Glossary

At the end of some entries related entries are given marked ¶.

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Aphelion: the point of a planet's orbit where distance from the sun is greatest. The distance between the earth and the sun at the aphelion is 152.1 million kilometers.
¶ ⇒Perihelion, ⇒Perigee, ⇒Apogee, ⇒Line of Apsides

Apogee: the point of the moon's orbit where the distance from the earth is greatest, being 406740 kilometers.
¶ ⇒Perihelion, ⇒Aphelion, ⇒Perigee, ⇒Line of Apsides

Baktun: name for a period of 20 ⇒katun or 144000 days in the Mayan ⇒long count. 1 baktun therefore equals about 394.3 tropical years.
¶ ⇒Kin, ⇒Uinal, ⇒Tun

Calendar: a system of division of time into different periods mostly according to astronomical events, but also sometimes arbitrarily. Almanacs showing the sequence of such periods as well as civil and religious feast days are also called calendar but need not concern us here.
Today's calendars with worldwide importance use astronomical events, of which mainly the tropical ⇒year and the synodic ⇒month are taken as the basis for the definition of calendrical periods. Depending on what events are taken into account there are three groups of calendars. ⇒Free lunar calendars (e. g. the Islamic calendar) only consider the synodic month, ⇒free solar calendars (e. g. the Gregorian calendar) only the tropical year. ⇒Bound lunar calendars (e. g. the Jewish calendar) use both the synodic month and the tropical year. There are calendars, too, that employ the sidereal year (mainly in India).
Another way of classification is to divide calendars into arithmetical and astronomical calendars. The first group contains calendars with fixed rules of intercalation, as e. g. the Gregorian, Persian, or Jewish calendars. The beginning of months or years of calendars belonging to the second group are determined by direct observation of certain astronomical events. The Islamic and the French Revolutionary calendars are two calendars of this group.

Callippic Cycle: 76-year cycle for ⇒bound lunar calendars that has 940 lunations in 76 Julian years. The cycle is attributed to the Greek scholar Callippus (Kallippos). Callippus assumed the lengths of the tropical ⇒year and of the synodic ⇒month to be (365 + ¹/₄) days and (29 + ⁴⁹⁹/₉₄₀) days, respectively. He achieved this by shortening the sequence of four ⇒Metonic cycles by one day. Thus 76 years as well as 940 lunar months had 27759 days. Of the 940 months 441 were "hollow", having 29 days each, and 499 were "full" with 30 days each. These months were arranged within 48 common years of 12 months each and 28 leap years of 13 months each. The assumed length of the synodic month is only about 22 s longer than the actual length.
¶ ⇒Oktaeteris, ¶ ⇒Metonic Cycle

Circle of Declination: any largest circle on the celestial sphere that is perpendicular to the celestial equator. The largest circle going through a certain fixed star and being perpendicular to the celestial equator is the circle of declination of that fixed star.
See Basics of Time Reckoning.
¶ ⇒Ecliptic, ⇒Great Circle

Common Year: a calendrical year that is not a ⇒leap year.

Conjunction: a special configuration in which a planet or the moon and the sun appear in the same direction for an observer on earth.
For planets with orbits within the earth's orbit there are the inferior cunjunction (the planet is between the earth and the sun) and the superior cunjunction (the sun is between the earth and the sun).
¶ ⇒Opposition, ⇒New Moon, ⇒Full Moon

Culmination: Moment in which a heavenly body reaches the highest point of its apparent daily path.

Cyclic Calculation of the Moon: calculation of ⇒new and ⇒full moons according to a lunar calendar. In this calculation, certain cycles are employed in which the phases of the moon repeat. The ⇒Metonic cycle was the basis of the lunar calendar of the Julian calendar. The cyclic calculation assumes the lunar month to have a constant and invariable (mean) length whereas the true lunar months vary in length considerably due to the irregularities of the moon's orbit. The calculated new and full moons are called cyclic new/full moons as against the true new/full moons determined by observation. The Gregorian reform of the calendar included an adptation of the lunar calendar in order to minimize the error of the cyclic moon against the true moon. See The Calculation of Easter.

