SOFAEo06a Method

public double Eo06a(
	double date1,
	double date2
)

Public Function Eo06a ( 
	date1 As Double,
	date2 As Double
) As Double

public:
virtual double Eo06a(
	double date1, 
	double date2
) sealed

Parameters

date1  Double

TT as a 2-part Julian Date (Note 1)

date2  Double

TT as a 2-part Julian Date (Note 1)

Return Value

1. The TT date date1+date2 is a Julian Date, apportioned in any convenient way between the two arguments. For example, JD(TT)=2450123.7 could be expressed in any of these ways, among others:

   Date 1	Date 2	Method
   2450123.7	0.0	JD method
   2451545.0	-1421.3	J2000 method
   2400000.5	50123.2	MJD method
   2450123.5	0.2	Date and time method

   The JD method is the most natural and convenient to use in cases where the loss of several decimal digits of resolution is acceptable. The J2000 method is best matched to the way the argument is handled internally and will deliver the optimum resolution. The MJD method and the date and time methods are both good compromises between resolution and convenience. For most applications of this function the choice will not be at all critical.

2. The equation of the origins is the distance between the true equinox and the celestial intermediate origin and, equivalently, the difference between Earth rotation angle and Greenwich apparent sidereal time (ERA-GST). It comprises the precession (since J2000.0) in right ascension plus the equation of the equinoxes (including the small correction terms).

Reference