SLA_MOON - Approx Moon Pos/Vel   

ACTION:

Approximate geocentric position and velocity of the Moon (single precision).

CALL:

CALL sla_MOON (IY, ID, FD, PV)

GIVEN:

IY year

ID I day in year (1 = Jan 1st)

FD R fraction of day

RETURNED:

$$[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$$ PV

NOTES:

1.

The date and time is TDB (loosely ET) in a Julian calendar which has been aligned to the ordinary Gregorian calendar for the interval 1900 March 1 to 2100 February 28. The year and day can be obtained by calling sla_CALYD or sla_CLYD.

2.

The position is accurate to better than 0.5 arcminute in direction and 1000 km in distance. The velocity is accurate to better than $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.5$ per hour in direction and 4 metres per socond in distance. (RMS figures with respect to JPL DE200 for the interval 1960-2025 are $14\hspace{-0.05em}^{'\hspace{-0.1em}'}$ and

$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.2$ per hour in longitude, $9\hspace{-0.05em}^{'\hspace{-0.1em}'}$ and $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.2$

per hour in latitude, 350 km and 2 metres per second in distance.) Note that the distance accuracy is comparatively poor because this routine is principally intended for computing topocentric direction.

3.

This routine is only a partial implementation of the original Meeus algorithm (reference below), which offers 4 times the accuracy in direction and 20 times the accuracy in distance when fully implemented (as it is in sla_DMOON).

REFERENCE:

Meeus, l'Astronomie, June 1984, p348.