# `mix astro.download_ephemeris`
[🔗](https://github.com/kipcole9/astro/blob/v2.5.0/lib/mix/tasks/astro.download_ephemeris.ex#L1)

Downloads the JPL DE440s ephemeris file (`de440s.bsp`) from NASA's
NAIF server and places it in the application's `priv` directory.

The file is approximately 32 MB and contains Chebyshev polynomial
segments for the Sun, Moon, Earth and Earth-Moon Barycenter used
by the scan-and-bisect rise/set algorithms.

This download is optional. A compact ephemeris covering 1900–2100
ships with the library, so it is needed only for dates outside that
range — the full kernel covers 1849–2150. Once downloaded it takes
precedence over the bundled ephemeris automatically.

To build a compact ephemeris over some other span of years instead,
see `mix astro.build_ephemeris`.

## Usage

    $ mix astro.download_ephemeris

If the file already exists, you will be prompted before overwriting.
Use the `--force` flag to skip the prompt:

    $ mix astro.download_ephemeris --force

To download to a custom location instead of the default `priv` directory:

    $ mix astro.download_ephemeris --dest /path/to/de440s.bsp

When using `--dest`, the target directory must already exist. You will
also need to configure the ephemeris path in your `runtime.exs`:

    config :astro, ephemeris: "/path/to/de440s.bsp"

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