# `Astro`
[🔗](https://github.com/kipcole9/astro/blob/v2.1.0/lib/astro.ex#L1)

High-level API for common astronomical observations.

This module is the primary public interface for the Astro library.
It provides functions for sunrise/sunset, moonrise/moonset,
equinoxes and solstices, lunar phases, and sun/moon position.
All functions accept standard Elixir `Date` or `DateTime` structs
and return `{:ok, value}` or `{:error, reason}` tuples.

For lower-level access see `Astro.Solar`, `Astro.Lunar`,
`Astro.Time`, `Astro.Earth` and `Astro.Ephemeris`.

## Specifying a location

Location is specified as a `{longitude, latitude}` tuple (note
the order, matching `Geo.Point`), a `Geo.Point.t` struct, or a
`Geo.PointZ.t` struct that includes elevation in meters.

* Longitude is `+` for east, `-` for west, in degrees.
* Latitude is `+` for north, `-` for south, in degrees.

## Time zone resolution

Rise/set functions (`sunrise/3`, `sunset/3`, `moonrise/3`,
`moonset/3`) return a `DateTime` in the local time zone of the
given location. By default the time zone is resolved via
`TzWorld` (if configured). This can be overridden with the
following options:

* `:time_zone` — a zone name string, `:utc`, or `:default`
  (resolve from coordinates via `TzWorld`).
* `:time_zone_database` — the time zone database module
  (e.g. Tz.TimeZoneDatabase).
* `:time_zone_resolver` — a custom 1-arity function
  `(%Geo.Point{}) → {:ok, String.t()}`.

## Function groups

### Solar

* `sunrise/3`, `sunset/3` — local sunrise and sunset times
* `solar_noon/2` — UTC solar noon for a location and date
* `hours_of_daylight/2` — duration of daylight
* `sun_position_at/1` — right ascension, declination and distance
* `sun_azimuth_elevation/2` — azimuth and altitude at a datetime
* `sun_apparent_longitude/1` — apparent ecliptic longitude

### Lunar

* `moonrise/3`, `moonset/3` — local moonrise and moonset times
* `moon_position_at/1` — right ascension, declination and distance
* `illuminated_fraction_of_moon_at/1` — fraction of the Moon illuminated
* `lunar_phase_at/1` — phase angle (0–360°)
* `lunar_phase_emoji/1` — Unicode emoji for a phase angle

### New moon search

* `date_time_new_moon_before/1`, `date_time_new_moon_at_or_after/1`
* `date_time_new_moon_nearest/1`

### Phase search

* `date_time_lunar_phase_at_or_before/2`
* `date_time_lunar_phase_at_or_after/2`

### Crescent visibility

* `new_visible_crescent/3` — predict visibility of the new crescent moon

### Equinoxes and solstices

* `equinox/2` — March or September equinox
* `solstice/2` — June or December solstice

# `altitude`

```elixir
@type altitude() :: float()
```

# `angle`

```elixir
@type angle() :: number()
```

# `astronomical_units`

```elixir
@type astronomical_units() :: number()
```

# `date`

```elixir
@type date() :: Calendar.date() | Calendar.datetime()
```

# `degrees`

```elixir
@type degrees() :: float()
```

# `kilometers`

```elixir
@type kilometers() :: number()
```

# `latitude`

```elixir
@type latitude() :: float()
```

# `location`

```elixir
@type location() :: {longitude(), latitude()} | Geo.Point.t() | Geo.PointZ.t()
```

# `longitude`

```elixir
@type longitude() :: float()
```

# `meters`

```elixir
@type meters() :: number()
```

# `method`

```elixir
@type method() :: :odeh | :yallop | :schaefer
```

# `options`

```elixir
@type options() :: keyword()
```

# `phase`

```elixir
@type phase() :: angle()
```

# `radians`

```elixir
@type radians() :: float()
```

# `date_time_lunar_phase_at_or_after`
*since 0.5.0* 

```elixir
@spec date_time_lunar_phase_at_or_after(date(), phase()) :: {:ok, Calendar.datetime()}
```

Returns the date time of a given
lunar phase at or after a given
date time or date.

