Astronomical units of length
Encyclopedia
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, and has been slightly updated since then.
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 constant
s also fix the different frames of reference
that are needed to report observations. The system is a conventional system, in that neither the unit of length nor the unit of mass are true physical constant
s, and there are at least three different measures of time.
, defined as 86400 second
s. 365.25 days make up one Julian year
. The symbol D is used in astronomy to refer to this unit.
. The symbol S is often used in astronomy to refer to this unit, although M☉ is also common.
The solar mass (M⊙), , is a standard way to express mass
in astronomy
, used to describe the masses of other star
s and galaxies
. It is equal to the mass of the Sun
, about 332,950 times the mass of the Earth
or 1,048 times the mass of Jupiter
.
equal to the total mass of the planet Jupiter
, . Jupiter mass is used to describe masses of the gas giant
s, such as the outer planets and extrasolar planet
s. It is also used in describing brown dwarfs.
equal to that of the Earth
. 1 M⊕ = 5.9742 × 1024 kg
. Earth mass is often used to describe masses of rocky terrestrial planet
s. One Earth mass is 0.00315 times a Jupiter mass.
(k) takes the value when the units of measurement are the astronomical units of length, mass and time. 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 radians per day. It is approximately equal to the mean Earth–Sun distance.
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.
Systems of measurement
A system of measurement is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce...
developed for use in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
. It was adopted by the 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) in 1976, and has been slightly updated since then.
The system was developed because of the difficulties in measuring and expressing astronomical data in 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 units). In particular, there is a huge quantity of very precise data relating to the positions of objects within the 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...
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
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, which is a necessary addition to 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...
in order to accurately treat astronomical data.
The astronomical system of units is a tridimensional
Dimensional analysis
In physics and all science, dimensional analysis is a tool to find or check relations among physical quantities by using their dimensions. The dimension of a physical quantity is the combination of the basic physical dimensions which describe it; for example, speed has the dimension length per...
system, in that it defines units of length
Length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...
, mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
and 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....
. The associated astronomical constant
Astronomical constant
An astronomical constant is a physical constant used in astronomy. A formal set of constants, along with recommended values, was defined by the International Astronomical Union in 1976, and a new set of recommended values was produced in 1994...
s also fix the different frames of reference
Frame of reference
A frame of reference in physics, may refer to a coordinate system or set of axes within which to measure the position, orientation, and other properties of objects in it, or it may refer to an observational reference frame tied to the state of motion of an observer.It may also refer to both an...
that are needed to report observations. The system is a conventional system, in that neither the unit of length nor the unit of mass are true physical constant
Physical constant
A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.There are many physical constants in...
s, and there are at least three different measures of time.
Astronomical unit of time
The astronomical unit of time is the dayDay
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...
, defined as 86400 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. 365.25 days make up one Julian year
Julian year
A Julian year can refer to:* The Julian year is a time interval of exactly 365.25 Earth days, used in astronomy.* The Julian year is a year in the Julian calendar which has started on different days, at different times, in different countries and is equal to either 365 or 366 days, or 365.25 days...
. The symbol D is used in astronomy to refer to this unit.
Astronomical unit of mass
The astronomical unit of mass is the solar massSolar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
. The symbol S is often used in astronomy to refer to this unit, although M☉ is also common.
The solar mass (M⊙), , is a standard way to express mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, used to describe the masses of other star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s and galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
. It is equal to the mass of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, about 332,950 times the mass of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
or 1,048 times the mass of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
.
Jupiter mass
Jupiter mass (MJ or MJUP), is the unit of massMass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
equal to the total mass of the planet Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, . Jupiter mass is used to describe masses of the gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, such as the outer planets and extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
s. It is also used in describing brown dwarfs.
Earth mass
Earth mass (M⊕) is the unit of massMass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
equal to that of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. 1 M⊕ = 5.9742 × 1024 kg
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
. Earth mass is often used to describe masses of rocky terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
s. One Earth mass is 0.00315 times a Jupiter mass.
| Solar mass | |
|---|---|
| Solar mass | 1 |
| Jupiter masses | 1,048 |
| Earth masses | 332,950 |
Astronomical unit of length
The astronomical unit of length is that length for which the Gaussian gravitational constantGaussian gravitational constant
The Gaussian gravitational constant is an astronomical constant first proposed by German polymath Carl Friedrich Gauss in his 1809 work Theoria motus corporum coelestium in sectionibus conicis solem ambientum , although he had already used the concept to great success in predicting the...
(k) takes the value when the units of measurement are the astronomical units of length, mass and time. 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 radians per day. It is approximately equal to the mean Earth–Sun distance.
Other units for astronomical distances
| Astronomical Range | Typical Units |
|---|---|
| Distances to satellites | kilometre Kilometre The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second... s |
| Distances to near-Earth object Near-Earth object A near-Earth object is a Solar System object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU. They include a few thousand near-Earth asteroids , near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to... s |
lunar distance Lunar distance (astronomy) In astronomy, a lunar distance is a measurement of the distance from the Earth to the Moon. The average distance from Earth to the Moon is 384,400 kilometers... |
| Planet Planet A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,... ary distances |
astronomical unit Astronomical unit An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance.... s, gigametre Gigametre A gigametre is a unit of length in the metric system, equal to one billion meters, the SI base unit of length, hence to 1,000,000 km or approximately 621,370 miles.... s |
| Distances to nearby star Star A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth... s |
parsec Parsec The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres .... s, light-year Light-year A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres... s |
| Distances at the galactic scale | kiloparsecs |
| Distances to nearby galaxies Galaxy A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a... |
megaparsecs |
The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms of redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
. 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.