The International Astronomical Union voted in August 2012 to change the definition of the astronomical unit to a plain old number: 149,597,870,700 meters. The measurement is based on the speed of light, a fixed distance that has nothing to do with the sun's mass. A meter is defined as the distance traveled by light in a vacuum in 1 / 299,792,458 of a second.
The International Astronomical Union voted in August 2012 to change the definition of the astronomical unit to a plain old number: 149,597,870,700 meters. The measurement is based on the speed of light, a fixed distance that has nothing to do with the sun's mass. A meter is defined as the distance traveled by light in a vacuum in 1 / 299,792,458 of a second.
Astronomical system of units From Wikipedia, the free encyclopedia Jump to: navigation, search This article's factual accuracy may be compromised due to out-of-date information. Please help improve the article by updating it. There may be additional information on the talk page. (February 2013)
The astronomical system of units, formally 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,[1] and has been significantly updated in 1994 and 2009 (see astronomical constant).
The system was developed because of the difficulties in measuring and expressing astronomical data in International System of Units (SI units). In particular, there is a huge quantity of very precise data relating to the positions of objects within the solar system which cannot conveniently be expressed or processed in SI units. Through a number of modifications, the astronomical system of units now explicitly recognizes the consequences of general relativity, which is a necessary addition to the International System of Units in order to accurately treat astronomical data.
The astronomical system of units is a tridimensional system, in that it defines units of length, mass and time. The associated astronomical constants also fix the different frames of reference that are needed to report observations.[2] The system is a conventional system, in that neither the unit of length nor the unit of mass are true physical constants, and there are at least three different measures of time.
The astronomical unit of time is the day, defined as 86400 seconds. 365.25 days make up one Julian year.[1] The symbol D is used in astronomy to refer to this unit. Astronomical unit of mass Main article: Solar mass
The astronomical unit of mass is the solar mass.[1] The symbol S is often used in astronomy to refer to this unit, although M☉ is also common. The solar mass (M⊙), 1.98892×1030 kg, is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, about 333,000 times the mass of the Earth or 1,048 times the mass of Jupiter.
In practice, the masses of celestial bodies appear in the dynamics of the solar system only through the products GM, where G is the constant of gravitation. In the past, GM of the sun could be determined experimentally with only limited accuracy. Its present accepted value is[3] GM☉=1.327 124 420 99 × 1020±1010 m3s−2 Non IAU units of mass
These are not SI or IAU units, but are used in astronomy. Jupiter mass Main article: Jupiter mass
Jupiter mass (MJ or MJUP), is the unit of mass equal to the total mass of the planet Jupiter, 1.8986×1027 kg. Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs. Earth mass Main article: Earth mass
Earth mass (M⊕) is the unit of mass equal to that of the Earth. 1 M⊕ = 5.9742 × 1024 kg. Earth mass is often used to describe masses of rocky terrestrial planets. One Earth mass is 0.00315 times a Jupiter mass. Equivalent Planetary masses Solar mass Solar mass 1 Jupiter masses 1,048 Earth masses 332,950 Astronomical unit of length Main article: Astronomical unit
The astronomical unit of length is that length for which the Gaussian gravitational constant (k) takes the value 0.017 202 098 95 when the units of measurement are the astronomical units of length, mass and time.[1] The dimensions of k2 are those of the constant of gravitation (G), i.e., L3M−1T−2. The term “unit distance” is also used for the length A while, in general usage, it is usually referred to simply as the “astronomical unit”, symbol AU, au or ua.
An equivalent definition of the astronomical unit is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.017 202 098 95 radians per day.[4] It is approximately equal to the mean Earth–Sun distance.