### Arguments

* `date_time` is a `t:DateTime.t/0` or a `t:Date.t/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

* `phase` is the required lunar phase expressed
  as a float number of degrees between `0.0` and
  `360.0`.

### Returns

* `{:ok, date_time}` at which the phase occurs or

* `{:error, {module, reason}}`

### Example

    iex> Astro.date_time_lunar_phase_at_or_after(~D[2021-08-01], Astro.Lunar.full_moon_phase())
    {:ok, ~U[2021-08-22 12:02:02.816534Z]}

# `date_time_lunar_phase_at_or_before`
*since 0.5.0* 

```elixir
@spec date_time_lunar_phase_at_or_before(date(), phase()) ::
  {:ok, Calendar.datetime()}
```

Returns the date time of a given
lunar phase at or before a given
date time or date.

### Arguments

* `date_time` is a `t:DateTime.t/0` or a `t:Date.t/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

* `phase` is the required lunar phase expressed
  as a float number of degrees between `0.0` and
  `3660.0`.

### Returns

* `{:ok, date_time}` at which the phase occurs or

* `{:error, {module, reason}}`

### Example

    iex> Astro.date_time_lunar_phase_at_or_before(~D[2021-08-01], Astro.Lunar.new_moon_phase())
    {:ok, ~U[2021-07-10 01:16:34.022607Z]}

# `date_time_new_moon_at_or_after`
*since 0.5.0* 

```elixir
@spec date_time_new_moon_at_or_after(date()) :: {:ok, Calendar.datetime()}
```

Returns the date time of the new
moon at or after a given date or
date time.

### Arguments

* `date_time` is a `DateTime` or a `Date` or
  any struct that meets the requirements of
  `t:Calendar.date` or `t:Calendar.datetime`.

### Returns

* `{:ok, date_time}` at which the new moon occurs or

* `{:error, {module, reason}}`

### Example

    iex> Astro.date_time_new_moon_at_or_after(~D[2021-08-23])
    {:ok, ~U[2021-09-07 00:51:44.267320Z]}

# `date_time_new_moon_at_or_before`
*since 0.5.0* 

Returns the date time of the new
moon before a given date or date time.

### Arguments

* `date_time` is a `t:DateTime.t/0` or a `t:Date.t/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

### Returns

* `{:ok, date_time}` at which the new moon occurs or

* `{:error, {module, reason}}`

### Example

    iex> Astro.date_time_new_moon_before(~D[2021-08-23])
    {:ok, ~U[2021-08-08 13:50:07.634598Z]}

# `date_time_new_moon_before`

```elixir
@spec date_time_new_moon_before(date()) :: {:ok, Calendar.datetime()}
```

# `date_time_new_moon_nearest`
*since 2.0.0* 

```elixir
@spec date_time_new_moon_nearest(date()) :: {:ok, Calendar.datetime()}
```

Returns the date time of the new
moon nearest to a given date or date time.

### Arguments

* `date_time` is a `t:DateTime.t/0` or a `t:Date.t/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

### Returns

* `{:ok, date_time}` at which the new moon occurs or

* `{:error, {module, reason}}`

### Example

    iex> Astro.date_time_new_moon_nearest(~D[2021-08-23])
    {:ok, ~U[2021-08-08 13:50:07.490242Z]}

# `equinox`

```elixir
@spec equinox(Calendar.year(), :march | :september) :: {:ok, DateTime.t()}
```

Returns the datetime in UTC for either the
March or September equinox.

### Arguments

* `year` is the gregorian year for which the equinox is
  to be calculated.

* `event` is either `:march` or `:september` indicating
  which of the two annual equinox datetimes is required.

### Returns

* `{:ok, datetime}` representing the UTC datetime of
  the equinox.

### Examples

    iex> {:ok, dt} = Astro.equinox 2019, :march
    iex> DateTime.truncate(dt, :second)
    ~U[2019-03-20 21:58:28Z]
    iex> {:ok, dt} = Astro.equinox 2019, :september
    iex> DateTime.truncate(dt, :second)
    ~U[2019-09-23 07:49:52Z]

### Notes

This equinox calculation is expected to be accurate
to within 2 minutes for the years 1000 CE to 3000 CE.

