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astronomyengine astronomyengine website

Lifecycle: experimental CRAN status R-CMD-check

The astronomyengine R package provides R bindings to the Astronomy Engine. It bundles an up-to-date copy of the Astronomy Engine C library by Don Cross as a single-file header + source pair. This provides low-level access to the C library so that other R packages can link against it via LinkingTo without bundling their own copy.

Astronomy Engine is an open-source library for calculating positions of the Sun, Moon, and planets, and for predicting astronomical events such as rise/set times, lunar phases, equinoxes, solstices, eclipses, and transits. It is based on the VSOP87 planetary model and is accurate to within approximately ±1 arcminute.

Installation

You can install the released version of astronomyengine from CRAN with:

install.packages("astronomyengine")

And the development version from GitHub with:

# install.packages("pak")
pak::pak("mitchelloharawild/astronomyengine")

Usage

library(astronomyengine)
# All functions accept a `POSIXct` time as input.
# Here we use a reference time of 2025-02-19 12:00 UTC for all examples.
now <- as.POSIXct("2025-02-19 12:00:00", tz = "UTC")

The R functions and documentation are designed to be as similar as possible to the C API. Therefore the most comprehensive resource for learning how to use the package is the Astronomy Engine C API documentation. If you need support for how to use the package, I suggest that you also search for information about the underlying C library.

Next sunrise

Find the next sunrise at a given location — here, Sydney Observatory (latitude −33.87°, longitude 151.21°):

astro_search_rise_set(
  astro_body[["SUN"]], now,
  latitude = -33.8688, longitude = 151.2093
)
#> [1] "2025-02-19 19:34:50 UTC"

Current moon phase

astro_moon_phase() returns the Moon’s phase as an angle in degrees: 0° = new moon, 90° = first quarter, 180° = full moon, 270° = third quarter.

astro_moon_phase(now)
#> [1] 256.4968

Next full moon

Search for the next time the Moon reaches a specific phase angle. Use 180° for a full moon:

astro_search_moon_phase(180, now, limit_days = 30)
#> [1] "2025-03-14 06:55:19 UTC"

Solar eclipse

Find the next solar eclipse visible anywhere on Earth:

eclipse <- search_global_solar_eclipse(now)
eclipse
#> $status
#> [1] 0
#> 
#> $kind
#> [1] 2
#> 
#> $peak
#> [1] "2025-03-29 10:47:25 UTC"
#> 
#> $distance
#> [1] 6637.035
#> 
#> $latitude
#> [1] NaN
#> 
#> $longitude
#> [1] NaN

The kind field indicates the eclipse type: 0 = partial, 1 = annular, 2 = total. This total solar eclipse peaks on 2025-03-29.

Note: latitude and longitude in the eclipse result indicate the point of greatest eclipse on Earth’s surface. These are only populated for annular eclipses; for total eclipses they will be NaN.

Transit of Mercury

A transit occurs when Mercury passes directly between the Earth and the Sun. These are rare — the next one after February 2025 is in November 2032:

astro_search_transit(astro_body[["MERCURY"]], now)
#> $start
#> [1] "2032-11-13 06:41:48 UTC"
#> 
#> $peak
#> [1] "2032-11-13 08:54:14 UTC"
#> 
#> $finish
#> [1] "2032-11-13 11:06:53 UTC"
#> 
#> $separation
#> [1] 9.594123

Equinoxes and solstices

astro_seasons() returns all four seasonal turning points for a given year:

astro_seasons(2025)
#> $mar_equinox
#> [1] "2025-03-20 09:01:26 UTC"
#> 
#> $jun_solstice
#> [1] "2025-06-21 02:42:17 UTC"
#> 
#> $sep_equinox
#> [1] "2025-09-22 18:19:34 UTC"
#> 
#> $dec_solstice
#> [1] "2025-12-21 15:03:03 UTC"

Using the C library from another package

To use the Astronomy Engine C API from another R package:

  1. Add astronomyengine to both Imports and LinkingTo in your DESCRIPTION:

    Imports: astronomyengine
LinkingTo: astronomyengine

  2. Include the header in your C or C++ source files:

    #include <astronomy/astronomy.h>

For detailed documentation of the C API, including key types, functions, supported bodies, and coordinate frame rotations, see the Astronomy Engine C API documentation.

These binaries (installable software) and packages are in development.
They may not be fully stable and should be used with caution. We make no claims about them.
Health stats visible at Monitor.