Day: a basic time unit of a calendar. Depending on different starting points there are different lengths. The solar day can be observed directly and is defined as the time between two consecutive ⇒culminations of the sun. Due to the varying movement of earth on her elliptic path around the sun the length of the solar day is not constant. To achieve a constant value the mean solar day is defined as the time between two consecutive culminations of a ‘mean sun’. The mean sun is an ideal sun travelling at a constant speed throughout the year. The length of such a mean solar day is 24 hours mean solar time.
The mean sidereal day is defined as the time between two consecutive culminations of the mean ⇒vernal equinox. This day is about 0.0084 s longer than the mean solar day, a difference caused by the ⇒precession. The true sidereal day is the time between two consecutive culminations of the true vernal equinox and differs from the mean sidereal day due to irregularities in the precession movement, s. Basics of Time Reckoning.
There are different times of the beginning of the day. The Julian calendar as well as the Gregorian calendar start their day at midnight, whereas the day starts at sunset in the Jewish calendar and the Islamic calendar. The day of the Julian date starts at 12 h GMT regardless of the actual location of the observer.
¶ ⇒Month, ⇒Year

Dominical Letter: letter of the first sunday of a year if the first seven days of the year are designated with the letters A to G. The dominical letter was used to determine the date of Easter.
See The Calculation of Easter.
¶ ⇒Golden Number, ⇒Easter, ⇒Epact

Easter: main feast of Christian churches. Interesting from a calendrical point of view is the determination of the date of the Easter sunday. The main rule for this was established at the Council of Nicaea in 325 BCE.
Detailed information on the calculation of Easter can be found in The Calculation of Easter. Easter Calculator is a form for the computation of Easter of both the Greogrian and the Julian calendars.
¶ ⇒Epact, ⇒Dominical Letter

Ecliptic: circle in which the earth's orbit's plane (ecliptical plane) intersects the celestial sphere. For an observer on earth the sun goes along the ecliptic during one tropical year.
The ecliptical plane is also called ecliptic sometimes.
See Basics of Time Reckoning.
¶ ⇒Vernal Equinox

Epact: number that gives the age of the cyclic moon on 22 March (old style epact of the Julian calendar) or 1 January (new style epact of the Gregorian calendar).
See The Calculation of Easter.
¶ ⇒Golden Number, ⇒Easter, ⇒Dominical Letter

Epoch: or era, a point of time on which the counting of days, months, and years of calendar starts. This can be defined by a real event or by a supposed event like the creation.
See Epochs and Eras.

Full Moon: astronomically the phase of the moon in which the moon is in ⇒opposition with the sun.
In the ecclesiastical lunar calendar the full moon determined by ⇒cyclic calculation (14th day after the cyclic new moon) was used for the calculation of the date of Easter.
¶ ⇒New Moon, ⇒Easter

Golden Number: a number indicating the position of a year in a 19-year cycle. If Y is the year the golden number GN is calculated by GN = 1 + (Y mod 19). The golden number was used to determine the date of Easter.
See The Calculation of Easter.
¶ ⇒Epact, ⇒Easter, ⇒Dominical Letter

Great Circle: A circle on the surface of a sphere that runs through two certain fix points and has the maximum diameter of all circles that exist running through these two points. The center of the great circle lies in the center of the sphere and its radius is the same as that of the sphere. The shorter one of the two arcs that are formed by the two points on the circle is the shortest direct distance between them on the surface of the sphere.
¶ ⇒Circle of Declination

Haab: name of the 365-day-year employed by the Mayans. Such a year contained 18 'months' of 20 days each and five additional days, see Mayan Calendar.
¶ ⇒Tzolkin

Ides: name of a certain day of the month of the Roman calendar. This day more or less marked the middle of a month. In the pre-Julian Roman calendar the Ides were the 15th day of March, May, Quintilis, October, November, and December, and the 13th day of the other months. After the Julian reform the 15th day of March, May, July, and October, as well as the 13th day of the remaining months were called the Ides. The word ‘Ides’ is used in its plural form only (latin Idus).
¶ ⇒Nones, ⇒Kalends

Indiction: a 15-year period having its roots in the rhythm of the assessment of taxes in the Roman empire. A five-year cycle introduced by emperor Diocletian later gave way to a 15-year period. Alongside with the number of a year only the number of that year within the current cycle was given. The indiction changed at various dates. In Alexandria the indiction year began on 1 May, in the rest of Egypt on 1 Thoth (29 August, in Julian years preceding a leap year 30 August). In Constantinople the indiction changed on 23 September first, but from some time in the 5th century 1 September was used so as to coincide with the civil New Year. There were more dates in use regionally in the medieval.

