Skip to contents

Convert Cartesian Vector to Horizontal Coordinates

Source: R/coordinates.R, R/rotation.R
astro_horizon_from_vector.Rd

Given a horizontal Cartesian vector, returns horizontal azimuth and altitude.

Given a horizontal Cartesian vector, returns horizontal azimuth and altitude.

Usage

astro_horizon_from_vector(vector, refraction = 1L)

astro_horizon_from_vector(vector, refraction = 1L)

Arguments

vector

A list with components:

x

North component in AU

y

West component in AU

z

Zenith (up) component in AU

t

The date and time (POSIXct)

refraction

Refraction option (0 = REFRACTION_NONE, 1 = REFRACTION_NORMAL, 2 = REFRACTION_JPLHOR). REFRACTION_NORMAL: correct altitude for atmospheric refraction (recommended). REFRACTION_NONE: no atmospheric refraction correction is performed. REFRACTION_JPLHOR: for JPL Horizons compatibility testing only; not recommended for normal use.

Value

A list representing horizontal coordinates with elements:

  • lat: Altitude in degrees (-90 to +90)

  • lon: Azimuth in degrees (0-360, clockwise from north)

  • dist: Distance in AU

  • status: Status code (0 = success)

A list with components:

lat

Altitude angle in degrees (corrected for refraction if requested)

lon

Azimuth in degrees clockwise from north

dist

Distance in AU

status

Status code (0 = success)

Details

IMPORTANT: This function differs from astro_sphere_from_vector in two ways:

  1. The longitude value represents azimuth defined clockwise from north (e.g., east = +90), preserving traditional navigational conventions.

  2. This function optionally corrects for atmospheric refraction.

The returned structure contains the azimuth in the lon field, measured in degrees clockwise from north: east = +90 degrees, west = +270 degrees.

IMPORTANT: This function differs from astro_sphere_from_vector() in two ways:

  • astro_sphere_from_vector() returns a lon value that represents azimuth defined counterclockwise from north (e.g., west = +90), but this function represents a clockwise rotation (e.g., east = +90). The difference is because astro_sphere_from_vector() is intended to preserve the vector "right-hand rule", while this function defines azimuth in a more traditional way as used in navigation and cartography.

  • This function optionally corrects for atmospheric refraction, while astro_sphere_from_vector() does not.

The returned structure contains the azimuth in lon. It is measured in degrees clockwise from north: east = +90 degrees, west = +270 degrees.

The altitude is stored in lat.

The distance to the observed object is stored in dist, and is expressed in astronomical units (AU).

Examples

# Vector pointing east at 45° altitude
vec <- list(x = 0, y = -0.707, z = 0.707,
            t = as.POSIXct("2024-01-01", tz = "UTC"))
hor <- astro_horizon_from_vector(vec, refraction = 1)

# Convert horizontal vector to angular coordinates
vec <- list(x = 0, y = -1, z = 0, t = as.POSIXct("2024-01-01", tz = "UTC"))
hor <- astro_horizon_from_vector(vec, refraction = 1)