Do You Know What a Year Is?

Timeby Michael Brackett

As the calendar year comes to a close, a new one begins, and the fear of a December 21, 2012, catastrophe fades away, the meaning of a year comes to mind.  Data resource managers deal with a wide variety of different years, and those different years need to be formally named and comprehensively defined to be meaningful and avoid any confusion.

Many people say “Well, everybody knows what a year is!”, and don’t see the need to precisely define a year.  However, the problem is that the term Year means many different things to many different people, and is often used without being formally named or comprehensively defined.  The result is misunderstanding, misuse of the data, and the creation of disparate data.

To comprehensively define the year being used in data resource design, the different types of years must be recognized and understood.  Years can either be astronomical years or calendar years.  The most prominent astronomical years are the sidereal year, anomalistic year, and equinoctial year.

The sidereal year is the time for the Earth to complete one revolution through its orbit, as measured against a fixed frame of reference.  That reference is usually a star (Latin name sidera)  resulting in the name sidereal year.  The duration of a sidereal year is 365.256363004 solar days (365 days, 6 hours, 9 minutes, and 9.76 seconds).

Then anomalistic year is the time for the Earth to complete one revolution through its elliptical orbit (ecliptic), with respect to the extreme points (apsides) of that elliptical orbit.  The apsides are the perihelion, where the Earth is closest to the Sun, and the aphelion where the Earth is farthest from the Sun.  The anomalistic year is usually the time between perihelion passages.  The duration of the anomalistic year is 365.259363 days (365 days, 6 hours, 13 minutes, and 53 seconds).

Then equinoctial year, also known as the solar tropical year, is the time for the ecliptic longitude of the sun to increase by 360 degrees, and is usually taken between vernal (spring) equinoxes.  Since the Sun’s ecliptic longitude is measured with respect to the equinox, the equinoctial year comprises a complete cycle of the seasons.  At 365.24219 days (365 days, 5 hours, 48 minutes, 45 seconds) it’s about 20 minutes shorter than the sidereal year.

Other less prominent, though often very useful, astronomical years are the draconic year, Sothic year, heliacal year, lunar year, Gaussian year, Besselian year, vague year, galactic year, and great year.

The draconic year, also known as the draconitic year and the eclipse year, is the length of time for the Sun, as seen from the Earth, to complete one revolution with respect to the same point where the Moon’s orbit intersects the Earth’s ecliptic (lunar node) and is associated with eclipses.  Eclipses occur when both the Sun and the Moon are near these lunar nodes.  Eclipses occur every half eclipse year giving two eclipse seasons in each eclipse year.  A draconic year is 346.620075883 days (346 days, 14 hours, 53 minutes, 54 seconds).

The Sothic year is the interval between heliacal risings of the star Sirius (ancient name Sothis).  It originated in ancient Egypt when astronomers noticed that the start of the annual Nile floods coincided with the first visibility of Sothis.  The Sothic year is shorter than the sidereal year and is very close to the Julian year.

The heliacal year is the interval between the heliacal risings of any specific star.  It differs from the sidereal year for stars away from the ecliptic due to the precession of the equinoxes.

The lunar year is twelve full cycles of the Moon phases, as seen from the Earth.  It is approximately 354.37 days.

The Gaussian year is the sidereal year for a planet of negligible mass compared to the Sun, that is unperturbed by other planets, and is governed by the Gaussian gravitational constant.  Such a planet would be slightly closer to the Sun than the Earth.  The Gaussian year would be 365.2568983 days (365 days, 6 hours, 9 minutes, and 56 seconds).

The Besselian year is a tropical year that starts when the Sun reaches an ecliptic mean longitude of 280 degrees.  It’s named after the 19th century German astronomer and mathematician Friedrich Bessel.  The Besselian year depends on the theory about the Earth’s orbit around the sun that was proposed by Newcomb in 1985, which is now obsolete.  Hence the Besselian year is no longer used.

