15.1 — Calendars and the Shape of the Year

In September 1752, Britain and its colonies went to bed on Wednesday the 2nd and woke up on Thursday the 14th.

Eleven days were removed from the calendar by Act of Parliament.

The story that crowds rioted demanding "give us our eleven days" is probably a later embellishment, drawn partly from a Hogarth painting. The genuine grievances were real and practical: people wanted to know whether they owed a full month's rent, whether wages were due, and whether a contract dated by the old system still ran.

And the reason for it is astronomical. The year does not divide evenly into days, and every calendar in history is an attempt to manage that.

The problem

Three natural cycles, none of which is a whole-number multiple of any other.

The day — one rotation of the Earth.

The lunar month — 29.53 days from one new moon to the next.

And the solar year — 365.2422 days, the time for the Earth to return to the same point relative to the Sun.

Twelve lunar months make about 354 days, which is 11 days short of a solar year.

So a purely lunar calendar drifts through the seasons, and a purely solar calendar loses the phases of the moon. Every calendar chooses which to keep.

The three types

Solar keeps the seasons and ignores the moon. The Gregorian calendar you use, and the Iranian calendar, which is astronomically the most accurate in use.

Lunar keeps the moon and lets the year drift. The Islamic Hijri calendar is the only major purely lunar calendar. Its year is about 354 days, so its months move through the seasons over a 33-year cycle — which is why Ramadan can fall in summer or winter (Chapter 7.1).

Lunisolar keeps both, by inserting an extra month periodically. Hindu calendars, the Hebrew calendar, and the traditional Chinese calendar. The Hebrew and Chinese calendars use a nineteen-year cycle with seven leap months — the Metonic cycle, named for a Greek astronomer who described it in 432 BCE, and known in Babylon earlier. 235 lunar months is almost exactly 19 solar years, which is a genuinely remarkable coincidence and is what makes lunisolar calendars possible at all.

The Gregorian calendar and how it got that way

Egyptian origin (Chapter 4.2): a 365-day year of twelve 30-day months plus five extra days. It drifted by about a quarter-day a year and the Egyptians largely accepted the drift.

Julius Caesar fixed it in 46 BCE, on the advice of the Alexandrian astronomer Sosigenes, by adding a leap day every four years. The transition year had to be 445 days long to realign it, and is known as the year of confusion.

The Julian year averages 365.25 days. The true year is 365.2422. The difference is about 11 minutes, which is one day every 128 years.

By the sixteenth century that had accumulated to ten days, and the visible problem was that the spring equinox — which the date of Easter depends on — had moved away from 21 March, where the Council of Nicaea had fixed it (Chapter 5.8).

Pope Gregory XIII's reform in 1582: drop ten days, and change the leap year rule. A century year is a leap year only if divisible by 400. So 1600 and 2000 were leap years; 1700, 1800 and 1900 were not.

That gives an average year of 365.2425 days, accurate to about one day in 3,300 years.

And adoption was religious rather than scientific. Catholic countries adopted immediately. Protestant countries delayed — Britain until 1752, as above. Orthodox countries later still: Russia adopted it in 1918, which is why the October Revolution is commemorated in November (Chapter 11.5), and Greece in 1923.

Several Orthodox churches still use the Julian calendar for liturgical purposes, which is why Orthodox Christmas falls on 7 January.

Indian calendars

India has a large number and they are lunisolar, and the diversity is the interesting part.

The structure. Twelve lunar months named Chaitra, Vaishakha, Jyeshtha and so on, with an extra month — adhika masa — inserted about every 32 to 33 months to keep them aligned with the solar year.

The eras. The Vikram Samvat, beginning 57 BCE, used in north and western India. The Shaka Samvat, beginning 78 CE, which is the basis of the Indian national calendar adopted in 1957. The Kollam era in Kerala. The Bengali San. And several regional variants.

The regional differences are substantial and produce a genuinely confusing situation. In some regions the month begins with the new moon — amanta, used in the south and west — and in others with the full moon — purnimanta, used in the north. So the same festival can fall in a differently named month depending on where you are.

And the new year is on different days in different regions. Ugadi and Gudi Padwa in the Deccan; Vaisakhi in Punjab; Poila Boishakh in Bengal; Puthandu in Tamil Nadu; Vishu in Kerala; Bihu in Assam; Navreh in Kashmir. Several of these fall in mid-April and several do not, because some follow the lunar reckoning and some the solar.

