Gaussian gravitational constant

The Gaussian gravitational constant (symbol ) is a parameter used in the orbital mechanics of the Solar System.
It relates the orbital period to the orbit's semi-major axis and the mass of the orbiting body in Solar masses.
The value of historically expresses the mean angular velocity of the system of Earth+Moon and the Sun considered as a two body problem,
with a value of about 0.986 degrees per day, or about 0.0172 radians per day.
As a consequence of law of gravitation and Kepler's third law, 
 is directly proportional to the square root of the standard gravitational parameter of the Sun,
and its value in radians per day follows by setting Earth's semi-major axis (the astronomical unit, au) to unity, :(rad/d) ()0.5·au−1.5.
A value of rad/day was determined by Carl Friedrich Gauss in his 1809 work "Theoria Motus Corporum Coelestium in Sectionibus Conicis Solem Ambientum" ("Theory of the Motion of the Heavenly Bodies Moving about the Sun in Conic Sections").
Gauss' value was introduced as a fixed, defined value by the IAU (adopted in 1938, formally defined in 1964), 
which detached it from its immediate representation of the (observable) mean angular velocity of the Sun–Earth system. Instead, the astronomical unit now became a measurable quantity slightly different from unity.
This was useful in 20th-century celestial mechanics to prevent the constant adaptation of orbital parameters to updated measured values, but it came at the expense of intuitiveness, as the astronomical unit, ostensibly a unit of length, was now dependent on the measurement of the strength of the gravitational force.
The IAU abandoned the defined value of in 2012 in favour of a defined value of the astronomical unit of exactly, while the strength of the gravitational force is now to be expressed in the separate standard gravitational parameter , measured in SI units of m3⋅s−2.
Discussion.
Gauss' constant is derived from the application of Kepler's third law to the 
system of Earth+Moon and the Sun considered as a two body problem,
relating the period of revolution () to the major semi-axis of the orbit () and the total mass of the orbiting bodies ().
Its numerical value was obtained by setting the major semi-axis and the mass of the Sun to unity and measuring the period in mean solar days:
The value represents the mean angular motion of the Earth-Sun system, in radians per day, equivalent to a value just below one degree (the division of the circle into 360 degrees in Babylonian astronomy was likely intended as approximating the number of days in a solar year). 
The correction due to the division by the square root of reflects the fact that the Earth–Moon system is not orbiting the Sun itself, but the center of mass of the system.
Isaac Newton himself determined a value of this constant which agreed with Gauss' value to six significant digits.
Gauss (1809) gave the value with nine significant digits, as 3548.18761 arc seconds.
Since all involved parameters, the orbital period, the Earth-to-Sun mass ratio, the semi-major axis and the length of the mean solar day, are subject to increasingly refined measurement, the precise value of the constant would have to be revised over time. 
But since the constant is involved in determining the orbital parameters of all other bodies in the Solar System, it was found to be more convenient to set it to a fixed value, by definition, implying that the value of would deviate from unity.
The fixed value of 0.01720209895 [rad] was taken to be the one set by Gauss (converted from degrees to radian), so that 
 42:(2 2 ) ≈ 1.
Gauss' 1809 value of the constant was thus used as an authoritative reference value for the orbital mechanics of the Solar System for two centuries.
From its introduction until 1938 it was considered a measured quantity, and from 1938 until 2012 it was used as a defined quantity, with measurement uncertainty delegated to the value of the astronomical unit. 
The defined value of was abandoned by the IAU in 2012, and the use of was deprecated, to be replaced by a fixed value of the astronomical unit, and the (measured) quantity of the standard gravitational parameter .
Role as a defining constant of Solar System dynamics.
Gauss himself stated the constant in arc seconds, with nine significant digits, as .
In the late 19th century, this value was adopted, and converted to radian, by Simon Newcomb, as . 
and the constant appears in this form in his "Tables of the Sun", published in 1898.
Newcomb's work was widely accepted as the best then available and his values of the constants were incorporated into a great quantity of astronomical research. Because of this, it became difficult to separate the constants from the research; new values of the constants would, at least partially, invalidate a large body of work. Hence, after the formation of the International Astronomical Union in 1919 certain constants came to be gradually accepted as "fundamental": defining constants from which all others were derived. In 1938, the VIth General Assembly of the IAU declared,
However, no further effort toward establishing a set of constants was forthcoming until 1950. An IAU symposium on the system of constants was held in Paris in 1963, partially in response to recent developments in space exploration. The attendees finally decided at that time to establish a consistent set of constants. Resolution 1 stated that
Resolution 4 recommended
Included in the list of fundamental constants was
These resolutions were taken up by a working group of the IAU, who in their report recommended two defining constants, one of which was
For the first time, the Gaussian constant's role in the scale of the Solar System was officially recognized. The working group's recommendations were accepted at the XIIth General Assembly of the IAU at Hamburg, Germany in 1964.
