalary) month has to be added every few years.

Egyptian Calendars

The ancient Egyptians began their year with the rising of Sirius (which they called Sopdet) above the horizon before sun rise.

• System dates to c. 3000 BC.
• Divided year into 365 days.
• Used two different calendars.
• Sirius, a stable star, and the brightest star in the night sky, was the basis for the Egyptian calendar.
• Ptolemaic rulers depended on calendars.

Other cultures developed independent calendars, notably China and Mesoamerican (Central American) civilizations.

Chinese Yin-Yang Li Traditional Calendar

Origins of the Chinese calendar can be traced back to 14th century BC, though legend says it was invented in 2637 BC.

• Literally ‘heaven-Earth’ calendar.
• Used alongside imported calendars such as the Hindu calendar.
• Used until 1912, when China officially adopted the Western Gregorian calendar.

How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes 
Anne Rooney

Background on Anne Rooney

Standardization of Easter Celebration Date

The Catholic Church fixes the date of Easter, its celebration to mark the resurrection of Christ, using a method set out in AD 325 by the Council of Nicaea. In the first centuries AD, Easter was celebrated on different days by different groups of Christians, the Council of Nicaea sought to standardize it.

Role of Full Moon

Easter is now celebrated on the first Sunday after the full moon occurring on or after the spring equinox. Early Christians couldn’t simply wait to find out when the full moon would fall, then quickly celebrate Easter. They had to fit in Lent – 40 days of fasting – immediately beforehand, so had to know several weeks in advance when that full moon would fall, a task that could only be achieved by keeping astronomical records and projecting into the future.

Easter Dates 

• Table of Easter Dates (ranging from 1998-2038).
• Easter may occur on different dates in the Gregorian Calendar (used by Catholic and Protestant churches) and the Julian Calendar (used by the Orthodox church).

How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes
Anne Rooney

Background on Anne Rooney