An equinox is commonly regarded as the instant of
time when the plane of earth's equator passes through
the center of the Sun. This occurs twice each year:
around 20 March and 23 September.

In other words, it is the moment at which the
center of the visible sun is directly above the equator.

# `hours_of_daylight`

```elixir
@spec hours_of_daylight(location(), Calendar.date()) :: {:ok, Time.t()}
```

Returns the number of hours of daylight for a given
location on a given date.

### Arguments

* `location` is the latitude, longitude and
  optionally elevation for the desired hours of
  daylight. It can be expressed as:

  * `{lng, lat}` - a tuple with longitude and latitude
    as floating point numbers. **Note** the order of the
    arguments.
  * a `Geo.Point.t` struct to represent a location without elevation
  * a `Geo.PointZ.t` struct to represent a location and elevation.

* `date` is any `t:Date.t/0` in the Gregorian
  calendar (for example, `Calendar.ISO`).

### Returns

* `{:ok, time}` where `time` is a `Time.t()`

### Examples

    iex> Astro.hours_of_daylight {151.20666584, -33.8559799094}, ~D[2019-12-08]
    {:ok, ~T[14:19:29]}

    # No sunset in summer
    iex> Astro.hours_of_daylight {-62.3481, 82.5018}, ~D[2019-06-07]
    {:ok, ~T[23:59:59]}

    # No sunrise in winter
    iex> Astro.hours_of_daylight {-62.3481, 82.5018}, ~D[2019-12-07]
    {:ok, ~T[00:00:00]}

### Notes

In latitudes above the polar circles (approximately
+/- 66.5631 degrees) there will be no hours of daylight
in winter and 24 hours of daylight in summer.

# `illuminated_fraction_of_moon_at`
*since 0.6.0* 

```elixir
@spec illuminated_fraction_of_moon_at(date()) :: number()
```

Returns the illumination of the moon
as a float for a given date or date time.

### Arguments

* `date_time` is a `t:DateTime.t/0` or a `t:Date.t/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

### Returns

* a `float` value between `0.0` and `1.0`
  representing the fractional illumination of
  the moon.

### Example

    iex> fraction = Astro.illuminated_fraction_of_moon_at(~D[2017-03-16])
    iex> Float.round(fraction, 4)
    0.8884

    iex> fraction = Astro.illuminated_fraction_of_moon_at(~D[1992-04-12])
    iex> Float.round(fraction, 4)
    0.6786

# `is_lunar_phase`
*macro* 

# `lunar_phase_at`
*since 0.5.0* 

```elixir
@spec lunar_phase_at(date()) :: phase()
```

Returns the lunar phase as a
float number of degrees at a given
date or date time.

### Arguments

* `date_time` is a `t:DateTime.t/0` or a `t:Date.t/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

### Returns

* the lunar phase as a float number of
  degrees.

### Example

    iex> Astro.lunar_phase_at ~U[2021-08-22 12:02:02.816534Z]
    180.00004404669988

    iex> Astro.lunar_phase_at(~U[2021-07-10 01:16:34.022607Z])
    3.6600909621461326e-6

# `lunar_phase_emoji`

```elixir
@spec lunar_phase_emoji(phase()) :: String.t()
```

Returns the moon phase as a UTF8 binary
representing an emoji of the moon phase.

### Arguments

* `phase` is a moon phase between `0.0` and `360.0`.

### Returns

* A single grapheme string representing the [Unicode
  moon phase emoji](https://unicode-table.com/en/sets/moon/).

### Examples

    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 12:02:02.816534Z]
    ...> |> Astro.lunar_phase_at()
    ...> |> Astro.lunar_phase_emoji()
    "🌕"

# `moon_position_at`
*since 0.6.0* 

```elixir
@spec moon_position_at(date()) :: Geo.PointZ.t()
```

Returns a `t:Geo.PointZ` containing
the right ascension and declination of
the moon at a given date or date time.

### Arguments

* `date_time` is a `t:DateTime.t/0` or a `t:Date.t/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

### Returns

* a `t:Geo.PointZ` struct with coordinates
  `{right_ascension, declination, distance}` with properties
  `%{reference: :celestial, object: :moon}`
  `distance` is in meters.

