Tropical year in the context of "Position of the Sun"

The Chinese calendar is a lunisolar calendar created by or commonly used by the Chinese people. While this description is generally accurate, it does not provide a definitive or complete answer. A total of 102 calendars have been officially recorded in classical historical texts. In addition, many more calendars were created privately, with others being built by people who adapted Chinese cultural practices, such as the Koreans, Japanese, Vietnamese, and many others, over the course of a long history.

A Chinese calendar consists of twelve months, each aligned with the phases of the moon, along with an intercalary month inserted as needed to keep the calendar in sync with the seasons. It also features twenty-four solar terms, which track the position of the sun and are closely related to climate patterns. Among these, the winter solstice is the most significant reference point and must occur in the eleventh month of the year. Each month contains either twenty-nine or thirty days. The sexagenary cycle for each day runs continuously over thousands of years and serves as a determining factor to pinpoint a specific day amidst the many variations in the calendar. In addition, there are many other cycles attached to the calendar that determine the appropriateness of particular days, guiding decisions on what is considered auspicious or inauspicious for different types of activities.

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.

A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) are more exact. The modern calendar year, as reckoned according to the Gregorian calendar, approximates the tropical year by using a system of leap years.

The term 'year' is also used to indicate other periods of roughly similar duration, such as the lunar year (a roughly 354-day cycle of twelve of the Moon's phases – see lunar calendar), as well as periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc.

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.

A lunar calendar is a calendar based on the monthly cycles of the Moon's phases (synodic months, lunations), in contrast to solar calendars, whose annual cycles are based on the solar year, and lunisolar calendars, whose lunar months are brought into alignment with the solar year through some process of intercalation – such as by insertion of a leap month. The most widely observed lunar calendar is the Islamic calendar. The details of when months begin vary from calendar to calendar, with some using new, full, or crescent moons and others employing detailed calculations.

Since each lunation is approximately 29+1⁄2 days, it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of 12 such lunations, a lunar year, is 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days), lunar calendars are 11 to 12 days shorter than the solar year. In lunar calendars, which do not make use of lunisolar calendars' intercalation, the lunar months cycle through all the seasons of a solar year over the course of a 33–34 lunar-year cycle (see, e.g., list of Islamic years).

The Twenty-Eight Mansions (Chinese: 二十八宿; pinyin: Èrshíbā Xiù), also called xiu or hsiu, are part of the Chinese constellations system. They can be considered as the equivalent to the zodiacal constellations in Western astronomy, though the Twenty-eight Mansions reflect the movement of the Moon through a sidereal month rather than the Sun in a tropical year.

The lunar mansion system was in use in other parts of East Asia, such as ancient Japan; the Bansenshūkai, written by Fujibayashi Yasutake, mentions the system several times and includes an image of the twenty-eight mansions.

The ecliptic or ecliptic plane is the orbital plane of Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making.

From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the background of stars – specifically the Zodiac constellations. The planets of the Solar System can also be seen along the ecliptic, because their orbital planes are very close to Earth's. The Moon's orbital plane is also similar to Earth's; the ecliptic is so named because the ancients noted that eclipses only occur when the Moon is crossing it.