Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a fiscal calendar or the astronomical year numbering system.

To reconcile the calendar year with an astronomical cycle ( which could not possibly be reckoned in a whole number of days ), certain years contain extra days.

SunWatch is a partially reconstructed 12th-century prehistoric American Indian village ; the village is organized around a central plaza dominated by wood posts forming an astronomical calendar.

One reason for this is that the full moon involved ( called the Paschal full moon ) is not an astronomical full moon, but the 14th day of a calendar lunar month.

The original form of this calendar would have determined Easter using precise astronomical calculations based on the meridian of Jerusalem.

A leap year ( or intercalary or bissextile year ) is a year containing one additional day ( or, in the case of lunisolar calendars, a month ) in order to keep the calendar year synchronized with the astronomical or seasonal year.

Because seasons and astronomical events do not repeat in a whole number of days, a calendar that had the same number of days in each year would, over time, drift with respect to the event it was supposed to track.

For some lunar calendars, such as the Chinese calendar, the first day of a month is the day when an astronomical new moon occurs in a particular time zone.

He was set to work on astronomical and calendar studies, a project which was intended to bring the year back into synchronization with the seasons.

An astronomical rule for Easter was proposed by the 1923 synod that also proposed the Revised Julian calendar: Easter was to be the Sunday after the midnight-to-midnight day at the meridian of the Church of the Holy Sepulchre in Jerusalem ( 35 ° 13 ' 47. 2 " E or UT + 2 < sup > h </ sup > 20 < sup > m </ sup > 55 < sup > s </ sup > for the small dome ) during which the first full moon after the vernal equinox occurs.

The word " year " is also used of periods loosely associated but not strictly identical with either the astronomical or the calendar year, such as the seasonal year, the fiscal year or the academic year, etc.

No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of intercalation such as leap years.

The Persian calendar, in use in Afghanistan and Iran, has its year begin on the day of the vernal equinox as determined by astronomical computation ( for the time zone of Tehran ), as opposed to using an algorithmic system of leap years.

* 1077: Chinese official Su Song is sent on a diplomatic mission to the Liao Dynasty and discovers that the Khitan calendar is more mathematically accurate than the Song calendar ; Emperor Zhezong later sponsors Su Song's astronomical clock tower in order to compete with Liao astronomers.

Mathematics were essential in drafting the astronomical calendar, a lunisolar calendar that used the Sun and Moon as time-markers throughout the year.

Additionally, calculation of the Karaite calendar is not based on astronomical calculations, but only on direct observation of the New Moon and the ripening of Barley.

Astronomers never preposed seriously to replace our era with their astronomical era ( which for that matter coincides exactly with the Christian era where it concerns the calendar years after the year 4 ).

therefore, only with precise foreknowledge of the line frequencies is an astronomical search for the radio spectra of these molecules feasible.

It is the basis for Coordinated Universal Time ( UTC ), which is used for civil timekeeping all over the Earth's surface, and for Terrestrial Time, which is used for astronomical calculations.

A range of astronomy software is available and used by amateur astronomers, including software that generates maps of the sky, software to assist with astrophotography, observation scheduling software, and software to perform various calculations pertaining to astronomical phenomena.

There is a large number of amateur astronomical societies around the world that serve as a meeting point for those interested in amateur astronomy, whether they be people who are actively interested in observing or " armchair astronomers " who may simply be interested in the topic.

They are grouped with the outer bodies — centaurs, Neptune trojans, and trans-Neptunian objects — as minor planets, which is the term preferred in astronomical circles.

In astronomy, a given point on the celestial sphere ( that is, the apparent position of an astronomical object ) can be identified using any of several astronomical coordinate systems, where the references vary according to the particular system.

An astronomical unit ( abbreviated as AU, au, a. u., or ua ) is a unit of length equal to exactly or approximately the mean Earth – Sun distance.

However, the use of AU to refer to the astronomical unit is widespread.

The astronomical constant whose value is one astronomical unit is referred to as unit distance and is given the symbol A.

This value of the astronomical unit had to be obtained experimentally and so is was not known exactly.