Jieqi: Name of a minor solar term of the Chinese calendar. A minor term is defined by a 15° sector of the ⇒ecliptic beginning at n · 30° + 15° ecliptical length. See The Chinese Calendar, Calendar in Japan and Vietnam.
¶ ⇒Zhongqi

Julian Date: also called Julian Day Number. Counting of days beginning on 1 January 4713 BCE (Julian) at 12 h UT. The time is expressed as a fraction of the day (e. g. 0.25 JD for 1 January 4713 BCE, 18 h UT). ⇒Modified Julian Date (MJD) and ⇒Truncated Julian Date (TJD) use different epochs.
See Julian Date.

Kalends: name of the first day of a month of the Roman calendar. The word ‘kalends’ is always used in its plural form although it specifies a single day (latin kalendae).
¶ ⇒Nones, ⇒Ides

Katun: name for a period of 20 ⇒tun = 7200 days in the Mayan ⇒long count. That is about 19.7 tropical years.
¶ ⇒Kin, ⇒Uinal, ⇒Baktun

Khalak: unit of time employed in the cyclic calculation of new moons in the Jewish Calendar. 1 Khalak has a length of 10/3 seconds.
¶ ⇒Molad

Kin: name for the ⇒day in the Mayan ⇒Long Count.
¶ ⇒Uinal, ⇒Tun, ⇒Katun, ⇒Baktun

Leap Year: calendar year which contains an additional day or an adiitional month to keep the calendar aligned with the actual ⇒year that is taken as the basis. Free ⇒solar or ⇒lunar calendars add a single leap day, while ⇒bound lunar calendars add a whole leap month to keep in synchrony with the moon's phases.
In the calendar used in most countries of the world, the Gregorian calendar, all years that are divisible by 4 and are no ⇒secular years are leap years, in which February has 29 days. Secular years are only leap years if they are divisible by 400. Thus, the year 2000 was a leap year whereas 2100 will be a common year.
Leap years of the Julian calendar are all years divisible by 4. Therefore the difference between both calendars increases by one day in every secular year not divisible by 400.
The schematic version of the Islamic calendar has 11 leap years within a 30-year cycle, the Persian calendar employs a 2820-year cycle with 683 leap years. The Jewish calendar has quite intricate leap year rules.
¶ ⇒Metonic Cycle

Line of Apsides: The apsides are the points of an orbit of one body around another is either greatest or least. As for the earth the line of apsides is the line betwenn ⇒aphelion and ⇒perihelion, while the moon's line of apsides goes from ⇒apogee to ⇒perigee.

Line of Nodes: line between the two ⇒nodes of a celestial body.

Long Count: a system for the designation of dates used by the Mayans but probably by other Central Americal peoples as well. Long before decimal notation came into use in Europe, the Mayans expressed their dates through powers of twenty. The time units ⇒uinal, ⇒katun, ⇒baktun, pictun, calabtun, and kinchiltun consisted of 20 of their next smallest units, a kinchiltun describing a period of nearly unimaginable 63 million years. The only exception was the ⇒tun which comprised not 20 but only 18 uinal or 360 days, thus roughly approximating the length of a tropical year.
Long count dates can be found on stelae probably erected on commemorable occasions.
See Mayan calendar.

Lunar Calendar, bound: or lunisolar calendar; a calendar with months kept in synchrony with the moon's phases and approximating the length of a tropical year. The beginning of each ⇒month is always near a certain phase of the moon, mostly a short time after ⇒new moon because the first visibility of the moon crescent after new moon is comparatively easy to observe and therefore used in observation based calendars. Because there is not a whole number of synodic months within a tropical ⇒year it is necessary to introduce ⇒leap years to which a whole leap month is added. The insertion of a single leap day would destroy the synchrony with the moon. Thus, common years have 12 and leap years 13 months. Examples for bound lunar calendars are the old Babylonian calendar and the Jewish calendar.
¶ ⇒(free) Lunar Calendar, ⇒(free) Solar Calendar

Lunar Calendar, free: a calendar with months kept in synchrony with the moon's phases that does not try to approximate the tropical ⇒year. The mean length of a ⇒month is a close approximation of the synodic month, twelve of which constitute a (lunar) year. This year is about 11 days shorter than the tropical year. The only calendar of worldwide importance following this pattern is the Islamic calendar.
¶ ⇒(free) Solar Calendar, ⇒(bound) Lunar Calendar

