xyz {astrolibR} | R Documentation |
Calculate geocentric X,Y, and Z and velocity coordinates of the Sun
xyz(date, equinox)
date |
reduced julian date (=JD - 2400000), scalar or vector |
equinox |
equinox of output (default = 1950) |
Calculates geocentric X,Y, and Z vectors and velocity coordinates (dx, dy and dz) of the Sun. The positive X axis is directed towards the equinox, the y-axis is directed towards the point on the equator at right ascension 6h, and the z axis is directed toward the north pole of the equator. Typical position accuracy is <1e-4 AU (15000 km).
The Earth-Sun distance is given by sqrt(x^2 + y^2 + z^2) for the given date. Note that velocities in the Astronomical Almanac are for Earth/Moon barycenter (a very minor offset); see AA 1999 page E3.
x,y,z |
geocentric rectangular coordinates, in Astronomical Units, scalar or vector |
xvel,yvel,zvel |
velocity vectors corresponding to X, Y and Z |
Written W. Landsman Raytheon ITSS 1989 and 2000
R adaptation by Arnab Chakraborty June 2013
Original algorithm from Almanac for Computers, Doggett et al. USNO 1978 Adapted from the book Astronomical Photometry by A. Henden
# What were the rectangular coordinates of the Sun on # Jan 22, 1999 0h UT (= JD 2451200.5) in J2000 coords? # NOTE: Astronomical Almanac (AA) is in TDT, so add 64 seconds to UT to convert. xyz(51200.5+64./86400, equinox=2000) # Compare to Astronomical Almanac (1999 page C20) # X (AU) Y (AU) Z (AU) # XYZ: 0.51456871 -0.76963263 -0.33376880 # AA: 0.51453130 -0.7697110 -0.3337152 # abs(err): 0.00003739 0.00007839 0.00005360 # abs(err) # (km): 5609 11759 8040