astrolibR (version 0.1)

moonpos: Compute the Right Ascension and Declination of the Moon at specified Julian date(s)

Description

Compute the Right Ascension and Declination of the Moon at specified Julian date(s)

Usage

moonpos(jd, radian=F)

Arguments

jd

Julian ephemeris date, scalar or vector

radian

if =TRUE, then all output variables are given in radians rather than degrees (default=FALSE)

Value

ra

apparent right ascension of the moon, referred to the true equator of the specified date(s), in degrees

dec

declination of the moon, in degrees

dis

Earth-moon distance between the center of the Earth and the center of the Moon, in km

geolong

apparent longitude of the moon referred to the ecliptic of the specified date(s), in degrees

geolat

apparent longitude of the moon referred to the ecliptic of the specified date(s), in degrees

Details

The method is derived from the Chapront ELP2000/82 Lunar Theory (Chapront-Touze and Chapront, 1983), as described by Jean Meeus (1998, Chpt. 47). Meeus quotes an approximate accuracy of 10" in longitude and 4" in latitude, but he does not give the time range for this accuracy. Comparison of this procedure with the example in ``Astronomical Algorithms'' reveals a very small discrepancy (~1 km) in the distance computation, but no difference in the position calculation.

References

Chaprint-Touze, M, and Chapront, J. 1983, The lunar ephemeris ELP 2000, Astron. Astrophys. 124, 50-62.

Meeus, J., 1998, ``Astronomical Algorithms'', 2nd ed., Willmann-Bell

See Also

cirrange nutate

Examples

# Find the position of the moon on April 12, 1992
# Result: 08 58 45.23  +13 46  6.1
# This is within 1" from the position given in the Astronomical Almanac

jd = jdcnv(1992,4,12,0)    # get Julian date
pos = moonpos(jd)     # get RA and Dec of moon
adstring(pos$ra,pos$dec,1)


# Plot the Earth-moon distance for every day at 0 TD in July, 1996

jd = jdcnv(1996,7,1,0)  # get Julian date of July 1
pos = moonpos(jd+rep(0,31))  # Moon position at all 31 days
plot(seq(1,31), pos$dis, xlab="July 1996 (day)", ylab="Lunar distance (km)")