### Example

    iex> Astro.moon_position_at(~D[1992-04-12]) |> Astro.Location.round(6)
    %Geo.PointZ{
      coordinates: {134.69343, 13.766512, 368409007.322444},
      properties: %{object: :moon, reference: :celestial},
      srid: nil
    }

# `moonrise`
*since 2.0.0* 

```elixir
@spec moonrise(location(), date(), options()) ::
  {:ok, DateTime.t()} | {:error, :moon_always_below_horizon}
```

Returns the datetime of moonrise for a given location and date.

Uses the JPL DE440s ephemeris with fully topocentric correction
to compute the Moon's altitude zero-crossing via scan-and-bisect.

### Arguments

* `location` is the location as a `{longitude, latitude}` tuple,
  a `Geo.Point.t` or a `Geo.PointZ.t`.

* `date` is a `t:Date.t/0` or `t:DateTime.t/0`.

* `options` is a keyword list of options.

### Options

* `:time_zone` is the time zone in which the sunrise
  is requested. The default is `:default` in which
  the sunrise time is reported in the time zone of
  the requested location. `:utc` can be specified or any
  other time zone name supported by the option
  `:time_zone_database` is acceptabe.

* `:time_zone_database` represents the module that
  implements the `Calendar.TimeZoneDatabase` behaviour.
  The default is the configured Elixir time zone database or
  one of Tzdata.TimeZoneDatabase or Tz.TimeZoneDatabase
  depending upon which dependency is configured.

* `:time_zone_resolver` is a 1-arity function that resolves the
  time zone name for a given location. The function will receive
  a `%Geo.Point{cordinates: {lng, lat}}` struct and is expected to
  return either `{:ok, time_zone_name}` or `{:error, :time_zone_not_found}`.
  The default is `TzWorld.timezone_at/1` if `:tz_world` is
  configured.

### Returns

* `{:ok, date_time}` with the local time of moonrise, or

* `{:error, :moon_always_below_horizon}` if the Moon does not
  rise on the given date at the given location.

# `moonset`
*since 2.0.0* 

```elixir
@spec moonset(location(), date(), options()) ::
  {:ok, DateTime.t()} | {:error, :moon_always_above_horizon}
```

Returns the datetime of moonset for a given location and date.

Uses the JPL DE440s ephemeris with fully topocentric correction
to compute the Moon's altitude zero-crossing via scan-and-bisect.

### Arguments

* `location` is the location as a `{longitude, latitude}` tuple,
  a `Geo.Point.t` or a `Geo.PointZ.t`.

* `date` is a `t:Date.t/0` or `t:DateTime.t/0`.

* `options` is a keyword list of options.

### Options

* `:time_zone` is the time zone in which the sunrise
  is requested. The default is `:default` in which
  the sunrise time is reported in the time zone of
  the requested location. `:utc` can be specified or any
  other time zone name supported by the option
  `:time_zone_database` is acceptabe.

* `:time_zone_database` represents the module that
  implements the `Calendar.TimeZoneDatabase` behaviour.
  The default is the configured Elixir time zone database or
  one of Tzdata.TimeZoneDatabase or Tz.TimeZoneDatabase
  depending upon which dependency is configured.

* `:time_zone_resolver` is a 1-arity function that resolves the
  time zone name for a given location. The function will receive
  a `%Geo.Point{cordinates: {lng, lat}}` struct and is expected to
  return either `{:ok, time_zone_name}` or `{:error, :time_zone_not_found}`.
  The default is `TzWorld.timezone_at/1` if `:tz_world` is
  configured.

### Returns

* `{:ok, date_time}` with the local time of moonset, or

* `{:error, :moon_always_above_horizon}` if the Moon does not
  set on the given date at the given location.

# `new_visible_crescent`
*since 2.1.0* 

```elixir
@spec new_visible_crescent(location(), date(), method()) ::
  {:ok, Astro.Lunar.CrescentVisibility.visibility()} | {:error, :no_sunset}
```

Predicts the visibility of the new crescent moon at a given location
on a given date using one of three published criteria.

