Qu'est-ce qu'un/une Month (mo) ?
Formal Definition
A month is a unit of time historically based on the period of the Moon's orbit around the Earth. In the Gregorian calendar — the civil calendar used in most of the world — a month is a named period ranging from 28 to 31 days. The average length of a Gregorian calendar month is approximately 30.4375 days (365.25 days / 12 months), which equals approximately 2,629,800 seconds. The month is not an SI unit of time, but it is widely used in civil, commercial, and legal contexts.
Several distinct types of months exist in astronomy. The synodic month (the period between successive new moons) averages 29.53059 days. The sidereal month (the time for the Moon to return to the same position relative to the stars) averages 27.32166 days. The tropical month, anomalistic month, and draconic month have slightly different values depending on the reference point used.
Calendar Month Lengths
In the Gregorian calendar, months have the following lengths: January (31), February (28 or 29), March (31), April (30), May (31), June (30), July (31), August (31), September (30), October (31), November (30), December (31). The irregular distribution of days across months is a historical artifact stemming from Roman calendar reforms, particularly those of Julius Caesar (46 BCE) and Augustus Caesar (8 BCE).
Etymology
Lunar Connection
The English word "month" derives from the Old English "monath," which is directly related to "mona" (moon). The Proto-Germanic root is "menoth-," from the Proto-Indo-European root "menes-" (moon, month), which also gave rise to the Latin "mensis" (month), the Greek "men" (month), and the Sanskrit "masa" (month, moon). This etymological connection between "month" and "moon" exists in virtually every Indo-European language, reflecting the universal practice of tracking time by lunar cycles.
Names of the Months
The names of the twelve Gregorian calendar months come from Latin via the Roman calendar. January honors Janus, the two-faced god of doorways and beginnings. February derives from "februa," purification rituals held in that month. March honors Mars, the god of war. April may come from "aperire" (to open, as flowers open in spring) or from the Etruscan form of Aphrodite. May honors Maia, a goddess of growth. June honors Juno, goddess of marriage.
July and August were renamed in honor of Julius Caesar and Augustus Caesar, respectively — they were originally Quintilis (fifth) and Sextilis (sixth). September through December retain their original Latin numerical names: "septem" (seven), "octo" (eight), "novem" (nine), "decem" (ten). These numbers are off by two because the original Roman calendar began in March, making September the seventh month, October the eighth, and so on.
Histoire
Lunar Calendars
The month is one of the oldest units of time, with roots in prehistoric observation of the Moon's phases. Archaeological evidence suggests that Upper Paleolithic people tracked lunar cycles as early as 30,000 years ago, making notches on bones and cave walls to record the passage of lunations. The Sumerian calendar (circa 2100 BCE) used months of 29 or 30 days based on direct observation of the new moon crescent, and many ancient civilizations — including those of Egypt, China, India, and Mesoamerica — developed lunar or lunisolar calendars.
The Roman Calendar
The Roman calendar underwent several major transformations that shaped the months we use today. The earliest Roman calendar, attributed to Romulus (8th century BCE), had only 10 months beginning with March and totaling 304 days, with an unnamed winter period. Numa Pompilius (circa 713 BCE) is traditionally credited with adding January and February, creating a 355-day lunar calendar with an intercalary month (Mercedonius) inserted every few years to keep the calendar aligned with the seasons.
By the 1st century BCE, the Roman calendar had drifted severely out of alignment with the solar year due to inconsistent intercalation (often manipulated for political purposes). In 46 BCE, Julius Caesar implemented a sweeping reform based on the Egyptian solar calendar, creating the Julian calendar with 365 days and a leap day every four years. Caesar assigned month lengths roughly similar to those we use today, with alternating months of 31 and 30 days (except February with 29, or 30 in leap years).
Gregorian Reform
The Julian calendar's leap year rule slightly overestimated the solar year (by about 11 minutes per year), causing the calendar to drift by approximately 1 day every 128 years. By the 16th century, the spring equinox had shifted by about 10 days from its expected date, affecting the calculation of Easter. In 1582, Pope Gregory XIII introduced the Gregorian calendar, which refined the leap year rule: years divisible by 100 are not leap years unless they are also divisible by 400. This correction reduced the average year length to 365.2425 days, closely approximating the tropical year of 365.2422 days.
The Gregorian calendar was adopted immediately by Catholic countries but only gradually by Protestant and Orthodox nations. Great Britain and its colonies (including what would become the United States) did not adopt it until 1752, Russia waited until 1918, Greece until 1923, and Turkey until 1926. Today, the Gregorian calendar is the de facto international standard for civil purposes, though many countries continue to use traditional lunar or lunisolar calendars for religious and cultural observances.
Utilisation actuelle
In Finance and Business
The month is the standard period for financial accounting, billing, and reporting. Salaries are typically paid monthly or bi-monthly. Rent, mortgage payments, insurance premiums, and subscription services are billed on a monthly basis. Financial statements (income statements, balance sheets) are prepared monthly for internal management and quarterly for public reporting. Economic indicators such as inflation rates, unemployment figures, and GDP growth are often reported as month-over-month or year-over-year monthly comparisons.
