asteRisk (version 1.4.3)

JPLephemerides: Calculate JPL main celestial objects ephemerides for a given Modified Julian Date

Description

NASA's Jet Propulsion Laboratory (JPL) provides mathematical models of the Solar System known as Development Ephemerides (DE). The models are given as sets of Chebyshev coefficients, which cam be used to calculate the position (and its derivatives) of the Sun, the eight major planets, Pluto and the Moon. This function employes JPL DE440 to calculate the position (and optionally velocities also) of the mentioned celestial objects, in ICRF frame. JPL DE440 covers the period from 1550 to 2650 AC. In addition to the position of celestial objects, lunar libration angles are also calculated. Internally, calculations are performed by employing Clenshaw's algorithm together with the Chebyshev coefficients provided by JPL DE440. The target time should be specified as a Modified Julian Date (MJD). MJD in different time systems can be used. Currently, UTC, UT1, TT and TDB are supported. Additionally, a central body with respect to which positions and velocities are calculated should be specified. By default, the Solar System Barycenter (SSB) is used, but additionally Mercury, Venus, Earth, Moon, Mars, Jupiter, Saturn, Uranus, Neptune or Pluto can be selected. Note that this function requires the additional package asteRiskData, which provides the Chebyshev coefficients, and can be installed by running install.packages("asteRiskData", repos="https://rafael-ayala.github.io/drat/")

Usage

JPLephemerides(MJD, timeSystem="UTC", centralBody="SSB", derivatives="acceleration")

Value

A list of vectors providing the positions (in meters), velocities (in m/s; only if requested), accelerations (in m/s^2; only if requested), Moon libration angles (in radians), first derivatives of Moon libration angles (in radians/s; only if velocities were requested) and second derivatives of Moon libration angles (in radians/s^2; only if accelerations were requested) of celestial objects with respect to the specified central body. For position, velocity and acceleration vectors, X, Y and Z components are given in this order. For Moon libration angles and their derivatives, they are given in the following order: phi, theta and psi.

Arguments

MJD

Modified Julian Date of the time for which celestial object ephemerides should be calculated. MJD are fractional number of days since midnight of the 17th of November, 1858. The MJD of a date-time string can be obtained with function dateTimeToMJD.

timeSystem

Time system into which the MJD is provided. Should be one from "UTC" (Coordinated Universal Time; default), "UT1" (Universal Time), "TT" (Terrestrial Time) and "TDB" (Barycentric Dynamical Time).

centralBody

String indicating the celestial object that will be taken as the center of coordinates to which positions and velocities are referred. Must be one of "SSB" (Solar System Barycenter), "Mercury", "Venus", "Earth", "Moon", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune" or "Pluto".

derivatives

String indicating what derivatives of positions should be calculated. Must be one of "none", "velocity" or "acceleration". If "none", only position is calculated. If "velocity", velocities are calculated, as well as first order derivatives of Moon libration angles. If "acceleration", both velocities and accelerations (as well as second order derivatives of Moon libration angles) are calculated.

References

https://gssc.esa.int/navipedia/index.php/Julian_Date https://gssc.esa.int/navipedia/index.php/Transformations_between_Time_Systems

Examples

if(requireNamespace("asteRiskData", quietly = TRUE)) {
# Let's calculate the MJD of the 12th of June, 2000 at 10:00:00 UTC time, in UTC

MJD_UTC <- dateTimeToMJD("2000-06-12 10:00:00", timeSystem = "UTC")

# Let's now calculate the JPL ephemerides using Earth as the central body:

ephemerides <- JPLephemerides(MJD_UTC, timeSystem = "UTC", centralBody="Earth")

# We can now calculate, for example, the exact distance between the barycenters
# of Earth and Moon

sqrt(sum(ephemerides$positionMoon^2))
}