Definition of the astronomical unit.
Gauss intended his constant to be defined using a mean distance of Earth from the Sun of 1 astronomical unit precisely. With the acceptance of the 1964 resolutions, the IAU, in effect, did the opposite: defined the constant as fundamental, and the astronomical unit as derived, the other variables in the definition being already fixed: mass (of the Sun), and time (the day of seconds). This transferred the uncertainty from the gravitational constant to uncertainty in the semi-major axis of the Earth-Sun system, which was no longer exactly one au (the au being defined as depending on the value of the gravitational constant).
The astronomical unit thus became a measured quantity rather than a defined, fixed one.
In 1976, the IAU reconfirmed the Gaussian constant's status at the XVIth General Assembly in Grenoble, declaring it to be a defining constant, and that
From this definition, the mean distance of Earth from the Sun works out to , but with perturbations by the other planets, which do not average to zero over time, the average distance is .
Abandonment.
In 2012, the IAU, as part of a new, self-consistent set of units and numerical standards for use in modern dynamical astronomy, redefined the astronomical unit as 
and hence abandoned the Gaussian constant as an indirect definition of scale in the Solar System, recommending
The value of "k" based on the defined value for the astronomical unit would now be subject to the measurement uncertainty of the standard gravitational parameter,
formula_1
Units and dimensions.
 is given as a unit-less fraction of the order of 1.7%, but it can be considered equivalent to the square root of the gravitational constant, in which case it has the units of au⋅d−1⋅−, where
Therefore, the dimensions of are
In spite of this is known to much greater accuracy than (or the square root of ).
The absolute value of is known to an accuracy of about 10−4, but the product (the gravitational parameter of the Sun) is known to an accuracy better than 10−10.
Derivation.
Gauss' original.
Gauss begins his "Theoria Motus" by presenting without proof several laws concerning the motion of bodies about the Sun. Later in the text, he mentions that Pierre-Simon Laplace treats these in detail in his "Mécanique Céleste". Gauss' final two laws are as follows:
He next defines:
and declares that 
is "constant for all heavenly bodies". He continues, "it is of no importance which body we use for determining this number," and hence uses Earth, defining
He states that the area swept out by Earth in its orbit "will evidently be" , and uses this to simplify his constant to
Here, he names the constant and plugging in some measured values, = days, = solar masses, achieves the result = .
In modern terms.
Gauss is notorious for leaving out details, and this derivation is no exception. It is here repeated in modern terms, filling out some of the details.
Define without proof
where
Next define
where
Note that every variable in the above equations is a constant for two-body motion. Combining these two definitions,
which is what Gauss was describing with the last of his laws. Taking the square root,
and solving for ,
At this point, define . Let be the entire area swept out by the body as it orbits, hence , the area of an ellipse, where is the semi-major axis and is the semi-minor axis. Let , the time for the body to complete one orbit. Thus,
Here, Gauss decides to use Earth to solve for . From the geometry of an ellipse, .   By setting Earth's semi-major axis, , reduces to and . Substituting, the area of the ellipse "is evidently" , rather than . Putting this into the numerator of the equation for and reducing,
Note that Gauss, by normalizing the size of the orbit, has eliminated it completely from the equation. Normalizing further, set the mass of the Sun to 1,
where now is in solar masses. What is left are two quantities: , the period of Earth's orbit or the sidereal year, a quantity known precisely by measurement over centuries, and , the mass of the Earth–Moon system. 
Again plugging in the measured values as they were known in Gauss's time, = days, = solar masses, yielding the result = .
Gauss' constant and Kepler's third law.
The Gaussian constant is closely related to Kepler's third law of planetary motion, and one is easily derived from the other. Beginning with the full definition of Gauss' constant,
where
From the geometry of an ellipse, the semi-latus rectum, can be expressed in terms of and thus: .   Therefore,
Substituting and reducing, Gauss' constant becomes
From orbital mechanics, is just , the mean motion of the body in its orbit. Hence,
which is the definition of Kepler's third law. In this form, it is often seen with , the Newtonian gravitational constant in place of .
Setting , , , and in radians per day results in , also in units of radians per day, about which see the relevant section of the mean motion article.
Other definitions.
The value of Gauss' constant, exactly as he derived it, had been used since Gauss' time because it was held to be a fundamental constant, as described above. The solar mass, mean solar day and sidereal year with which Gauss defined his constant are all slowly changing in value. If modern values were inserted into the defining equation, a value of would result. 
It is also possible to set the gravitational constant, the mass of the Sun, and the astronomical unit to 1. This defines a unit of time with which the period of the resulting orbit is equal to . These are often called "canonical units".