The time to traverse an AU is found to be τ < sub > A </ sub > =, resulting in the astronomical unit in metres as c < sub > 0 </ sub > τ < sub > A </ sub > =.

These measured positions are then compared with those calculated by the laws of celestial mechanics: an assembly of calculated positions is often referred to as an ephemeris, in which distances are commonly calculated in astronomical units.

The comparison of the ephemeris with the measured positions leads to a value for the speed of light in astronomical units, which is AU / d ( TDB ).

As the speed of light in meters per second ( c < sub > 0 </ sub >) is fixed in the International System of Units, this measurement of the speed of light in AU / d ( c < sub > AU </ sub >) also determines the value of the astronomical unit in meters ( A ):

The best current ( 2009 ) estimate of the International Astronomical Union ( IAU ) for the value of the astronomical unit in meters is A = m, based on a comparison of JPL and IAA – RAS ephemerides.

With the definitions used before 2012, the astronomical unit was dependent on the heliocentric gravitational constant, that is the product of the gravitational constant G and the solar mass M < sub >☉</ sub >.

Only the product is required to calculate planetary positions for an ephemeris, which explains why ephemerides are calculated in astronomical units and not in SI units.

At a summit in Aleppo, Syria, in 1997, the World Council of Churches ( WCC ) proposed a reform in the calculation of Easter which would have replaced the present divergent practices of calculating Easter with modern scientific knowledge taking into account actual astronomical instances of the spring equinox and full moon based on the meridian of Jerusalem, while also following the Council of Nicea position of Easter being on the Sunday following the full moon.

Thus the aim developed, to provide a new time scale for astronomical and scientific purposes, to avoid the unpredictable irregularities of the mean solar time scale, and to replace for these purposes Universal Time ( UT ) and any other time scale based on the rotation of the Earth around its axis, such as sidereal time.

When ephemeris time was first adopted, time scales were still based on astronomical observation, as they always had been.

The steady progression of sunset around the world and seasonal changes results in gradual civil time changes from one day to the next based on observable astronomical phenomena ( the sunset ) and not on man-made laws and conventions.

His goal was to achieve a definition based on astronomical constants, using a pendulum.

The second thus defined is consistent with the ephemeris second, which was based on astronomical measurements.

The Earth's motion was described in Newcomb's Tables of the Sun ( 1895 ), which provided a formula for estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892.

From an astronomical view, the equinoxes and solstices would be the middle of the respective seasons, but a variable seasonal lag means that the meteorological start of the season, which is based on average temperature patterns, occurs several weeks later than the start of the astronomical season.

Some authorities attempt to define an astronomical winter, often offset, which is based solely on the position of the Earth in its orbit around the sun.

Time standards based on Earth rotation were replaced ( or initially supplemented ) for astronomical use from 1952 onwards by an ephemeris time standard based on the Earth's orbital period and in practice on the motion of the Moon.

The invention in 1955 of the caesium atomic clock has led to the replacement of older and purely astronomical time standards, for most practical purposes, by newer time standards based wholly or partly on atomic time.

Apparent solar time (' apparent ' is often used in English-language sources, but ' true ' in French astronomical literature ) is based on the solar day, which is the period between one solar noon ( passage of the real Sun across the meridian ) and the next.

A third method, using a completely different astronomical approach based on a lunar Crucifixion darkness and eclipse model ( consistent with Apostle Peter's reference to a " moon of blood " in Acts 2: 20 ), points to Friday, 3 April AD 33.

The Chinese calendar is based on astronomical observations and therefore dependent on what is considered the local standard time.

Since the calendar uses astronomical calculation for determining the vernal equinox, it has no intrinsic error, but this makes it an observation based calendar.

Unlike most other calendar systems in India, the New Year's Day on the Malayalam Calendar is not based on any astronomical event.

In 1821, Alexis Bouvard had published astronomical tables of the orbit of Uranus, making predictions of future positions based on Newton's laws of motion and gravitation.

On this is based a method, astrogeodetic levelling, for deriving the local figure of the geoid by measuring deflections of the vertical by astronomical means over an area.

Alfonso X of Castile commissioned the Alfonsine tables, composed of astronomical data based on observation, from which the date of the creation has been calculated to be either 6984 BC.