In astrometry, an International Celestial Reference Frame (ICRF) is a realization of the International Celestial Reference System (ICRS) using reference celestial sources observed at radio wavelengths. In the context of the ICRS, a reference frame is the physical realization of a reference system, i.e., the reference frame is the set of reported coordinates of the reference sources, with the coordinates derived using the procedures spelled out by the ICRS.
The ICRF creates a quasi-inertial frame of reference centered at the barycenter of the Solar System, whose axes are defined by the measured positions of extragalactic sources (mainly quasars) observed using very long baseline interferometry. Although general relativity implies that there are no true inertial frames around gravitating bodies, the ICRF is important because it does not exhibit any measurable angular motion since the extragalactic sources used to define the ICRF are so far away. The ICRF is now the standard reference frame used to define the positions of the planets (including the Earth) and other astronomical objects.
The ICRF, now called ICRF1, was adopted by the International Astronomical Union (IAU) on 1 January 1998. ICRF1 had an angular noise floor of approximately 250 microarcseconds (µas) and a reference axis stability of approximately 20 µas; this was an order-of-magnitude improvement over the previous reference frame derived from Fifth Fundamental Catalog (FK5). The ICRF1 contains 212 defining sources and also contains positions of 396 additional non-defining sources for reference. The positions of these sources have been adjusted in later extensions to the catalogue. The ICRF1 agrees with the orientation of the Fifth Fundamental Catalog (FK5) "J2000.0" frame to within the (lower) precision of the latter.
An updated reference frame ICRF2 was created in 2009. The update was a joint collaboration of the International Astronomical Union, the International Earth Rotation and Reference Systems Service, and the International VLBI Service for Geodesy and Astrometry. ICRF2 is defined by the position of 295 compact radio sources (97 of which also define ICRF1). Alignment of ICRF2 with ICRF1-Ext2, the second extension of ICRF1, was made with 138 sources common to both reference frames. Including non-defining sources, it comprises 3414 sources measured using very-long-baseline interferometry. The ICRF2 has a noise floor of approximately 40 µas and an axis stability of approximately 10 µas. Maintenance of the ICRF2 will be accomplished by a set of 295 sources that have especially good positional stability and unambiguous spatial structure.
The data used to derive the reference frame come from approximately 30 years of VLBI observations, from 1979 to 2009. Radio observations in both the S-band (2.3GHz) and X-band (8.4GHz) were recorded simultaneously to allow correction for ionospheric effects. The observations resulted in about 6.5 million group-delay measurements among pairs of telescopes. The group delays were processed with software that takes into account atmospheric and geophysical processes. The positions of the reference sources were treated as unknowns to be solved for by minimizing the mean squared error across group-delay measurements. The solution was constrained to be consistent with the International Terrestrial Reference Frame (ITRF2008) and earth orientation parameters (EOP) systems.
ICRF3 is the third major revision of the ICRF, and was adopted by the IAU in August 2018, to become effective January 1, 2019. The modeling incorporates the effect of the galactocentric acceleration of the solar system, a new feature over and above ICRF2. ICRF3 contains positions for 4536 extragalactic sources. Of these 303 of which have been identified as defining sources. ICRF3 also increases the number of defining sources in the southern sky.
3C 286, also known by its position as 1328+307 (B1950 coordinates) is a quasar at redshift 0.8493 with a radial velocity of 164,137 km/s. It is part of the Third Cambridge Catalogue of Radio Sources.
3C 286 is one of four primary calibrators used by the Very Large Array (along with 3C 48, 3C 138, and 3C 147). Visibilities of all other sources are calibrated using observed visibilities of one of these four calibrators.Barycentric celestial reference system
The barycentric celestial reference system (BCRS) is a coordinate system used in astrometry to specify the location and motions of astronomical objects. It was created in 2000 by the International Astronomical Union (IAU) to be the global standard reference system for objects located outside the gravitational vicinity of Earth: planets, moons, and other Solar System bodies, stars and other objects in the Milky Way galaxy, and extra-galactic objects.
