# Astro
[](https://hex.pm/packages/astro)
[](https://hex.pm/packages/astro)
[](https://hex.pm/packages/astro)
[](https://hex.pm/packages/astro)
Astro is a library to provide basic astromonomical functions with a focus on functions that support solar, lunar and lunisolar calendars such as the Chinese, Hebrew and Persian calendars.
## Usage
**NOTE: It's important to install and configure `Astro` correctly before use. See the [installation](#installation) notes below.**
The primary functions are:
### Solar function
* `Astro.sunrise/3`
* `Astro.sunset/3`
* `Astro.solstice/2`
* `Astro.equinox/2`
* `Astro.hours_of_daylight/2`
### Lunar functions
* `Astro.moon_position_at/1`
* `Astro.illuminated_fraction_of_moon_at/1`
* `Astro.date_time_new_moon_at_or_after/1`
* `Astro.lunar_phase_at/1`
* `Astro.lunar_phase_emoji/1`
### Examples
```elixir
# 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}
# Hours of daylight on December 7th in Sydney
iex> Astro.hours_of_daylight {151.20666584, -33.8559799094}, ~D[2019-12-07]
{:ok, ~T[14:18:45]}
# No sunset in summer at high latitudes
iex> Astro.hours_of_daylight {-62.3481, 82.5018}, ~D[2019-06-07]
{:ok, ~T[23:59:59]}
# No sunrise in winter at high latitudes
iex> Astro.hours_of_daylight {-62.3481, 82.5018}, ~D[2019-12-07]
{:ok, ~T[00:00:00]}
# Calculate solstices for 2019
iex> Astro.solstice 2019, :december
{:ok, ~U[2019-12-22 04:18:57Z]}
iex> Astro.solstice 2019, :june
{:ok, ~U[2019-06-21 15:53:45Z]}
# Calculate equinoxes for 2019
iex> Astro.equinox 2019, :march
{:ok, ~U[2019-03-20 21:58:06Z]}
iex> Astro.equinox 2019, :september
{:ok, ~U[2019-09-23 07:49:30Z]}
```
### Specifying a location
The desired location of sunrise or sunset can be specified as either:
* a tuple of longitude and latitude (note the order) such as `{-62.3481, 82.5018}`
* a tuple of longitude, latitude and elevation (note the order) such as `{-62.3481, 82.5018, 0}`.
* a `Geo.Point.t` struct
* a `Geo.PointZ.t` struct
### Location units and direction
For this implementation, the latitude and longitude of the functions in `Astro` are specified as follows:
* Longitude is `+` for eastern longitudes and `-` for western longitudes and specified in degrees
* Latitude is `+` for northern latitudes and `-` for southern latitudes and specified in degrees
* Elevation is specified in meters
## References
* Thanks to @pinnymz for the [ruby-zmanim](https://github.com/pinnymz/ruby-zmanim) gem which has a well structured ruby implementation of sunrise / sunset and some core astronomical algorithms.
* Eventually all roads lead to the canonical book on the subject by Jean Meeus: [Astronomical Algorithms](https://www.amazon.com/Astronomical-Algorithms-Jean-Meeus/dp/0943396352)
* For the intersection of calendars and astronomy, [Calendrical Calculations](https://www.amazon.com/Calendrical-Calculations-Ultimate-Edward-Reingold/dp/1107683165) by Nachum Dershowitz and Edward M. Reingold remains the standard reference.
* On the web, [timeanddate.com](https://www.timeanddate.com/astronomy/) is a great reference. The sunrise/sunset calculations in this library are tested to return times within 1 minute of timeanddate.com results.
* [Wikipedia](https://wikipedia.com) for content to help describe the understanding behind some of the functions
## Installation
### Configure Astro
Astro can be installed by adding `astro` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:astro, "~> 1.0"}
]
end
```
### Install a time zone database
A time zone database is required in order to support time zone conversions. Two popular options are [tzdata](https://hex.pm/packages/tzdata) and [tz](https://hex.pm/packages/tz). Then the appropriate database must be configured in `config.exs` or `runtime.exs` as the default time zone database. For example:
```elixir
# If using tzdata
config :elixir,
:time_zone_database, Tzdata.TimeZoneDatabase
# If using tz
config :elixir,
:time_zone_database, Tz.TimeZoneDatabase
```
### Install TzWorld Data
Then get dependencies and install the data required to determine a time zone from a location which is used by the dependency `tz_world`.
```
mix deps.get
mix tz_world.update
```
### Add TzWorld to supervision tree
It is also required that `tz_world` be added to your applications supervision tree by adding the relevant `tz_world` backend to it in your `MyApp.Application` module:
```
defmodule MyApp.Application do
use Application
def start(_type, _args) do
children = [
.....
# See the documentation for tz_world for the
# various available backends. This is the recommended
# backend.
TzWorld.Backend.DetsWithIndexCache
]
opts = [strategy: :one_for_one, name: Astro.Supervisor]
Supervisor.start_link(children, opts)
end
end
```
### Configure your application module
Make sure that you have configured your application in `mix.exs`:
```elixir
def application do
[
mod: {MyApp.Application, [strategy: :one_for_one]},
.....
]
end
```
Documentation can be found at [https://hexdocs.pm/astro](https://hexdocs.pm/astro).