The speed of light in IAU is the defined value c0 = 299 792 458 m/s of the SI units. In terms of this speed, the astronomical unit of length has the presently accepted value:[3] 1 ua = c0τA = 1.495 978 707 00 × 1011±3 m., where τA is the transit time of light across the astronomical unit. The astronomical unit of length is determined by the condition that the measured data in the ephemeris match observations, and that in turn decides the transit time τA. Other units for astronomical distances Astronomical Range Typical Units Distances to satellites kilometres Distances to near-Earth objects lunar distance Planetary distances astronomical units, gigametres Distances to nearby stars parsecs, light-years Distances at the galactic scale kiloparsecs Distances to nearby galaxies megaparsecs
The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms of redshift. The reasons for this are that converting redshift to distance requires knowledge of the Hubble constant which was not accurately measured until the early 21st century, and that at cosmological distances, the curvature of space-time allows one to come up with multiple definitions for distance. For example, the distance as defined by the amount of time it takes for a light beam to travel to you is different from the distance as defined by the apparent size of an object. See also
Measuring the Universe The IAU and astronomical units
Scientists use units all the time. The concept of an internationally standardised system of units is one of the most fundamental in experimental science. Everyone uses familiar units such as kilograms, kilometres and seconds and they are indispensable in daily life. Scientists may need more exotic units such as measures of current, frequency and other scientific quantities, but the principle is the same, without an agreed scheme of measurement, scientists could not share results and there could be disastrous and costly mistakes.
The International Astronomical Union (IAU) is responsible for maintaining and approving a special set of units in astronomy, formally defined in 1976. One of the most important of these is the astronomical unit. It is a unit of length approximating the Sun-Earth distance (of about 150 million kilometres) which is of convenient use in astronomy. According to its definition adopted by the XXVIIIth General Asssembly of the IAU (IAU 2012 Resolution B2), the astronomical unit is a conventional unit of length equal to 149 597 870 700 m exactly. This definition is valid irrespective of the used time scale. The unique symbol for the astronomical unit is au. The IAU also defines other astronomical units: the astronomical unit of time is 1 day (d) of 86,400 SI seconds (s) (SI is the International System of Units) and the astronomical unit of mass is equal to the mass of the Sun, 1.9891×1030 kg.
Beyond the Solar System the distances in astronomy are so great that using the au becomes too cumbersome. The IAU recognises several other distance units to be used on different scales. For studies of the structure of the Milky Way, our local galaxy, the parsec (pc) is the usual choice. This is equivalent to about 30.857×1012 km, or about 206,000 aus, and is itself defined in terms of the au – as the distance at which one Astronomical Unit subtends an angle of one arcsecond. Alternatively the light-year (ly) is sometimes used in scientific papers as a distance unit, although its use is mostly confined to popular publications and similar media. The light-year is roughly equivalent to 0.3 parsecs, and is equal to the distance traveled by light in one Julian year in a vacuum, according to the IAU. To think of it in easily accessible terms, the light-year is 9,460,730,472,580.8 km or 63,241 au. While smaller than the parsec, it is still an incredibly large distance.
Defining a unit is often more complex than first appears. For instance, to define a light-year it is necessary to understand exactly what a year is. When referring to a year in the precisely defined astronomical sense, it should be written with the indefinite article “a” as “a year”. Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified. The IAU also recognises a Julian century of 36,525 days in the fundamental formulas for precession (more info). Other measurements of time such as sidereal, solar and universal time are not suitable for measuring precise intervals of time, since the rate of rotation of Earth, on which they ultimately depend, is variable with respect to the second.
How do you know where you are now? How do we know where we are in space? How does the International Space Station or the latest space probe keep track of its location in the Universe? The best answer would be – with great difficulty! Ever since the earliest philosophers first considered our place in the Universe, it has always been a natural first step to define our position in the overall order and structure of the cosmos
he International Astronomical Union (IAU) is responsible for defining a Universal Frame of Reference. This work touches on many aspects of our daily lives, so much so, that without a standard reference frame many of our modern gadgets would, at best, be incompatible with each other and, at worst, inaccurate or not fit for purpose.
Many people nowadays use the Global Positioning System (GPS) in their everyday lives. GPS requires several aspects of the Universal Frame of Reference to be defined. For example, the systems that controlled the launches of the GPS satellites had to have an excellent understanding of the positions of the stars, orbital elements and the definitions of various units in order to position the satellite in the correct orbit needed to complete the “constellation” of satellites. The IAU Commission 8 (Astrometry) and 4 (Ephemerides) provide valuable information about “physical position” and “position in time” respectively, mainly to astronomers and space scientists. Astronomers also need to have accurate definitions for concepts such as the celestial equator — the imaginary line on the sky above the equator on Earth — and the ecliptic — the path of the Sun across the sky — as some earlier reference frames were based on these.