The geocentric celestial reference system (GCRS), also created by the IAU in 2000, is a similar standard coordinate system used to specify the location and motions of near-earth objects, such as satellites.These systems make it easier for scientists and engineers to compile, share, compare, and convert accurate measurements worldwide, by establishing standards both of measure and of methodology, and providing a consistent framework of operations. The focus of the BCRS is on astronomy: exploration of the Solar System and the universe. The BCRS is the system currently used for expressing positional data in astronomical references, such as the Hipparcos star catalog.
The focus of the GCRS is somewhat more on the navigation of Earth satellites and the geophysical applications they support. The proper functioning of the Global Positioning System (GPS) is directly dependent upon the accuracy of satellite measurements as supported by the GCRS.Catalogues of Fundamental Stars
The Catalogue of Fundamental Stars is a series of six astrometric catalogues of high precision positional data for a small selection of stars to define a celestial reference frame, which is a standard coordinate system for measuring positions of stars.Earth-centered inertial
Earth-centered inertial (ECI) coordinate frames have their origins at the center of mass of Earth and do not rotate with respect to the stars. ECI frames are called inertial, in contrast to the Earth-centered, Earth-fixed (ECEF) frames, which remain fixed with respect to Earth's surface in its rotation. It is convenient to represent the positions and velocities of terrestrial objects in ECEF coordinates or with latitude, longitude, and altitude. However, for objects in space, the equations of motion that describe orbital motion are simpler in a non-rotating frame such as ECI. The ECI frame is also useful for specifying the direction toward celestial objects.
The extent to which an ECI frame is actually inertial is limited by the non-uniformity of the surrounding gravitational field. For example, the Moon's gravitational influence on a high-Earth orbiting satellite is significantly different than its influence on Earth, so observers in an ECI frame would have to account for this acceleration difference in their laws of motion. The closer the observed object is to the ECI-origin, the less significant the effect of the gravitational disparity is.Epoch (astronomy)
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as the celestial coordinates or elliptical orbital elements of a celestial body, because these 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 main use of astronomical quantities specified in this way is to calculate other relevant parameters of motion, in order to predict future positions and velocities. The applied tools of the disciplines of celestial mechanics or its subfield orbital mechanics (for predicting orbital paths and positions for bodies in motion under the gravitational effects of other bodies) can be used to generate an ephemeris, a table of values giving the positions and velocities of astronomical objects in the sky at a given time or times.
Astronomical quantities can be specified in any of several ways, for example, as a polynomial function of the time-interval, with an epoch as a temporal point of origin (this is a common current way of using an epoch). Alternatively, the time-varying astronomical quantity can be expressed as a constant, equal to the measure that it had at the epoch, leaving its variation over time to be specified in some other way—for example, by a table, as was common during the 17th and 18th centuries.
The word epoch was often used in a different way in older astronomical literature, e.g. during the 18th century, in connection with astronomical tables. At that time, it was customary to denote as "epochs", not the standard date and time of origin for time-varying astronomical quantities, but rather the values at that date and time of those time-varying quantities themselves. In accordance with that alternative historical usage, an expression such as 'correcting the epochs' would refer to the adjustment, usually by a small amount, of the values of the tabulated astronomical quantities applicable to a fixed standard date and time of reference (and not, as might be expected from current usage, to a change from one date and time of reference to a different date and time).Equatorial coordinate system
The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention.The origin at the center of Earth means the coordinates are geocentric, that is, as seen from the centre of Earth as if it were transparent. The fundamental plane and the primary direction mean that the coordinate system, while aligned with Earth's equator and pole, does not rotate with the Earth, but remains relatively fixed against the background stars. A right-handed convention means that coordinates increase northward from and eastward around the fundamental plane.Equinox (celestial coordinates)
In astronomy, equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator. Although there are two intersections of the ecliptic with the celestial equator, by convention the equinox associated with the sun's ascending node is used as the origin of celestial coordinate systems and referred to simply as the equinox. In contrast to the common usage of spring and fall, or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.