In Legal and Contractual Contexts
Legal documents and contracts routinely specify durations in months. Notice periods, probationary periods, warranty terms, and statute of limitations are commonly expressed in months. In many jurisdictions, a "month" in legal contexts is defined as a calendar month (from a given date in one month to the same date in the next), though some contracts specify 30-day months to avoid ambiguity.
In Government and Administration
Government operations are organized around monthly cycles. Tax returns may be filed monthly (for businesses) or annually. Census data, crime statistics, and economic indicators are tabulated by month. Immigration visas and work permits specify their duration in months. Military deployments, prison sentences, and probation periods are measured in months.
Everyday Use
Personal Finance
For most people, the month is the fundamental unit of budgeting. Monthly income, monthly rent, monthly utility bills, monthly loan payments — personal finance revolves around the monthly cycle. Budgeting apps and financial advisors organize spending into monthly categories, and savings goals are often expressed as monthly contributions. Credit card statements arrive monthly, and interest on savings accounts is typically compounded monthly.
Health and Development
Infant age is measured in months for the first two years of life, reflecting the rapid developmental changes that occur during this period. Pediatricians schedule well-child visits at specific monthly intervals (1, 2, 4, 6, 9, 12, 15, 18, and 24 months). Pregnancy is commonly described in months ("she's six months along"), though medical professionals use weeks for greater precision. Menstrual cycles average approximately one month (28 days), and fertility tracking relies on monthly patterns.
Subscriptions and Services
The subscription economy is built on the monthly model. Streaming services (Netflix, Spotify), software subscriptions (Microsoft 365, Adobe Creative Cloud), gym memberships, and cloud storage plans are priced monthly. The phrase "per month" appears in virtually every consumer pricing discussion, making the month the default unit for comparing recurring costs.
In Science & Industry
In Climatology and Meteorology
Climatologists analyze temperature, precipitation, and other weather variables on a monthly basis. Monthly mean temperatures and monthly precipitation totals form the foundation of climate normals — typically calculated as 30-year averages by month. Climate classification systems (such as the Koppen system) use monthly temperature and precipitation thresholds to categorize climate types. Seasonal forecasts extend one to three months ahead, and climate projections for future decades are often presented as monthly averages.
In Astronomy
The synodic month (29.53 days) remains important in astronomy for predicting lunar phases, eclipses, and tidal patterns. The Saros cycle — a period of approximately 18 years, 11 days, and 8 hours (or 223 synodic months) after which solar and lunar eclipses repeat — was known to Babylonian astronomers and remains used in modern eclipse prediction. Tidal patterns follow a roughly monthly cycle, with spring tides occurring near new and full moons (twice per synodic month) and neap tides occurring near the quarter moons.
In Ecology and Environmental Science
Ecological studies frequently use monthly sampling intervals. Animal migration patterns, plant phenology (flowering, fruiting, leaf fall), and insect population dynamics are tracked by month. Environmental monitoring programs measure water quality, air pollution, and soil conditions on monthly schedules. Satellite-derived vegetation indices (such as NDVI) are typically produced as monthly composites for large-scale ecological assessment.
Interesting Facts
February is the only month whose length changes from year to year. In 28 of every 400 Gregorian years, February has 29 days instead of 28 — a correction that keeps the calendar aligned with Earth's orbit around the Sun.
The Roman emperor Augustus allegedly took a day from February to add to August (which was named after him), making August 31 days instead of 30. However, some historians dispute this account, suggesting the month lengths were already established before Augustus.
The word "month" and "moon" share the same Proto-Indo-European root, and this connection exists in nearly every Indo-European language — a testament to the universal importance of lunar observation in early human timekeeping.
A "blue moon" — the second full moon within a single calendar month — occurs approximately once every 2.7 years. This is possible because the synodic month (29.53 days) is shorter than all calendar months except February.
The Islamic calendar uses purely lunar months of 29 or 30 days, making the Islamic year about 354 days long. This means Islamic holidays (such as Ramadan) migrate through the Gregorian calendar, completing a full cycle in approximately 33 years.
September, October, November, and December literally mean "seventh," "eighth," "ninth," and "tenth" month in Latin, even though they are now the 9th through 12th months. The discrepancy arose when January and February were added to the beginning of the Roman calendar.
The shortest interval between two full moons in a calendar month occurred in some time zones on January 1 and January 31 — a span of exactly 30 days. The average synodic month of 29.53 days means full moons can occasionally fit twice into months of 30 or 31 days.
The Gregorian calendar reform of 1582 required skipping 10 days: in Catholic countries, October 4, 1582, was followed directly by October 15, 1582. When Britain adopted the reform in 1752, 11 days had to be skipped, leading to public protests under the slogan "Give us our eleven days!"