What Is a Month? Gregorian, Lunar and Hindu Calendar Months Explained

A month is a unit of time used in calendars that is roughly as long as one natural phase cycle of the Moon — in fact, the English words "month" and "Moon" share the same root, as the Wikipedia article on the month explains. The term appeared as a rising query in Google Trends India data observed on 10 October 2026, with an interest score of 2 and 40% growth over the past 12 months. That signal confirms search curiosity only; it does not point to any specific news event. Here is a clear, evergreen explanation of how a month is defined and how the Gregorian, lunar and Hindu calendars handle it differently.

Where the idea of a month comes from

The original month was the time between two new moons (or two full moons). Astronomers call this the synodic month, and its average length is 29.53059 days — about 29 days, 12 hours and 44 minutes. Because of the Moon's slightly elliptical orbit, an individual synodic month can vary by up to seven hours. A related measure, the sidereal month — the time the Moon takes to return to the same position against the stars — is shorter, at about 27.32166 days. The synodic month is longer because Earth itself moves around the Sun while the Moon orbits Earth, so the Moon needs roughly 2.2 extra days to line up with the Sun again.

Archaeological evidence suggests humans tracked days against the Moon's phases as early as the Paleolithic age, using tally marks on bones and sticks.

How the Gregorian calendar divides the year

The Gregorian calendar, used internationally including for official purposes in India, divides the year into 12 months of 28 to 31 days. These months no longer follow the Moon; they are a conventional split of the solar year inherited from the Roman calendar.

February is the only month whose length changes, gaining a 29th day in leap years. The average Gregorian month length works out to 30.436875 days, and any five consecutive months that skip February always total 153 days. The names September through December come from the Latin numerals for seven to ten, because they were originally the seventh to tenth months of the old Roman calendar.

Why lunar calendars need a fix

Twelve lunar months total only about 354 days — roughly 10 to 11 days short of a solar year. Calendars handle this gap in different ways:

• Pure lunar calendars, such as the Islamic calendar, keep 12 lunar months and let the year drift through the seasons over a cycle of about 33 solar years.
• Lunisolar calendars, such as the Hebrew and Hindu calendars, insert an extra (leap) month every two or three years to stay aligned with the seasons. The Metonic cycle formalises this: 235 lunar months closely match 19 solar years.
• Solar calendars, such as the Gregorian and Persian calendars, ignore the Moon entirely for month lengths.

Months in the Hindu calendar and Adhik Maas

India's traditional Hindu calendar is lunisolar. Its 12 lunar months run from Chaitra (चैत्र) through Vaiśākha (वैशाख) — including Jyeṣṭha, Āshāḍha, Śrāvaṇa, Bhādrapada, Āśvina, Kārtika, Mārgaśīrṣa, Pauṣa, Māgha and Phālguna — and these same names are used in the Indian national calendar. Regional languages have their own forms: for example, Chaitra is Chithirai in Tamil and Chaithramu in Telugu.

To keep the lunar cycle aligned with the solar year, the calendar periodically inserts an extra month called Adhik Maas or Purushottam Maas (from adhika, "extra", and māsa, "month"). The month is dedicated to Vishnu. Because leap-month years are not fixed to a simple rule visible in these sources, and because lunar month start dates vary by regional panchang and by year, readers should check a current panchang for exact dates rather than rely on fixed Gregorian equivalents.

The bottom line

Whether you mean a 30-day page on a wall calendar or the span between two new moons, a month is humanity's oldest way of slicing the year. The Gregorian version is a fixed convention averaging about 30.44 days; the lunar version averages about 29.53 days; and the Hindu lunisolar system blends the two, using Adhik Maas to stop festivals from drifting out of their seasons.