# Accurate RA and DEC

1. Apr 4, 2010

### Philosophaie

Could someone do an equation check:

r,v,w and N are computed from JPL data

Sun's true longitude:
lonsun = v + w

Convert lonsun,r to ecliptic rectangular geocentric coordinates xs,ys:
xecl = r * cos(lonsun)
yecl = r * sin(lonsun)

(since the Sun always is in the ecliptic plane, zs is of course zero). xs,ys is the Sun's position in a coordinate system in the plane of the ecliptic. To convert this to equatorial, rectangular, geocentric coordinates, compute:
xequ = xecl
yequ= yecl * cos(ecl)
zequ = yecl * sin(ecl)

Finally, compute the Sun's Right Ascension (RA) and Declination (Dec):
RA = atan2( yequ, xequ )
Dec = atan2( zequ, sqrt(xequ*xequ+yequ*yequ) )

I have been using:

xecl = (Cos(w) *Cos(N) -Sin(w) *Sin(N) * Cos(i)) * x(k) + (-Sin(w) *Cos(N) - Cos(w) *Sin(N) *Cos(i)) * y
yecl = (Cos(w) * Sin(N) -Sin(w) *Sin(N) *Cos(i) * x + (Sin(w) * Sin(N) -Cos(w) * Cos(N) *Cos(i) )* y
zecl(k) = (Sin(w) * Sin(i)) * x + (Cos(w) *Sin(i)) * y

xeq = xecl(k)
yeq = Cos(oe) * yecl - Sin(oe) * zecl
zeq = Sin(oe) * yecl + Cos(oe) * zecl
where oe is the obiliqity of the planet

also RA=N+W+v is the equation true.

Looking for an accurate RA and DEC derived from JPL data.

Last edited: Apr 5, 2010