At the optimal observation time after sunset, the function evaluates
the geometric and photometric conditions to classify the crescent
into one of five visibility categories.

### Arguments

* `location` is the latitude, longitude and optionally elevation for
  the observation site. It can be expressed as:

  * `{lng, lat}` - a tuple with longitude and latitude as floating
    point numbers. **Note** the order of the arguments.

  * a `t:Geo.Point.t/0` struct to represent a location without elevation.

  * a `t:Geo.PointZ.t/0` struct to represent a location and elevation.

* `date` is a `t:Date.t/0` or `t:DateTime.t/0` indicating the evening
  on which crescent visibility is to be evaluated.

* `method` selects the prediction criterion. Default `:odeh`.

  * `:odeh` — Odeh (2006). Empirical criterion based on 737 observations.
    Uses topocentric ARCV with a Danjon limit of 6.4°. The most widely
    used modern criterion.

  * `:yallop` — Yallop (1997). Empirical criterion based on 295
    observations. Uses geocentric ARCV. The original single-parameter
    approach that Odeh later refined.

  * `:schaefer` — Schaefer (1988/2000). Physics-based model computing
    the contrast between crescent brightness and twilight sky brightness
    against the human contrast detection threshold. The best observation
    time is found by scanning from sunset to moonset.

### Options

When `method` is `:schaefer`, the following options are accepted as
an optional fourth argument (a keyword list):

* `:extinction` — V-band zenith extinction coefficient. Default `0.172`
  (clean sea-level site). Typical values: `0.12` (high mountain),
  `0.17` (sea level), `0.25` (hazy conditions).

### Returns

* `{:ok, visibility}` where `visibility` is one of:

  * `:A` — Visible to the naked eye.

  * `:B` — Visible with optical aid.

  * `:C` — May need optical aid.

  * `:D` — Not visible with optical aid.

  * `:E` — Not visible.

* `{:error, :no_sunset}` if no sunset occurs on the given date at
  the given location (e.g. polar day).

### Method comparison

| Aspect | Yallop (1997) | Odeh (2006) | Schaefer (1988/2000) |
|--------|---------------|-------------|----------------------|
| Basis | Empirical polynomial | Empirical polynomial | Physical model |
| Observations | 295 | 737 | N/A (theory) |
| ARCV type | Geocentric | Topocentric | N/A |
| Best time | Sunset + 4/9 lag | Sunset + 4/9 lag | Scanned (max Rs) |
| Atmosphere | Not modelled | Not modelled | Extinction coefficient |

### Examples

    iex> location = {-0.1275, 51.5072}
    iex> Astro.new_visible_crescent(location, ~D[2025-03-31])
    {:ok, :A}

    iex> location = {-0.1275, 51.5072}
    iex> Astro.new_visible_crescent(location, ~D[2025-03-31], :yallop)
    {:ok, :A}

# `new_visible_crescent`
*since 2.1.0* 

```elixir
@spec new_visible_crescent(location(), date(), :schaefer, keyword()) ::
  {:ok, Astro.Lunar.CrescentVisibility.visibility()} | {:error, :no_sunset}
```

Same as `new_visible_crescent/3` with `method: :schaefer` but
accepts additional atmospheric options.

See `new_visible_crescent/3` for full documentation.

### Options

* `:extinction` — V-band zenith extinction coefficient. Default `0.172`.

### Example

    iex> location = {39.8579, 21.3891}
    iex> {:ok, visibility} = Astro.new_visible_crescent(location, ~D[2025-03-31], :schaefer, extinction: 0.25)
    iex> visibility in [:A, :B, :C, :D, :E]
    true

# `solar_noon`

```elixir
@spec solar_noon(location(), Calendar.date()) :: {:ok, DateTime.t()}
```

Returns solar noon for a
given date and location as
a UTC datetime

### Arguments

* `location` is the latitude, longitude and
  optionally elevation for the desired solar noon
  time. It can be expressed as:

  * `{lng, lat}` - a tuple with longitude and latitude
    as floating point numbers. **Note** the order of the
    arguments.
  * a `Geo.Point.t` struct to represent a location without elevation.
  * a `Geo.PointZ.t` struct to represent a location and elevation.

