Observer position and velocity vector from geographic coordinates — astro_observer

Calculates the geocentric equatorial position and velocity vectors of an observer on the surface of the Earth, taking into account the Earth's rotation.

Usage

astro_observer_state(time, latitude, longitude, height, of_date = FALSE)

Arguments

time: A POSIXct date and time for which to calculate the observer's state vector.
latitude: The geographic latitude of the observer in degrees north of the equator (range: -90 to +90).
longitude: The geographic longitude of the observer in degrees east of the prime meridian (range: 0 to 360 or -180 to +180).
height: The elevation of the observer above sea level in meters.
of_date: Logical. If TRUE, uses equator-of-date coordinates (Earth's equator at the given time). If FALSE (default), uses J2000 coordinates (Earth's equator on 2000-01-01).

Value

A list with components:

x: Equatorial x-coordinate in AU
y: Equatorial y-coordinate in AU
z: Equatorial z-coordinate in AU
vx: Equatorial x-velocity in AU/day
vy: Equatorial y-velocity in AU/day
vz: Equatorial z-velocity in AU/day
t: The time (POSIXct) at which the state vector is valid
status: Status code (0 = success)

The position vector components are expressed in Astronomical Units (AU). Multiply by #KM_PER_AU to convert to kilometers. The velocity components are in AU per day.

If only position is needed without velocity, astro_observer_vector() is slightly more efficient.

Examples

# Get observer state at Sydney Observatory
time <- as.POSIXct("2024-01-01 12:00:00", tz = "UTC")
obs_state <- astro_observer_state(time, latitude = -33.8688, longitude = 151.2093, height = 0)
obs_state
#> $x
#> [1] 1.11619e-05
#> 
#> $y
#> [1] 3.361475e-05
#> 
#> $z
#> [1] -2.365291e-05
#> 
#> $vx
#> [1] -0.0002117903
#> 
#> $vy
#> [1] 7.067011e-05
#> 
#> $vz
#> [1] 4.893798e-07
#> 
#> $t
#> [1] "2024-01-01 12:00:00 UTC"
#> 
#> $status
#> [1] 0
#>