Lunation: name of a month in the cyclic Easter calculation of the Julian and Gregorian calendars. A lunation has a length of 29 or 30 days.
In astronomy, a synodic ⇒month is sometimes called lunation as well.
See The Calculation of Easter.
¶ ⇒Easter

Lunisolar Calendar: ⇒(bound) lunar calendar

Metonic Cycle: 19-year cycle employed in ⇒bound lunar calendars that has 235 lunations in 19 Julian years. The cycle is attributed to the Greek scholar Meton who lived in the 5th century BCE. Meton assumed the lengths of the tropical ⇒year and of the synodic ⇒month to be (365 + ⁵/₁₉) days and (29 + ²⁵/₄₇) days, respectively. Thus, 19 years as well as 235 lunar months had 6940 days. Of the 235 months 110 were "hollow", having 29 days each, and 125 were "full" with 30 days each. These months were arranged within 12 common years of 12 months each and 7 leap years of 13 months each. In fact, 19 tropical years have (19 · 365.242199) days = 6939.601781 days, and 235 synodic months have 235 · 29.530589 days = 6939,688443 days. The difference is about 2 hours in 19 years. The assumed year however is more than 30 min too long.
A lunisolar calendar based on the Metonic cycle was in use for the ⇒cyclic calculation of the moon to compute the date of ⇒Easter until the Gregorian reform in 1582.
¶ ⇒Oktaeteris, ¶ ⇒Callipic Cycle

Modified Julian Date: abbr. MJD; ⇒Julian Date

Molad: a cyclic ⇒new moon of the Jewish calendar. The ⇒cyclic calculation of the moon of the Jewish calendar takes the time between two consecutive new moons as 29 days 12 hours 793 ⇒khalakim = (29 days 12 hours 44 minutes 3,33.. seconds).

Month: astronomically the duration of one revolution of the moon around the earth. Depending on the coice of a starting point there are different lengths. The most obvious month is the synodic month with 29 days 12 hours 44 minutes und 2.9 seconds and is defined as the time between two consecutive ⇒conjunctions of the moon. The siderereal month is defined as the time between two consecutive passes of the moon through the ⇒circle of declination of a fixed star, which is 27 days 7 hours 43 minutes 11,5 seconds. The time between two consecutive passes of the moon through the circle of declination of the vernal equinox is called tropic month with a length of 27 days 7 hours 43 minutes 4,7 seconds. The anomalistic month has 27 days 13 hours 18 minutes 33,2 seconds which is the time between two consecutive passes of the moon through the ⇒perigee. The draconic month is the time between two consecutive passes of the moon through the ascending node of the moon's orbit and has 27 days 5 hours 5 minutes 35,8 seconds.
See Basics of Time Reckoning.
In calendars, months approximate either the length of a synodic month (in ⇒free and ⇒bound lunar calendars) or 1/12 of the length of the tropical year (in ⇒free solar calendars).
¶ ⇒year, ⇒day

Nakshâtra: 27 or 28 stars near the ⇒ecliptic with their ⇒circles of declination dividing the ecliptic into 27 or 28 sections. See Calendars of India for further information.
¶ ⇒Tithi, ⇒Sankrânti

New Moon: astronomically the phase of the moon in which the moon is in ⇒conjunction with the sun.
Calendrical new moons are determined by a ⇒ cyclic calculation, e. g. for The Calculation of Easter and in the Jewish calendar. These notional new moons can differ quite substantial from the actual astronomical new moons.
¶ ⇒Full Moon, ⇒Easter

New Style: used to designate the Gregorian calendar as against the Julian calendar.
¶ ⇒Old Style

Node: point in which the orbit of a body intersects the ⇒Ecliptic. The node in which the body passes from south to north is called ascending node, the point where the body passes from north to south is the descending node.
¶ ⇒Line of Nodes

Nones: name for the day eight days before the ⇒ides in the Roman calendar. The word ‘nones’ is used only in its plural form. (latin nonae).
¶ ⇒Kalenden

Oktaeteris: name of a leap year cycle employed for bound lunar calendars in Ancient Greece. The cycle had a length of eight years, of which five were common years and three ⇒leap years with 12 and 13 months respectively. The cycle approximates eight tropical ⇒years with 99 lunar months, i. e. 8 · 365.242199 days = 2921.937592 days with 99 · 29.530589 days = 2923.528311 days. The error amounts to more than one and a half day in eight years already.
¶ ⇒Metonic Cycle, ⇒Callippic Cycle