* `date` is any `t:Date.t/0` in the Gregorian
  calendar (for example, `Calendar.ISO`).

### Returns

* a UTC datetime representing solar noon
  at the given location for the given date.

### Example

    iex> Astro.solar_noon {151.20666584, -33.8559799094}, ~D[2019-12-06]
    {:ok, ~U[2019-12-06 01:45:42Z]}

### Notes

Solar noon is the moment when the sun passes a
location's meridian and reaches its highest position
in the sky. In most cases, it doesn't happen at 12 o'clock.

At solar noon, the Sun reaches its
highest position in the sky as it passes the
local meridian.

# `solstice`

```elixir
@spec solstice(Calendar.year(), :june | :december) :: {:ok, DateTime.t()}
```

Returns the datetime in UTC for either the
June or December solstice.

### Arguments

* `year` is the gregorian year for which the solstice is
  to be calculated.

* `event` is either `:june` or `:december` indicating
  which of the two annual solstice datetimes is required.

### Returns

* `{:ok, datetime}` representing the UTC datetime of
  the solstice.

### Examples

    iex> {:ok, dt} = Astro.solstice 2019, :december
    iex> DateTime.truncate(dt, :second)
    ~U[2019-12-22 04:19:19Z]
    iex> {:ok, dt} = Astro.solstice 2019, :june
    iex> DateTime.truncate(dt, :second)
    ~U[2019-06-21 15:54:07Z]

### Notes

This solstice calculation is expected to be accurate
to within 2 minutes for the years 1000 CE to 3000 CE.

A solstice is an event occurring when the Sun appears
to reach its most northerly or southerly excursion
relative to the celestial equator on the celestial
sphere. Two solstices occur annually, around June 21
and December 21.

The seasons of the year are determined by
reference to both the solstices and the equinoxes.

The day of a solstice in either hemisphere has either the most
sunlight of the year (summer solstice) or the least
sunlight of the year (winter solstice) for any place
other than the Equator.

Alternative terms, with no ambiguity as to which
hemisphere is the context, are "June solstice" and
"December solstice", referring to the months in
which they take place every year.

# `sun_apparent_longitude`

```elixir
@spec sun_apparent_longitude(Calendar.date()) :: degrees()
```

Returns solar longitude for a
given date. Solar longitude is used
to identify the seasons.

### Arguments

* `date` is any `t:Date.t/0` in the Gregorian
  calendar (for example, `Calendar.ISO`).

### Returns

* a `float` number of degrees between 0 and
  360 representing the solar longitude
  on `date`.

### Examples

    iex> Astro.sun_apparent_longitude ~D[2019-03-21]
    0.08035853207991295
    iex> Astro.sun_apparent_longitude ~D[2019-06-22]
    90.32130455695378
    iex> Astro.sun_apparent_longitude ~D[2019-09-23]
    179.68691978440197
    iex> Astro.sun_apparent_longitude ~D[2019-12-23]
    270.83941087483504

### Notes

Solar longitude (the ecliptic longitude of the sun)
in effect describes the position of the earth in its
orbit, being zero at the moment of the March
equinox.

Since it is based on how far the earth has moved
in its orbit since the equinox, it is a measure of
what time of the tropical year (the year of seasons)
we are in, but without the inaccuracies of a calendar
date, which is perturbed by leap years and calendar
imperfections.

# `sun_azimuth_elevation`
*since 0.11.0* 

```elixir
@spec sun_azimuth_elevation(location(), Calendar.datetime()) ::
  {azimuth :: float(), altitude :: float()}
```

Returns a tuple `{azimuth, altitude}` for a given
date time and location.

### Arguments

* `location` is the latitude, longitude and
  optionally elevation for the desired sunrise
  azimuth and altitude. It can be expressed as:

  * `{lng, lat}` - a tuple with longitude and latitude
    as floating point numbers. **Note** the order of the
    arguments.
  * a `Geo.Point.t` struct to represent a location without elevation
  * a `Geo.PointZ.t` struct to represent a location and elevation

* `date_time` is a `t:DateTime.t/0` or a `t:Date.t/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

### Returns

* a tuple of the format `{azimith, altitude}` which are
  expressed in float degrees.