However, much more than basic positional input needs to be considered to establish a Universal Frame of Reference. Scientists have to agree on definitions for certain key reference units or parameters. These topics are covered by other IAU commissions, including Commission 31 (Time ). In the context of the Universal Frame of Reference the work of Commission 31 is also closely linked with Commission 19 on the Rotation of the Earth, as knowledge of the Earth's ever-changing orientation in space is necessary to link terrestrial and celestial frames. The Earth is not fixed, nor in moving in a way that is simply described, so much work goes into measuring and defining this complex movement. Phenomena such as precession, the slow, roughly 25 000 year cycle, of movement of the direction of the Earth’s axis, and nutation, the continual “nodding” of the Earth’s axis, due mainly to tidal forces from the Sun and Moon, all have to be taken into account when defining a Universal Reference Frame.
In 1997 and 1998 the IAU, in collaboration with the International Earth Rotation and Reference Systems Service (IERS) and the International Very Long Baseline Interferometry Service (IVS)International Celestial ReferenceFrame(ICRF). The ICRF uses the relative positions of 212 extragalactic radio sources to establish an origin for the system at the centre of mass of the Solar System, and coordinate axes that are aligned with the conventional axes of the celestial equator and equinox ( the point at which the Sun crosses the equatorial plane moving from south to north ) of the epoch J2000.0 (1200 hours Terrestrial Time on 1 January 2000), but are obtained in a way that is independent of the dynamics of the Earth’s rotation . On 20 August1997, a t the 23rd IAU General Assembly in Kyoto, Japan, the IAU adopted the ICRF, and the celestial equator and the ecliptic were no longer central in establishing a celestial or Universal Reference Frame.
In recent years more precise measurements have allowed the ICRF to be refined, allowing for a much more accurate system. At the IAU General Assembly in 2003 the IAU Working Group (WG) on the ICRF was dissolved and its work is now covered by the main Reference Frame Working Groups: Commission 8, Densification of the Optical Reference Frame and Division 1, Second Realization of International Celestial Reference Frame. On 24 August 2006, at the IAU General Assembly in Prague, new resolutions were adopted that aim to improve our definition of the Universal Frame of Reference.
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The International Astronomical Union voted in August 2012 to change the definition of the astronomical unit to a plain old number: 149,597,870,700 meters. The measurement is based on the speed of light, a fixed distance that has nothing to do with the sun's mass. A meter is defined as the distance traveled by light in a vacuum in 1 / 299,792,458 of a second.
The International Astronomical Union voted in August 2012 to change the definition of the astronomical unit to a plain old number: 149,597,870,700 meters. The measurement is based on the speed of light, a fixed distance that has nothing to do with the sun's mass. A meter is defined as the distance traveled by light in a vacuum in 1 / 299,792,458 of a second.
Astronomical system of units
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article's factual accuracy may be compromised due to out-of-date information. Please help improve the article by updating it. There may be additional information on the talk page. (February 2013)
The astronomical system of units, formally 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,[1] and has been significantly updated in 1994 and 2009 (see astronomical constant).
The system was developed because of the difficulties in measuring and expressing astronomical data in International System of Units (SI units). In particular, there is a huge quantity of very precise data relating to the positions of objects within the solar system which cannot conveniently be expressed or processed in SI units. Through a number of modifications, the astronomical system of units now explicitly recognizes the consequences of general relativity, which is a necessary addition to the International System of Units in order to accurately treat astronomical data.
The astronomical system of units is a tridimensional system, in that it defines units of length, mass and time. The associated astronomical constants also fix the different frames of reference that are needed to report observations.[2] The system is a conventional system, in that neither the unit of length nor the unit of mass are true physical constants, and there are at least three different measures of time.
Astronomical unit of time
Main article: Day
The astronomical unit of time is the day, defined as 86400 seconds. 365.25 days make up one Julian year.[1] The symbol D is used in astronomy to refer to this unit.
Astronomical unit of mass
Main article: Solar mass
The astronomical unit of mass is the solar mass.[1] The symbol S is often used in astronomy to refer to this unit, although M☉ is also common. The solar mass (M⊙), 1.98892×1030 kg, is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, about 333,000 times the mass of the Earth or 1,048 times the mass of Jupiter.
