VSOP87 Theory Equations Summary Part 2
Spherical Coordinates (L,B,R)
VSOP87 series D is used to compute the heliocentric spherical LBR-coordinates or series C is used to compute the equivalent heliocentric rectangular XYZ-coordinates (see Part 1 ).
In the following VSOP87 spherical equations, we will encounter the following variables:
JD = Julian Day number corresponding to a given date and time (TT)
t = Julian millennia, corresponding to JD and reckoned from J2000.0
When we want to compute the position of a planet for a given date and time, we first compute the JD value and then compute the corresponding (t) value from it. This is the actual numerical value used as the time variable in the equations.
The A,B,C values in each series term in the equations are taken from the original FORTRAN program data tables. There may be thousands of terms in a sub-series, each with its own set of A,B,C values. These values are used by the code generator tool to generate the numerical terms in the output source code.
A = Amplitude
B = Phase (not to be confused with the spherical B-coordinate)
C = Frequency
Indices used in the summation equations:
n = Sub-series order from 0 to 5
j = Index of term within sub-series (from 1 to k)
k = Total number of terms in sub-series (can be from 1 to multiple thousands)
L, B, R = Spherical Heliocentric Ecliptical Coordinates
L = Heliocentric ecliptical longitude in radians
B = Heliocentric ecliptical latitude in radians (not to be confused with phase B)
R = Heliocentric radius vector (distance) in AU
These spherical coordinates are computed using the VSOP87 sub-series function modules generated by the Multi-Language VSOP87 Source Code Generator Tool.
Computing the Planetary Spherical Heliocentric Ecliptical LBR-Coordinates
In the following case of spherical coordinates, the heliocentric longitude and latitude (L, B) will be expressed in radians and the heliocentric distance (R) in astronomical units.
VSOP87 Series B = Spherical heliocentric ecliptical LBR-coordinates - Equinox J2000.0
VSOP87 Series D = Spherical heliocentric ecliptical LBR-coordinates - Equinox of Date
The VSOP87 B and D series correspond to the A and C series. The only difference is the type of coordinates used.
These spherical coordinate equations are analogous to the equations used to compute the rectangular coordinates in part 1, with the computational roles of (X, Y, Z) being replaced by (L, B, R) respectively.
For each order (n), the VSOP87 (Ln Bn Rn) sub-series summations may be expressed as
The six separate partial summations returned by the modules for the respective sub-series are in turn summed to obtain the final, complete ecliptical spherical LBR-coordinates of the planet.
Mathematically, the complete VSOP87 solutions in terms of spherical variables can be expressed by the following double summations.
Eq. 17 = Eq. 14
Eq. 18 = Eq. 15
Eq. 19 = Eq. 16
The double summation equations 17,18 and 19 can be written in terms of the following pseudocode with each of the terms representing functions (inner summations) generated by the VSOP87 Source Code Generator Tool. The heliocentric longitude and latitude (L, B) will be expressed in radians and the distance (R) in astronomical units.
Algorithm For Complete Heliocentric Spherical (L,B,R) Coordinates of Planet
Example For The Planet Venus
The following block of pseudocode equates to the double summations defined above. It can easily be translated into the source code of several programming languages. The heliocentric longitude and latitude (L, B) will be expressed in radians and the distance (R) in astronomical units.
Algorithm For Complete Heliocentric Spherical (L,B,R) Coordinates of Venus
© Jay Tanner - PHP Science Labs - 2011
FORTRAN is for dinos