The equinox moves because of perturbing forces, therefore in order to define a coordinate system it is necessary to specify the date for which the equinox is chosen. This date should not be confused with the epoch. Astronomical objects show real movements such as orbital and proper motions, and the epoch defines the date for which the position of an object applies. Therefore a complete specification of the coordinates for an astronomical objects requires both the date of the equinox and of the epoch.The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.Frame of reference
In physics, a frame of reference (or reference frame) consists of an abstract coordinate system and the set of physical reference points that uniquely fix (locate and orient) the coordinate system and standardize measurements.
In n dimensions, n + 1 reference points are sufficient to fully define a reference frame. Using rectangular (Cartesian) coordinates, a reference frame may be defined with a reference point at the origin and a reference point at one unit distance along each of the n coordinate axes.
In Einsteinian relativity, reference frames are used to specify the relationship between a moving observer and the phenomenon or phenomena under observation. In this context, the phrase often becomes "observational frame of reference" (or "observational reference frame"), which implies that the observer is at rest in the frame, although not necessarily located at its origin. A relativistic reference frame includes (or implies) the coordinate time, which does not correspond across different frames moving relatively to each other. The situation thus differs from Galilean relativity, where all possible coordinate times are essentially equivalent.Fundamental ephemeris
A fundamental ephemeris of the Solar System is a model of the objects of the system in space, with all of their positions and motions accurately represented. It is intended to be a high-precision primary reference for prediction and observation of those positions and motions, and which provides a basis for further refinement of the model. It is generally not intended to cover the entire life of the Solar System; usually a short-duration time span, perhaps a few centuries, is represented to high accuracy. Some long ephemerides cover several millennia to medium accuracy.
They are published by the Jet Propulsion Laboratory as Development Ephemeris. The latest releases include DE430 which covers planetary and lunar ephemeris from Dec 21, 1549 to Jan 25, 2650 with high precision and is intended for general use for modern time periods . DE431 was created to cover a longer time period Aug 15, -13200 to March 15, 17191 with slightly less precision for use with historic observations and far reaching forecasted positions. DE432 was released as a minor update to DE430 with improvements to the Pluto barycenter in support of the New Horizons mission.ICRF
ICRF can refer to:
Imperial Cancer Research Fund, a cancer research organization in the United Kingdom
International Celestial Reference Frame, a reference frame in astrometry and astronomy
The Israel Cancer Research Fund, a North American charitable organization that supports cancer research in Israel
Ion cyclotron resonance frequencyInternational Celestial Reference System
The International Celestial Reference System (ICRS) is the current standard celestial reference system adopted by the International Astronomical Union (IAU). Its origin is at the barycenter of the Solar System, with axes that are intended to be "fixed" with respect to space. ICRS coordinates are approximately the same as equatorial coordinates: the mean pole at J2000.0 in the ICRS lies at 17.3±0.2 mas in the direction 12 h and 5.1±0.2 mas in the direction 18 h. The mean equinox of J2000.0 is shifted from the ICRS right ascension origin by 78±10 mas (direct rotation around the polar axis).
The defining extragalactic reference frame of the ICRS is the International Celestial Reference Frame (currently ICRF2) based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured with the utmost accuracy by Very Long Baseline Interferometry (VLBI). The positions of most are known to 0.001 arcsecond or better, which is orders of magnitude more precise than the best optical measurements. At optical wavelengths, the ICRS is currently realized by the Hipparcos Celestial Reference Frame (HCRF), a subset of about 100,000 stars in the Hipparcos Catalogue. A more accurate optical realization of the ICRS (Gaia-CRF2), based on the observation by the Gaia spacecraft of almost 500,000 extragalactic objects believed to be quasars, is under preparation.International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.Jet Propulsion Laboratory Development Ephemeris
The name Jet Propulsion Laboratory Development Ephemeris (followed by a number), the abbreviation JPL DE(number), or just DE(number) designates one of a series of models of the Solar System produced at the Jet Propulsion Laboratory in Pasadena, California, primarily for purposes of spacecraft navigation and astronomy. The models consist of computer representations of positions, velocities and accelerations of major Solar System bodies, tabulated at equally spaced intervals of time, covering a specified span of years. Barycentric rectangular coordinates of the Sun, eight major planets and Pluto, and geocentric coordinates of the Moon are tabulated.