### Example

    iex> {:ok, date_time} = DateTime.new(~D[2023-05-17], ~T[12:47:00], "Australia/Sydney")
    iex> location = {151.1637781, -33.5145852}
    iex> {_azimuth, _altitude} = Astro.sun_azimuth_elevation(location, date_time)

# `sun_position_at`
*since 0.6.0* 

```elixir
@spec sun_position_at(date()) :: Geo.PointZ.t()
```

Returns a `t:Geo.PointZ` containing
the right ascension and declination of
the sun at a given date or date time.

### Arguments

* `date_time` is a `t:DateTime.t/0` or a `t:Date/0` or
  any struct that meets the requirements of
  `t:Calendar.date/0` or `t:Calendar.datetime/0`.

### Returns

* a `t:Geo.PointZ.t/0` struct with coordinates
  `{right_ascension, declination, distance}` with properties
  `%{reference: :celestial, object: :sun}`.
  `distance` is in meters.

### Example

    iex> Astro.sun_position_at(~D[1992-10-13])
    %Geo.PointZ{
      coordinates: {-161.61854343627374, -7.785324796344723, 149169604737.93973},
      properties: %{object: :sun, reference: :celestial},
      srid: nil
    }

# `sunrise`

```elixir
@spec sunrise(location(), date(), options()) ::
  {:ok, DateTime.t()}
  | {:error, :time_zone_not_found | :time_zone_not_resolved | :no_time}
```

Calculates the sunrise for a given location and date.

Sunrise is the moment when the upper limb of
the sun appears on the horizon in the morning.

### Arguments

* `location` is the latitude, longitude and
  optionally elevation for the desired sunrise
  time. It can be expressed as:

  * `{lng, lat}` - a tuple with longitude and latitude
    as floating point numbers. **Note** the order of the
    arguments.
  * a `t:Geo.Point.t/0` struct to represent a location without elevation
  * a `t:Geo.PointZ.t/0` struct to represent a location and elevation

* `date` is a `t:Date.t/0`, `t:NaiveDateTime.t/0` or `t:DateTime.t/0`
  to indicate the date of the year in which
  the sunrise time is required.

* `options` is a keyword list of options.

### Options

* `solar_elevation` represents the type of sunrise
  required. The default is `:geometric` which equates to
  a solar elevation of 90°. In this case the calculation
  also accounts for refraction and elevation to return a
  result which accords with the eye's perception. Other
  solar elevations are:

    * `:civil` representing a solar elevation of 96.0°. At this
      point the sun is just below the horizon so there is
      generally enough natural light to carry out most
      outdoor activities.

    * `:nautical` representing a solar elevation of 102.0°
      This is the point at which the horizon is just barely visible
      and the moon and stars can still be used for navigation.

    * `:astronomical` representing a solar elevation of 108.0°.
      This is the point beyond which astronomical observation
      becomes impractical.

    * Any floating point number representing the desired
      solar elevation.

* `:time_zone` is the time zone in which the sunrise
  is requested. The default is `:default` in which
  the sunrise time is reported in the time zone of
  the requested location. `:utc` can be specified or any
  other time zone name supported by the option
  `:time_zone_database` is acceptabe.

* `:time_zone_database` represents the module that
  implements the `Calendar.TimeZoneDatabase` behaviour.
  The default is the configured Elixir time zone database or
  one of Tzdata.TimeZoneDatabase or Tz.TimeZoneDatabase
  depending upon which dependency is configured.

* `:time_zone_resolver` is a 1-arity function that resolves the
  time zone name for a given location. The function will receive
  a `%Geo.Point{cordinates: {lng, lat}}` struct and is expected to
  return either `{:ok, time_zone_name}` or `{:error, :time_zone_not_found}`.
  The default is `TzWorld.timezone_at/1` if `:tz_world` is
  configured.

### Returns

* a `t:DateTime.t/0` representing the time of sunrise in the
  requested time zone at the requested location.

* `{:error, :time_zone_not_found}` if the requested
  time zone is unknown.

* `{:error, :time_zone_not_resolved}` if it is not possible
  to resolve a time zone name from the location. This can happen
  if `:tz_world` is not configured as a dependency and no
  `:time_zone_resolver` option is specified.