In practice, the masses of celestial bodies appear in the dynamics of the solar system only through the products GM, where G is the constant of gravitation. In the past, GM of the sun could be determined experimentally with only limited accuracy. Its present accepted value is[3] GM☉=1.327 124 420 99 × 1020±1010 m3s−2
Non IAU units of mass
These are not SI or IAU units, but are used in astronomy.
Jupiter mass
Main article: Jupiter mass
Jupiter mass (MJ or MJUP), is the unit of mass equal to the total mass of the planet Jupiter, 1.8986×1027 kg. Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs.
Earth mass
Main article: Earth mass
Earth mass (M⊕) is the unit of mass equal to that of the Earth. 1 M⊕ = 5.9742 × 1024 kg. Earth mass is often used to describe masses of rocky terrestrial planets. One Earth mass is 0.00315 times a Jupiter mass.
Equivalent Planetary masses Solar mass
Solar mass 1
Jupiter masses 1,048
Earth masses 332,950
Astronomical unit of length
Main article: Astronomical unit
The astronomical unit of length is that length for which the Gaussian gravitational constant (k) takes the value 0.017 202 098 95 when the units of measurement are the astronomical units of length, mass and time.[1] The dimensions of k2 are those of the constant of gravitation (G), i.e., L3M−1T−2. The term “unit distance” is also used for the length A while, in general usage, it is usually referred to simply as the “astronomical unit”, symbol AU, au or ua.
An equivalent definition of the astronomical unit is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.017 202 098 95 radians per day.[4] It is approximately equal to the mean Earth–Sun distance.
The speed of light in IAU is the defined value c0 = 299 792 458 m/s of the SI units. In terms of this speed, the astronomical unit of length has the presently accepted value:[3] 1 ua = c0τA = 1.495 978 707 00 × 1011±3 m., where τA is the transit time of light across the astronomical unit. The astronomical unit of length is determined by the condition that the measured data in the ephemeris match observations, and that in turn decides the transit time τA.
Other units for astronomical distances
Astronomical Range Typical Units
Distances to satellites kilometres
Distances to near-Earth objects lunar distance
Planetary distances astronomical units, gigametres
Distances to nearby stars parsecs, light-years
Distances at the galactic scale kiloparsecs
Distances to nearby galaxies megaparsecs
The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms of redshift. The reasons for this are that converting redshift to distance requires knowledge of the Hubble constant which was not accurately measured until the early 21st century, and that at cosmological distances, the curvature of space-time allows one to come up with multiple definitions for distance. For example, the distance as defined by the amount of time it takes for a light beam to travel to you is different from the distance as defined by the apparent size of an object.
See also
Measuring the Universe
The IAU and astronomical units
Scientists use units all the time. The concept of an internationally standardised system of units is one of the most fundamental in experimental science. Everyone uses familiar units such as kilograms, kilometres and seconds and they are indispensable in daily life. Scientists may need more exotic units such as measures of current, frequency and other scientific quantities, but the principle is the same, without an agreed scheme of measurement, scientists could not share results and there could be disastrous and costly mistakes.
The International Astronomical Union (IAU) is responsible for maintaining and approving a special set of units in astronomy, formally defined in 1976. One of the most important of these is the astronomical unit. It is a unit of length approximating the Sun-Earth distance (of about 150 million kilometres) which is of convenient use in astronomy. According to its definition adopted by the XXVIIIth General Asssembly of the IAU (IAU 2012 Resolution B2), the astronomical unit is a conventional unit of length equal to 149 597 870 700 m exactly. This definition is valid irrespective of the used time scale. The unique symbol for the astronomical unit is au. The IAU also defines other astronomical units: the astronomical unit of time is 1 day (d) of 86,400 SI seconds (s) (SI is the International System of Units) and the astronomical unit of mass is equal to the mass of the Sun, 1.9891×1030 kg.
Beyond the Solar System the distances in astronomy are so great that using the au becomes too cumbersome. The IAU recognises several other distance units to be used on different scales. For studies of the structure of the Milky Way, our local galaxy, the parsec (pc) is the usual choice. This is equivalent to about 30.857×1012 km, or about 206,000 aus, and is itself defined in terms of the au – as the distance at which one Astronomical Unit subtends an angle of one arcsecond. Alternatively the light-year (ly) is sometimes used in scientific papers as a distance unit, although its use is mostly confined to popular publications and similar media. The light-year is roughly equivalent to 0.3 parsecs, and is equal to the distance traveled by light in one Julian year in a vacuum, according to the IAU. To think of it in easily accessible terms, the light-year is 9,460,730,472,580.8 km or 63,241 au. While smaller than the parsec, it is still an incredibly large distance.