The astronomical basis is genuinely sophisticated. Aryabhata and later astronomers computed the lengths of the year and the month to considerable accuracy (Chapter 6.6), and the panchanga — the traditional almanac — gives five elements for each day: the lunar day, the lunar mansion, the weekday, and two derived astronomical quantities.

And one persistent divergence is worth explaining because it puzzles people. Most Indian calendars use the sidereal zodiac — measured against the fixed stars — while Western astrology uses the tropical zodiac, measured against the equinoxes. Because of the precession of the equinoxes (Chapter 2.8's orbital cycles), the two have drifted apart by about 24 degrees over roughly 1,700 years, which is why an Indian and a Western astrologer will give a person different sun signs.

The week

The seven-day week has no astronomical basis at all. It does not divide the month or the year evenly.

Its origins are Babylonian (Chapter 4.1): the seven visible moving objects in the sky — Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn — each given a day.

And the transmission is visible in the names.

Day	Planet	Latin/Romance	English	Sanskrit
1	Sun	dies Solis	Sunday	Ravivara
2	Moon	lunes	Monday	Somavara
3	Mars	martes	Tuesday (Tiw)	Mangalavara
4	Mercury	miércoles	Wednesday (Woden)	Budhavara
5	Jupiter	jueves	Thursday (Thor)	Guruvara
6	Venus	viernes	Friday (Frigg)	Shukravara
7	Saturn	sábado	Saturday	Shanivara

The English names for Tuesday to Friday substituted Germanic gods for the Roman ones — Tiw for Mars, Woden for Mercury, Thor for Jupiter, Frigg for Venus — which is why the Romance and English names diverge for exactly those four.

And the Sanskrit names are the same seven bodies in the same order, which is strong evidence of transmission from the Hellenistic world rather than independent invention, and it is dated to roughly the early centuries CE (Chapter 6.5).

The Jewish sabbath gave the week its religious weight, and Christianity and Islam adopted the seven-day cycle with different rest days — Saturday, Sunday and Friday respectively.

Attempts to change it have failed. The French revolutionary calendar imposed a ten-day week in 1793; it was abandoned by 1806, partly because a one-in-ten rest day was unpopular for obvious reasons. The Soviet Union tried five and six-day cycles in the early 1930s with staggered rest days to keep factories running continuously, and reverted in 1940.

Both failures are informative. The week is arbitrary and it is deeply embedded, and coordinating a change requires everyone to change at once.

Time zones and the clock

Before railways, every town kept its own local time by the sun. Noon was when the sun was highest, which differs by four minutes per degree of longitude.

Railways made that unworkable. A timetable requires everyone on the line to agree what time it is. Britain standardised on Greenwich time in the 1840s, driven by the railway companies.

The International Meridian Conference of 1884 in Washington adopted Greenwich as the prime meridian, largely because most existing shipping charts already used it.

Time zones are political rather than astronomical. China spans about five geographical hours and uses one time zone. India uses a single zone at UTC+5:30 — the half-hour offset chosen as a compromise between the longitudes of the country — which means sunrise in Arunachal Pradesh can be nearly two hours earlier than in Gujarat, and there is a long-running argument for a separate northeastern zone on grounds of daylight use and productivity.

Daylight saving time was proposed in various forms from the nineteenth century and adopted widely during the First World War to save fuel. The evidence for energy savings is weak and contested; the evidence for a short-term increase in heart attacks and accidents around the spring transition is reasonably good. India does not use it, which is sensible at its latitude, where day length varies little.

Where this shows up in your life

Every date you write. Every festival that moves — Easter, Diwali, Ramadan, Chinese New Year — moves because it follows a lunar or lunisolar reckoning inside a solar calendar.

The financial year. India's runs April to March, inherited from British practice, which itself traces to the old English year beginning on 25 March and being shifted by the eleven-day correction of 1752 — which is why the British tax year ends on 5 April, a date that makes no sense except as a historical residue.

And the general observation. A calendar looks like a natural fact and is a negotiated construction, decided by astronomers, imposed by rulers, resisted by populations and adopted at different times for religious reasons. The eleven days removed in 1752 are the clearest illustration: time itself is administered.

What the next page covers

Chapter 15.2 covers India's festivals — why the same religion produces such different celebrations in different regions, what each major festival actually commemorates and where the story comes from, how agricultural cycles underlie most of them, and why the emotional attachment to a regional festival runs deeper than to a national one.