* `{:error, :no_time}` if for the requested date
  and location there is no sunrise. This can occur at
  very high and very low latitudes during summer and winter.

### Notes

* If the resolved UTC date time is ambiguous because of a daylight savings
  transition, the second of the two possibilities is applied. See
  the `DateTime.from_naive/3` for more information.

### Examples

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

    # Sunrise in Alert, Nanavut, Canada
    Astro.sunrise({-62.3481, 82.5018}, ~D[2019-12-04])
    {:error, :no_time}

# `sunset`

```elixir
@spec sunset(location(), date(), options()) ::
  {:ok, DateTime.t()}
  | {:error, :time_zone_not_found | :time_zone_not_resolved | :no_time}
```

Calculates the sunset for a given location and date.

Sunset is the moment when the upper limb of
the sun disappears below the horizon in the evening.

### Arguments

* `location` is the latitude, longitude and
  optionally elevation for the desired sunrise
  time. It can be expressed as:

  * `{lng, lat}` - a tuple with longitude and latitude
    as floating point numbers. **Note** the order of the
    arguments.
  * a `Geo.Point.t` struct to represent a location without elevation
  * a `Geo.PointZ.t` struct to represent a location and elevation

* `date` is a `t:Date.t/0`, `t:NaiveDateTime.t/0` or `t:DateTime.t/0`
  to indicate the date of the year in which
  the sunset time is required.

* `options` is a keyword list of options.

### Options

* `solar_elevation` represents the type of sunset
  required. The default is `:geometric` which equates to
  a solar elevation of 90°. In this case the calulation
  also accounts for refraction and elevation to return a
  result which accords with the eyes perception. Other
  solar elevations are:

    * `:civil` representing a solar elevation of 96.0°. At this
      point the sun is just below the horizon so there is
      generally enough natural light to carry out most
      outdoor activities.

    * `:nautical` representing a solar elevation of 102.0°
      This is the point at which the horizon is just barely visible
      and the moon and stars can still be used for navigation.

    * `:astronomical`representing a solar elevation of 108.0°.
      This is the point beyond which astronomical observation
      becomes impractical.

    * Any floating point number representing the desired
      solar elevation.

* `:time_zone` is the time zone in which the sunset
  is requested. The default is `:default` in which
  the sunrise time is reported in the time zone of
  the requested location. `:utc` can be specified or any
  other time zone name supported by the option
  `:time_zone_database` is acceptabe.

* `:time_zone_database` represents the module that
  implements the `Calendar.TimeZoneDatabase` behaviour.
  The default is the configured Elixir time zone database or
  one of Tzdata.TimeZoneDatabase or Tz.TimeZoneDatabase
  depending upon which dependency is configured.

* `:time_zone_resolver` is a 1-arity function that resolves the
  time zone name for a given location. The function will receive
  a `%Geo.Point{cordinates: {lng, lat}}` struct and is expected to
  return either `{:ok, time_zone_name}` or `{:error, :time_zone_not_found}`.
  The default is `TzWorld.timezone_at/1` if `:tz_world` is
  configured.

### Returns

* a `t:DateTime.t/0` representing the time of sunset in the
  requested time zone at the requested location.

* `{:error, :time_zone_not_found}` if the requested
  time zone is unknown.

* `{:error, :time_zone_not_resolved}` if it is not possible
  to resolve a time zone name from the location. This can happen
  if `:tz_world` is not configured as a dependency and no
  `:time_zone_resolver` option is specified.

* `{:error, :no_time}` if for the requested date
  and location there is no sunset. This can occur at
  very high and very low latitudes during summer and winter.

### Notes

* If the resolved UTC date time is ambiguous because of a daylight savings
  transition, the second of the two possibilities is applied. See
  the `DateTime.from_naive/3` for more information.

### Examples

    # Sunset in Sydney, Australia
    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 Alert, Nanavut, Canada
    Astro.sunset({-62.3481, 82.5018}, ~D[2019-12-04])
    {:error, :no_time}

---

*Consult [api-reference.md](api-reference.md) for complete listing*