Defining a unit is often more complex than first appears. For instance, to define a light-year it is necessary to understand exactly what a year is. When referring to a year in the precisely defined astronomical sense, it should be written with the indefinite article “a” as “a year”. Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified. The IAU also recognises a Julian century of 36,525 days in the fundamental formulas for precession (more info). Other measurements of time such as sidereal, solar and universal time are not suitable for measuring precise intervals of time, since the rate of rotation of Earth, on which they ultimately depend, is variable with respect to the second.
what is the latest value of the astronomical unit by the standards maintained by the USNO? to cross chck
http://www.iau.org/public/place_in_cosmos/
defining our place in the cosmos
How do you know where you are now? How do we know where we are in space? How does the International Space Station or the latest space probe keep track of its location in the Universe? The best answer would be – with great difficulty! Ever since the earliest philosophers first considered our place in the Universe, it has always been a natural first step to define our position in the overall order and structure of the cosmos
he International Astronomical Union (IAU) is responsible for defining a Universal Frame of Reference. This work touches on many aspects of our daily lives, so much so, that without a standard reference frame many of our modern gadgets would, at best, be incompatible with each other and, at worst, inaccurate or not fit for purpose.
Many people nowadays use the Global Positioning System (GPS) in their everyday lives. GPS requires several aspects of the Universal Frame of Reference to be defined. For example, the systems that controlled the launches of the GPS satellites had to have an excellent understanding of the positions of the stars, orbital elements and the definitions of various units in order to position the satellite in the correct orbit needed to complete the “constellation” of satellites. The IAU Commission 8 (Astrometry) and 4 (Ephemerides) provide valuable information about “physical position” and “position in time” respectively, mainly to astronomers and space scientists. Astronomers also need to have accurate definitions for concepts such as the celestial equator — the imaginary line on the sky above the equator on Earth — and the ecliptic — the path of the Sun across the sky — as some earlier reference frames were based on these.
However, much more than basic positional input needs to be considered to establish a Universal Frame of Reference. Scientists have to agree on definitions for certain key reference units or parameters. These topics are covered by other IAU commissions, including Commission 31 (Time ). In the context of the Universal Frame of Reference the work of Commission 31 is also closely linked with Commission 19 on the Rotation of the Earth, as knowledge of the Earth's ever-changing orientation in space is necessary to link terrestrial and celestial frames. The Earth is not fixed, nor in moving in a way that is simply described, so much work goes into measuring and defining this complex movement. Phenomena such as precession, the slow, roughly 25 000 year cycle, of movement of the direction of the Earth’s axis, and nutation, the continual “nodding” of the Earth’s axis, due mainly to tidal forces from the Sun and Moon, all have to be taken into account when defining a Universal Reference Frame.
In 1997 and 1998 the IAU, in collaboration with the International Earth Rotation and Reference Systems Service (IERS) and the International Very Long Baseline Interferometry Service (IVS)International Celestial ReferenceFrame(ICRF). The ICRF uses the relative positions of 212 extragalactic radio sources to establish an origin for the system at the centre of mass of the Solar System, and coordinate axes that are aligned with the conventional axes of the celestial equator and equinox ( the point at which the Sun crosses the equatorial plane moving from south to north ) of the epoch J2000.0 (1200 hours Terrestrial Time on 1 January 2000), but are obtained in a way that is independent of the dynamics of the Earth’s rotation . On 20 August1997, a t the 23rd IAU General Assembly in Kyoto, Japan, the IAU adopted the ICRF, and the celestial equator and the ecliptic were no longer central in establishing a celestial or Universal Reference Frame.
In recent years more precise measurements have allowed the ICRF to be refined, allowing for a much more accurate system. At the IAU General Assembly in 2003 the IAU Working Group (WG) on the ICRF was dissolved and its work is now covered by the main Reference Frame Working Groups: Commission 8, Densification of the Optical Reference Frame and Division 1, Second Realization of International Celestial Reference Frame. On 24 August 2006, at the IAU General Assembly in Prague, new resolutions were adopted that aim to improve our definition of the Universal Frame of Reference.
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