Julian year (astronomy)
Encyclopedia
In astronomy
, a Julian year (symbol: a) is a unit of measurement
of time
defined as exactly 365.25 day
s of 86 400 SI
second
s each, totaling 31 557 600 seconds. The Julian year is the average length of the year
in the Julian calendar
used in Western societies in previous centuries, and for which the unit is named. Nevertheless, because a Julian year measures duration rather than designating a date, the Julian year does not correspond to years in the Julian calendar
or any other calendar
. Nor does it correspond to the many other ways of defining a year
.
of measurement, nor is it sanctioned in the International System of Units
(SI), nor does it correspond very exactly to the length of a solar year. Nevertheless, astronomers and other scientists use it for convenience to measure lengthy durations, which would be unwieldy to express as a number of seconds (the SI unit of time). Astronomers before the mid-20th century used the so called Besselian year instead, which corresponds much more closely to the real length of a solar year, but which is also not totally exact, and which has the disadvantage of being much harder to determine than the simple 365.25 day Julian year. Simplicity is also the reason why the Julian year is used instead of a "Gregorian Year" of 365.2425 days.
Since the Julian year corresponds roughly to the duration of what most people think of as a year
, its use also aids comprehension. For example, it is easier to express and to comprehend the orbital period
of Pluto
as 248 Julian years (248 a) than as 7,828 million seconds (7.828 × 10^{9} s). For this reason, its use is recommended by International Astronomical Union
(IAU).
The Julian year is the basis of the definition of the light-year
as a unit of measurement of distance.
One hundred Julian years (36,525 days) are called a Julian century. One Julian century is 3,155,760,000 seconds. One thousand Julian years (365,250 days) are called a Julian millennium. These units are used in calculating Solar System
ephemerides
.
The standard epoch in use today is Julian epoch J2000.0. It is exactly 12:00 TT
(close to but not exactly Greenwich mean
noon) on January 1, 2000 in the Gregorian
(not Julian) calendar. Julian within its name indicates that other Julian epochs can be a number of Julian years of 365.25 days each before or after J2000.0. For example, the future epoch J2100.0 will be exactly 36,525 days (one Julian century) from J2000.0 on January 1, 2100 (the dates will still agree because the Gregorian century 2000–2100 will have the same number of days as a Julian century).
Because Julian years are not exactly the same length as years on the Gregorian calendar, astronomical epochs will diverge noticeably from the Gregorian calendar in a few hundred years.
for events since its introduction on October 15, 1582 (or later, depending on country), and the Julian calendar for events before that date.
as the integer number of days that have elapsed since a reference date or initial epoch. The Julian day uniquely specifies a date without reference to its day, month, or year in any particular calendar. A specific time within a day is specified via a decimal fraction.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, a Julian year (symbol: a) is a unit of measurement
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
defined as exactly 365.25 day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
s of 86 400 SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s each, totaling 31 557 600 seconds. The Julian year is the average length of the year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....
in the Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...
used in Western societies in previous centuries, and for which the unit is named. Nevertheless, because a Julian year measures duration rather than designating a date, the Julian year does not correspond to years in the Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...
or any other calendar
Calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...
. Nor does it correspond to the many other ways of defining a year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....
.
Usage
The Julian year is not a fundamental unitFundamental unit
A set of fundamental units is a set of units for physical quantities from which every other unit can be generated.In the language of measurement, quantities are quantifiable aspects of the world, such as time, distance, velocity, mass, momentum, energy, and weight, and units are used to describe...
of measurement, nor is it sanctioned in the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
(SI), nor does it correspond very exactly to the length of a solar year. Nevertheless, astronomers and other scientists use it for convenience to measure lengthy durations, which would be unwieldy to express as a number of seconds (the SI unit of time). Astronomers before the mid-20th century used the so called Besselian year instead, which corresponds much more closely to the real length of a solar year, but which is also not totally exact, and which has the disadvantage of being much harder to determine than the simple 365.25 day Julian year. Simplicity is also the reason why the Julian year is used instead of a "Gregorian Year" of 365.2425 days.
Since the Julian year corresponds roughly to the duration of what most people think of as a year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....
, its use also aids comprehension. For example, it is easier to express and to comprehend the orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
of Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
as 248 Julian years (248 a) than as 7,828 million seconds (7.828 × 10^{9} s). For this reason, its use is recommended by International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
(IAU).
The Julian year is the basis of the definition of the light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
as a unit of measurement of distance.
One hundred Julian years (36,525 days) are called a Julian century. One Julian century is 3,155,760,000 seconds. One thousand Julian years (365,250 days) are called a Julian millennium. These units are used in calculating Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
ephemerides
Ephemeris
An ephemeris is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Different kinds of ephemerides are used for astronomy and astrology...
.
Epochs
In astronomy, an epoch specifies a precise moment in time. The positions of celestial objects and events, as measured from earth, change over time, so when measuring or predicting celestial positions, the epoch to which they pertain must be specified. A new standard epoch is chosen about every 50 years.The standard epoch in use today is Julian epoch J2000.0. It is exactly 12:00 TT
Terrestrial Time
Terrestrial Time is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth....
(close to but not exactly Greenwich mean
Greenwich Mean Time
Greenwich Mean Time is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is arguably the same as Coordinated Universal Time and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United...
noon) on January 1, 2000 in the Gregorian
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...
(not Julian) calendar. Julian within its name indicates that other Julian epochs can be a number of Julian years of 365.25 days each before or after J2000.0. For example, the future epoch J2100.0 will be exactly 36,525 days (one Julian century) from J2000.0 on January 1, 2100 (the dates will still agree because the Gregorian century 2000–2100 will have the same number of days as a Julian century).
Because Julian years are not exactly the same length as years on the Gregorian calendar, astronomical epochs will diverge noticeably from the Gregorian calendar in a few hundred years.
Julian calendar distinguished
The Julian year, being a uniform measure of duration, should not be confused with the variable length historical years in the Julian calendar. An astronomical Julian year is never individually numbered. Astronomers follow the same calendar conventions that are accepted in the world community: They use the Gregorian calendarGregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...
for events since its introduction on October 15, 1582 (or later, depending on country), and the Julian calendar for events before that date.
Julian day distinguished
A Julian year should not be confused with the Julian day (also Julian day number or JDN), which is also used in astronomy. Despite the similarity of names, there is little connection between the two. It is a way of expressing a dateCalendar date
A date in a calendar is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "24 " is ten days after "14 " in the Gregorian calendar. The date of a...
as the integer number of days that have elapsed since a reference date or initial epoch. The Julian day uniquely specifies a date without reference to its day, month, or year in any particular calendar. A specific time within a day is specified via a decimal fraction.
Other sources
- Explanatory supplement to the Astronomical Almanac. P. Kenneth Seidelmann, editor. Mill Valley, Cal.: University Science Books, 1992. Pages 8, 696, 698-9, 704, 716, 730.