planet_coords: Calculate low precision Right Ascension and declination for the planets given a date

Description

Calculate low precision Right Ascension and declination for the planets given a date

Usage

planet_coords(date, planet=planet, jd = FALSE)

Arguments

date

If jd=FALSE, then date is a 3-6 element vector containing year,month (1-12), day, and optionally hour, minute, & second. If jd=TRUE, then date is a vector of Julian dates.

planet

scalar string giving name of a planet, e.g. 'venus' (default = planet that computes coordinates for all planets except Earth)

If =TRUE, then the date parameter should be supplied as one or more Julian dates (default = FALSE)

Value

ra: Right Ascension of planet(s), J2000 degrees
dec: declination of planet(s), J2000 degrees

Details

For low precision, this routine uses function helio to get the heliocentric ecliptic coordinates of the planets at the given date, then converts these to geocentric ecliptic coordinates following Meeus (1991, p.209). These are then converted to Right Ascension and declination using the function euler. The function returns astrometric coordinates, i.e. no correction for aberration. The accuracy between the years 1800 and 2050 is better than 1 arcminute for the terrestial planets, but reaches 10 arcminutes for Saturn. Before 1850 or after 2050 the accuracy can get much worse.

The high precision option available in the IDL procedure based on JPL planetary ephemerides is not current available in the R astrolib package. The helio function is based on the two-body problem and neglects interactions between the planets. This is why the worst results are for Saturn.

References

Meeus, J. 1991, "Astronomical Algorithms"

Examples

Run this code

# Find the RA, Dec of Venus on 1992 Dec 20
# Result: RA = 21 05  2.66  Dec = -18 51 45.7

planet_coords(c(1992,12,20))    # compute for all planets
adstring(ra[2],dec[2],1)        # Venus is second planet
# This position is 37" from the full DE406 ephemeris position of
# RA = 21 05  5.24        -18 51 43.1

# Plot the declination of Mars for every day in the year 2001

jd = jdcnv(2001,1,1,0)      # get Julian date of midnight on Jan 1
out = planet_coords(jd+seq(0,365), planet='mars')     
plot(jd+seq(0,365), out$dec, pch=20, xlab='Day of 2001', ylab='Declination of Mars (degrees)')

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