Calendar in the context of "Month"

Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is the branch of astronomy that studies the universe as a whole.

Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Egyptians, Babylonians, Greeks, Indians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars.

An almanac (also spelled almanack and almanach) is a regularly published listing of a set of current information about one or multiple subjects. It includes information like weather forecasts, farmers' planting dates, tide tables, and other tabular data often arranged according to the calendar. Celestial figures and various statistics are found in almanacs, such as the rising and setting times of the Sun and Moon, dates of eclipses, hours of high and low tides, and religious festivals. The set of events noted in an almanac may be tailored for a specific group of readers, such as farmers, sailors, or astronomers.

A lunisolar calendar is a calendar in many cultures, that combines monthly lunar cycles with the solar year. As with all calendars which divide the year into months, there is an additional requirement that the year have a whole number of months (Moon cycles). The majority of years have twelve months but every second or third year is an embolismic year, which adds a thirteenth intercalary, embolismic, or leap month.

In contrast to purely lunar calendars such as the Islamic calendar, lunisolar calendars have additional intercalation rules that reset them periodically into a rough agreement with the solar year and thus with the seasons.

Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future. Time dictates all forms of action, age, and causality, being a component quantity of various measurements used to sequence events, to compare the duration of events (or the intervals between them), and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.

Time is primarily measured in linear spans or periods, ordered from shortest to longest. Practical, human-scale measurements of time are performed using clocks and calendars, reflecting a 24-hour day collected into a 365-day year linked to the astronomical motion of the Earth. Scientific measurements of time instead vary from Planck time at the shortest to billions of years at the longest. Measurable time is believed to have effectively begun with the Big Bang 13.8 billion years ago, encompassed by the chronology of the universe. Modern physics understands time to be inextricable from space within the concept of spacetime described by general relativity. Time can therefore be dilated by velocity and matter to pass faster or slower for an external observer, though this is considered negligible outside of extreme conditions, namely relativistic speeds or the gravitational pulls of black holes.

A calendar era is the period of time elapsed since one epoch of a calendar and, if it exists, before the next one. For example, the current year is numbered 2025 in the Gregorian calendar, which numbers its years in the Western Christian era (the Coptic Orthodox and Ethiopian Orthodox churches have their own Christian eras).

In antiquity, regnal years were counted from the accession of a monarch. This makes the chronology of the ancient Near East very difficult to reconstruct, based on disparate and scattered king lists, such as the Sumerian King List and the Babylonian Canon of Kings. In East Asia, reckoning by era names chosen by ruling monarchs ceased in the 20th century except for Japan, where they are still used.

The French Republican calendar (French: calendrier républicain français), also commonly called the French Revolutionary calendar (calendrier révolutionnaire français), was a calendar created and implemented during the French Revolution and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871, meant to replace the Gregorian calendar. The calendar consisted of twelve 30-day months, each divided into three 10-day cycles similar to weeks, plus five or six intercalary days at the end to fill out the balance of a solar year. It was designed in part to remove all religious and royalist influences from the calendar, and it was part of a larger attempt at dechristianisation and decimalisation in France (which also included decimal time of day, decimalisation of currency, and metrication). It was used in government records in France and other areas under French rule, including Belgium, Luxembourg, and parts of the Netherlands, Germany, Switzerland, Malta, and Italy.

The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years slightly differently to make the average calendar year 365.2425 days long rather than the Julian calendar's 365.25 days, thus more closely approximating the 365.2422-day "tropical" or "solar" year that is determined by the Earth's revolution around the Sun.

The rule for leap years is that every year divisible by four is a leap year, except for years that are divisible by 100, except in turn for years also divisible by 400. For example, 1800 and 1900 were not leap years, but 1600 and 2000 were leap years.

The calendrical systems devised and used by the pre-Columbian cultures of Mesoamerica, primarily a 260-day year, were used in religious observances and social rituals, such as divination.

These calendars have been dated to early as ca. 1100 BCE. By 500 BCE at the latest, the essentials were fully defined and functional. 260-day calendars are still used in the Guatemalan highlands, Veracruz, Oaxaca and Chiapas, Mexico.

The September equinox (or southward equinox) is the moment when the Sun appears to cross the celestial equator, heading southward. Because of differences between the calendar year and the tropical year, the September equinox may occur from September 21 to 24.

At the equinox, the Sun as viewed from the equator rises due east and sets due west. Before the Southward equinox, the Sun rises and sets more northerly, and afterwards, it rises and sets more southerly.