Astro - astronomy calculations (sunrise, sunset, moonrise, moonset, moon phase equinox, solstice, ...)

Please say hi to a new lib, Astro that aims to deliver easy-to-consume astronomy calculations of practical use. For now it only calculates sunrise and sunset. In testing its precision is within 1 minute of the results returns from

It makes use of tz_world to dereference geo coordinates to a timezone and tzdata for time zone conversions.

This lib started our as a requirement to support certain solar, lunar and lunisolar calendars but it has wider applicability.


# Sunrise in Sydney on December 4th
iex> Astro.sunrise({151.20666584, -33.8559799094}, ~D[2019-12-04])
{:ok, #DateTime<2019-12-04 05:37:00.000000+11:00 AEDT Australia/Sydney>}

# Sunset in Sydney on December 4th
iex> Astro.sunset({151.20666584, -33.8559799094}, ~D[2019-12-04])
{:ok, #DateTime<2019-12-04 19:53:00.000000+11:00 AEDT Australia/Sydney>}

# Sunset in the town of Alert in Nunavut, Canada
# ...doesn't exist since there is no sunset in summer
iex> Astro.sunset({-62.3481, 82.5018}, ~D[2019-07-01])
{:error, :no_time}

# ...or sunrise in winter
iex> Astro.sunrise({-62.3481, 82.5018}, ~D[2019-12-04])
{:error, :no_time}

Next steps (as of 0.5.0)

  • [X] Sunrise
  • [X] Sunset
  • [X] Solstice
  • [X] Equinox
  • [X] Moon phase
  • [ ] Moon rise
  • [ ] Moon set

New version 0.2.0 is now out:


  • Add Astro.equinox/2and Astro.solstice/2 to calculate solstices and equinoxes for a year. From these can be derived the seasons.

  • Add Astro.Time.datetime_from_julian_days/1

  • Add Astro.Time.utc_datetime_from_terrestrial_datetime/1

Now on to lunar calculations (phase, moonrise, moonset)… And to implement the Persian calendar (which is a Solar based calendar).


New version 0.3.0 is now out:

Change in behaviour

  • Seconds are no longer truncated to zero when calculating datetimes and durations


  • Add Astro.solar_noon/2 to return the true solar noon for a location and date

  • Add Astro.hours_of_daylight/2 to return hours, minutes and seconds as a Time.t() representing the number of daylight hours for a give location and date

  • Add Astro.sun_apparent_longitude/1 to return the apparent solar longitude on a given date. The result, a number of degrees between 0 and 360, can be used to determine the seasons.

This completes the work required to support the Persian calendar which will be landing on in the next couple of days.

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Astro version 0.5.0 is now out. The release adds functions that calculate the phase of the moon which, apart from anything else, is needed to calculate the Chinese calendar. Please note that Elixir 1.11 or later is required.

Bug Fixes

  • Updates documentation to be clear about installation and setup requirements for tz_world

  • Fixes test data for São Paulo now that it no longer uses DST

  • Ensure :astro is started in test mode


This primary focus of this release is to add lunar calculations for moon phase.

  • Adds Astro.date_time_new_moon_before/1

  • Adds Astro.date_time_new_moon_at_or_after/1

  • Adds Astro.lunar_phase_at/1

  • Adds Astro.date_time_lunar_phase_at_or_before/2

  • Adds Astro.date_time_lunar_phase_at_or_after/2


Astro version 0.6.0 is now out. This version avoids requiring some dependencies which makes the footprint smaller and compilation much faster. I have been using it to support the upcoming Chinese calendar with all tests passing so I consider the basic solar and lunar algorithms to be in solid shape.

Please note that Elixir 1.11 or later is required.


Roadmap to version 1.0 is to add Astro.moonrise/2 and Astro.moonset/2.

Additional development proposals or suggestions are very welcome.

Bug Fixes

  • Fix Astro.Math.atan_r/2

  • Fix Astro.Earth.ephemeris/1

Breaking changes

  • Change Astro.Time.date_time_{from, to}_iso_days/1 to Astro.Time.date_time_{from, to}_moment/1


  • Remove dependency on ex_cldr_calendar and jason

  • Add Astro.sun_position_at/1 which is a public API for returning the right ascension, declination and radius (distance to the sun) at a given date/time.

  • Add Astro.moon_position_at/1 which is a public API for returning the right ascension, declination and radius (distance to the moon) at a given date/time.

  • Add Astro.illuminated_fraction_of_moon_at/1 to return the fraction of the moon that is lit at a given date/time.


A small update resulting in Astro version 0.9.0 that adds Astro.lunar_phase_emoji/1 to return the phase of the moon as an emoji. For example:

iex> Astro.lunar_phase_emoji 0
iex> Astro.lunar_phase_emoji 45
iex> Astro.lunar_phase_emoji 90
iex> Astro.lunar_phase_emoji 135
iex> Astro.lunar_phase_emoji 180
iex> Astro.lunar_phase_emoji 245
iex> Astro.lunar_phase_emoji 270
iex> Astro.lunar_phase_emoji 320
iex> Astro.lunar_phase_emoji 360

iex> ~U[2021-08-22 12:01:02.170362Z]
...> |> Astro.lunar_phase_at()
...> |> Astro.lunar_phase_emoji()