DE405, created in May 1997, include both nutations and librations, and is considered the fundamental planetary and lunar ephemerides of The Astronomical Almanac. It is very large at 62.4 Megabytes, so smaller, more targeted versions have been created based on DE405.Lunar month
In lunar calendars, a lunar month is the time between two successive syzygies (new moons or full moons). The precise definition varies, especially for the beginning of the month.
This article deals with the definitions of a 'month' that are mainly of significance in astronomy. For other definitions, including a description of a month in the calendars of different cultures around the world, see: month.Poles of astronomical bodies
The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere. Astronomical bodies include stars, planets, dwarf planets and small Solar System bodies such as comets and minor planets (i.e. asteroids), as well as natural satellites and minor-planet moons.Quantum reference frame
A quantum reference frame is a reference frame which is treated quantum theoretically. It, like any reference frame, is an abstract coordinate system which defines physical quantities, such as time, position, momentum, spin, and so on. Because it is treated within the formalism of quantum theory, it has some interesting properties which do not exist in a normal classical reference frame.Sidereal time
Sidereal time is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky. Briefly, sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars"
Viewed from the same location, a star seen at one position in the sky will be seen at the same position on another night at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the Sun. Just as the Sun and Moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earth's rotation about its polar axis, solar time following the Sun while sidereal time roughly follows the stars.
More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox and both celestial poles, and is usually expressed in hours, minutes, and seconds. Common time on a typical clock measures a slightly longer cycle, accounting not only for Earth's axial rotation but also for Earth's orbit around the Sun.
A sidereal day is approximately 23 hours, 56 minutes, 4.0905 SI seconds or also (24 hours - 4 minutes + 4 seconds). The March equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 26,000 years, so the misnamed sidereal day ("sidereal" is derived from the Latin sidus meaning "star") is 0.0084 seconds shorter than the stellar day, Earth's period of rotation relative to the fixed stars.
The slightly longer "true" sidereal period is measured as the Earth Rotation Angle (ERA), formerly the stellar angle. An increase of 360° in the ERA is a full rotation of the Earth.
Because Earth orbits the Sun once a year, the sidereal time at any given place and time will gain about four minutes against local civil time, every 24 hours, until, after a year has passed, one additional sidereal "day" has elapsed compared to the number of solar days that have gone by.Tropical year
A tropical year (also known as a solar year) is the time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. Because of the precession of the equinoxes, the seasonal cycle does not remain exactly synchronized with the position of the Earth in its orbit around the Sun. As a consequence, the tropical year is about 20 minutes shorter than the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars (the sidereal year).
Since antiquity, astronomers have progressively refined the definition of the tropical year. The entry for "year, tropical" in the Astronomical Almanac Online Glossary (2015) states:
the period of time for the ecliptic longitude of the Sun to increase 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of seasons, and its length is approximated in the long term by the civil (Gregorian) calendar. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds.
An equivalent, more descriptive, definition is "The natural basis for computing passing tropical years is the mean longitude of the Sun reckoned from the precessionally moving equinox (the dynamical equinox or equinox of date). Whenever the longitude reaches a multiple of 360 degrees the mean Sun crosses the vernal equinox and a new tropical year begins" (Borkowski 1991, p. 122).
The mean tropical year in 2000 was 365.24219 ephemeris days; each ephemeris day lasting 86,400 SI seconds. This is 365.24217 mean solar days (Richards 2013, p. 587).Universal Time
Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, England. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see § Versions). All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.