Old Style: used to designate the Julian calendar as against the Gregorian calendar.
¶ ⇒New Style

Opposition: a configuration in which a planet or the moon appears in the direction opposite to the sun for an observer on earth.
¶ ⇒Conjunction, ⇒New Moon, ⇒Full Moon

Perigee: the point of the moon's orbit nearest to the earth, where the distance of the moon from the earth is 356410 km.
¶ ⇒Perihelion, ⇒Aphelion, ⇒Apogee, ⇒Line of Apsides

Perihelion: the point of the earth's orbit nearest to the sun, the distance between earth and sun being 147.1 million km there.
¶ ⇒Aphelion, ⇒Perigee, ⇒Apogee, ⇒Line of Apsides

Platonic Year: duration of a whole revolution of the earth axis because of the ⇒precession; about 25700 years.

Precession: slow rotation of the earth axis around an axis perpendicular to the ecliptical plane (⇒Ecliptic). It is caused by the earth's shape which slightly differs from that of an ideal sphere and the gravitational forces between earth and both moon and sun.
The precession causes the shift of the ⇒vernal equinox and the celestial poles, one revolution lasting aboute 25700 years.
See Basics of Time Reckoning.

Sankrânti: The sun's entry into one of 27 or 28 sections into which the ecliptic is divided for Indian calendars. The limits of these sections are defined by stars called ⇒Nakshâtra. See Calendars of India.
¶ ⇒Tithi

Secular Year: a year whose number leaves no remainder when divided by 100. In the Gregorian calendar secular years not divisible by 400 are no ⇒Leap Years.

Solar Calendar, free: a calendar which is based solely on the annual movement of the sun, or the tropical year. That results in the months being independent of the moon's phases which means that the beginning of the months normally do not coincide with a ⇒ or a ⇒. In leap years a single leap day can be inserted to approximate the true length of the year. The Julian calendar, the Gregorian calendar, and the ancient Egyptian calendar are examples for free solar calendars.
¶ ⇒(free) Lunar Calendar, ⇒(bound) Lunar Calendar

Tithi: The time it takes the angle between the sun and the moon to wane or wax by 12°; used in Indian calendars.
¶ ⇒Sankrânti, ⇒Nakshâtra

Truncated Julian Date: abbr. TJD, ⇒Julian Date

Tun: name of a period of 18 ⇒Uinal = 360 days in the Mayan ⇒long count.
¶ ⇒Kin, ⇒Katun, ⇒Baktun

Tzolkin: he 260-day-'year' of the Mayans, consisting of 13 'months' with 20 days each, see Mayan calendar.
¶ ⇒Haab

Uinal: name of a period of 20 days in the Mayan ⇒long count.
¶ ⇒Kin, ⇒Tun, ⇒Katun, ⇒Baktun

Vernal Equinox: the point of the ⇒ecliptic in which the sun passes the celstial equator from south to north.
¶ ⇒Precession

Wandering Year: The ancient Egyptian year of constantly 365 days was 6 ours shorter than the Julian year. This caused the beginning of the Egyptian year ‘wandering’ through the whole Julian year, which is why it was also called ‘wandering year’.
¶ ⇒Year

Year: in astronomy the duration of one revolution of the earth around the sun. Depending on different starting points there are years of different lengths. The year on which most calendars are based is the tropical year with a length of currently 365 days 5 hours 48 minutes 46 seconds, or 365.242199 days. It is defined as the time between two consecutive passes of the sun through the ⇒vernal equinox. The sidereal year is the time between two consecutive passes of the sun through the ⇒circle of declination of a fixed star and has a length of 365 days 6 hours 9 minutes 9 seconds. The anomalistic year of 365 days 6 hours 13 minutes 53 seconds is the time between two consecutive passes of the earth through the ⇒Perihelion.
See Basics of Time Reckoning.
The years of most of the calendars try to approximate the tropical year. The calendrical year of the Julian calendar has 365.25 days, that of the Gregorian calendar has 365.2425 days. The Persian calendar employs a year of 365.2422 days.
¶ ⇒Day, ⇒Month, ⇒Leap Year

Zhongqi: Name of a major solar term of the Chinese calendar. A major term is defined by a 15° section of the ⇒ecliptic beginning at n · 30° ecliptical length. See The Chinese Calendar, Calendar in Japan and Vietnam.
¶ ⇒Jieqi

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