The vague year, also known as a wandering year, is an approximation to a year equaling 365 days, but wanders in relation to an exact year.  A vague year consists of 12 schematic months of 30 days each plus 5 epagomenal days, totaling 365 days.

The galactic year is the time for the Earth’s Solar System to revolve once around the Milky Way galactic center.  The duration of that revolution is approximately 230 million Earth years.

The great year, also known as the equinoctial cycle or precession of the equinoxes, is one complete revolution of the equinoxes around the ecliptic.  Precession is the movement of the rotational axis of the Earth, where the axis slowly marks a cone around the ecliptic pole with an angular radius of 23.5 degrees, and moves one degree every 72 years.   During precession the stars move slowly through equatorial coordinates and ecliptic longitudes.  The length of a great year is slightly less than 26,000 years.  However, it cannot be determined exactly because the precession speed is not constant.

The calendar year is an approximation of the Earth’s orbital period stated as a calendar.  Calendar years are typically Gregorian or Julian consisting of common years containing 365 days and leap years containing 366 days.  However, other calendars, such as Chinese, Persian, Islamic, Hindu, and so on, also exist.

The equinoctial year is the basis for the Gregorian calendar.  The Gregorian calendar can be used to form a variety of different years, such as a seasonal year, calendar year, fiscal year, academic year, and Julian year.

A seasonal year is the time between recurrences of a seasonal event, such as a river flooding, seasonal bird migration, flowering of plants, first frost, a sports event, and so on.  A seasonal year can vary as much as a month or more from one calendar year to another year.

A fiscal year is a financial twelve-month period useful for preparing financial statements.  Many jurisdictions have regulations requiring financial statements every twelve months, although those twelve months do not need to correspond to a calendar year.  For example, in the United States state fiscal years are July 1 through June 30, and federal fiscal years are October 1 through September 30.  Other countries have different fiscal years, such as the United Kingdom’s fiscal year runs from April 1 to March 31 for corporation tax and government financial statements, and April 6 to April 5 for personal taxation and payment of state benefits.  A corporate fiscal year is exactly 52 weeks divided into four 13-week quarters.  Organizations using the corporate fiscal year usually file for a change in the dates of their fiscal year every calendar year.

An academic year is the annual period that a student attends an educational institution, and is usually divided into academic terms consisting of either four quarters or three semesters.  Academic year dates vary widely among educational institutions, generally starting and ending sometime during the early Fall.

A Julian year is defined as exactly 365.25 days, and is used in astronomy and other scientific contexts.  A Julian century is 36525 days and a Julian millennium is 365250 days.  Expressing time in Julian years precisely specifies the number of days, not the number of years, and is used for long time intervals.  Julian year is used in the computation of distances based on a light year.

For a large number of Julian years, the notations kiloannum (Ka) is 103 (thousand) years, megaannum (Ma) is 106 (million) years, gigaannum (Ga) is 109 (billion) years, teraannum (Ta) is 1012 (trillion) years, petaannum (Pa) 1015 (quadrillion), and exaannum (Ea) 1018 (quintillion) years are used.  Kiloannum is used for geology, archeology, and paleontology.  Megaannum is used for geology, paleontology, and celestial mechanics.  Gigaaannum is used for geology and cosmology where the age of the Earth is 4.57 Ga.  Tetraannum is 70 times longer than the age of the universe.  Petaannum is the half-life of cadmium 113 at 8 Pa.  Exaannum is the half-life of tungsten 180 at 1.8 Ea.

The years described above are with respect to the Earth and its Solar System.  Other planets in the Solar System, and planets in other solar systems, will have different values for these years, or may have different years altogether.

Professional data managers, particularly data architects and data modelers, must provide a formal name and comprehensive definition of the specific year being used in the data resource, and must specify the location, such as the Earth, the Solar System, the Milky Way Galaxy, or the universe.

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