Julian year (astronomy)

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In astronomy, a Julian year (symbol: a or aj) is a unit of measurement of time defined as exactly 365.25 days of 86400 SI seconds each. [1] [2] [3] [4] The length of the Julian year is the average length of the year in the Julian calendar that was used in Western societies until the adoption of the Gregorian Calendar, and from which the unit is named. Nevertheless, because astronomical Julian years are measuring duration rather than designating dates, this 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.

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Usage

The Julian year is not a unit of measurement in the International System of Units (SI), but it is recognized by the International Astronomical Union (IAU) as a non-SI unit for use in astronomy. [3] Before 1984, both the Julian year and the mean tropical year were used by astronomers. In 1898, Simon Newcomb used both in his Tables of the Sun in the form of the Julian century (36 525 days) and the "solar century" (36524.22 days), a rounded form of 100 mean tropical years of 365.24219879 days each according to Newcomb. [5] However, the mean tropical year is not suitable as a unit of measurement because it varies from year to year by a small amount, 6.14×10−8 days according to Newcomb. [5] In contrast, the Julian year is defined in terms of the SI unit one second, so is as accurate as that unit and is constant. It approximates both the sidereal year and the tropical year to about ±0.008 days. The Julian year is the basis of the definition of the light-year as a unit of measurement of distance. [2]

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 (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 at 12:00 TT 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 example, in the next 1000 years, seven days will be dropped from the Gregorian calendar but not from 1000 Julian years, so J3000.0 will be January 8, 3000 12:00 TT.

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. When not using Julian day numbers (see next §), astronomers follow the same conventional calendars that are accepted in the world community: They use the Gregorian calendar for events since its introduction on October 15, 1582 (or later, depending on country), and the Julian calendar for events before that date, and occasionally other, local calendars when appropriate for a given publication.

Julian day distinguished

A Julian year should not be confused with the Julian day, which is also used in astronomy (more properly called the Julian day number or JDN). The JDN uniquely specifies a place in time, without becoming bogged down in its date-in-month, week, month, or year in any particular calendar. Despite the similarity of names, there is almost no connection between the Julian day numbers and Julian years.

The Julian day number is a simplified time-keeping system originally intended to ease calculation with historical dates which involve a diversity of local, idiosyncratic calendars. It was adopted by astronomers in the mid-1800s, and identifies each date as the integer number of days that have elapsed since a reference date ("epoch"), chosen to precede most, if not all, historical records. A specific time within a day, always using UTC, is specified via a decimal fraction.

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<span class="mw-page-title-main">Declination</span> Astronomical coordinate analogous to latitude

In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or south (negative) of the celestial equator, along the hour circle passing through the point in question.

<span class="mw-page-title-main">Ecliptic</span> Apparent path of the Sun on the celestial sphere

The ecliptic or ecliptic plane is the orbital plane of Earth around the Sun. 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. The ecliptic is an important reference plane and is the basis of the ecliptic coordinate system.

The term ephemeris time can in principle refer to time in association with any ephemeris. In practice it has been used more specifically to refer to:

  1. a former standard astronomical time scale adopted in 1952 by the IAU, and superseded during the 1970s. This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating mean solar time. The intent was to define a uniform time based on Newtonian theory. Ephemeris time was a first application of the concept of a dynamical time scale, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion.
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The Revised Julian calendar, or less formally the new calendar and also known as the Milanković calendar, is a calendar proposed in 1923 by the Serbian scientist Milutin Milanković as a more accurate alternative to both Julian and Gregorian calendars. At the time, the Julian calendar was still in use by all of the Eastern Orthodox Church and affiliated nations, while the Catholic and Protestant nations were using the Gregorian calendar. Thus, Milanković's aim was to discontinue the divergence between the naming of dates in Eastern and Western churches and nations. It was intended to replace the Julian calendar in Eastern Orthodox Churches and nations. From 1 March 1600 through 28 February 2800, the Revised Julian calendar aligns its dates with the Gregorian calendar, which had been proclaimed in 1582 by Pope Gregory XIII.

<span class="mw-page-title-main">Second</span> SI unit of time

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Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth. For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time, which succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.

<span class="mw-page-title-main">Year</span> Time of one planets orbit around a star

A year is the time taken for astronomical objects to complete one orbit. For example, a year on Earth is the time taken for Earth to revolve around the Sun. Generally, a year is taken to mean a calendar year, but the word is also used for periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. The term can also be used in reference to any long period or cycle, such as the Great Year.

The Julian day is a continuous count of days from the beginning of the Julian period; it is used primarily by astronomers, and in software for easily calculating elapsed days between two events.

A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.

A sidereal year, also called a sidereal orbital period, is the time that Earth or another planetary body takes to orbit the Sun once with respect to the fixed stars.

In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to perturbations and vary with time. These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane, the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit.

The astronomical system of units, formerly called the IAU (1976) System of Astronomical Constants, is a system of measurement developed for use in astronomy. It was adopted by the International Astronomical Union (IAU) in 1976 via Resolution No. 1, and has been significantly updated in 1994 and 2009.

<span class="mw-page-title-main">Timekeeping on Mars</span> Proposed approaches to tracking date and time on the planet Mars

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In astronomy, an equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator. Although there are two such intersections, the equinox associated with the Sun's ascending node is used as the conventional origin of celestial coordinate systems and referred to simply as "the equinox". In contrast to the common usage of spring/vernal and autumnal equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.

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 differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.

<span class="mw-page-title-main">Light-year</span> Distance that light travels in one year

A light-year, alternatively spelled light year, is a unit of length used to express astronomical distances and is equal to exactly 9460730472580.8 km, which is approximately 5.88 trillion mi. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year. Despite its inclusion of the word "year", the term should not be misinterpreted as a unit of time.

A tropical year or solar year is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronomical seasons. For example, it is the time from vernal equinox to the next vernal equinox, or from summer solstice to the next summer solstice. It is the type of year used by tropical solar calendars.

<span class="mw-page-title-main">Lunar month</span> Time between successive new moons

In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.

References

  1. ↑ P. Kenneth Seidelmann, ed., The explanatory supplement to the Astronomical Almanac, (Mill Valley, Cal.: University Science Books, 1992), pp. 8, 696, 698–9, 704, 716, 730.
  2. 1 2 "Measuring the Universe". International Astronomical Union . Retrieved March 22, 2012.
  3. 1 2 International Astronomical Union. "Recommendations Concerning Units". Archived from the original on February 16, 2007. Retrieved February 18, 2007. Reprinted from the "IAU Style Manual" by G.A. Wilkinson, Comm. 5, in IAU Transactions XXB (1987).
  4. ↑ Harold Rabinowitz and Suzanne Vogel, The manual of scientific style (Burlington, MA: Academic Press, 2009) 369.
  5. 1 2 Simon Newcomb, Tables of the Four Inner Planets, vol. 6 of Astronomical Papers Prepared for the Use of the American Ephemeris and Nautical Almanac (Washington, DC: 1898), pp. 10–11.
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