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

Builds a compact ephemeris kernel from JPL's `de440s.bsp`.

The full DE440s kernel is ~31 MB because it carries every planetary
barycenter over 1849–2150. `Astro` uses only the Sun, Moon and Earth,
so this task extracts those segments over a chosen span of years and
writes a much smaller DAF/SPK file. Over the default 1900–2100 window
the result is about 8.4 MB.

Extraction is lossless: the original Chebyshev coefficients are copied
verbatim, so results computed from the compact file are identical to
those computed from the full kernel for any date it covers.

This task generates the ephemeris that ships with the library. Run it
to regenerate that file, or to build one covering a different span.

## Usage

    $ mix astro.build_ephemeris

The full kernel is downloaded to a temporary file, subset, and then
discarded. To subset a copy you already have, avoiding the download:

    $ mix astro.build_ephemeris --source priv/de440s.bsp

To cover a different span of years:

    $ mix astro.build_ephemeris --from 1900 --to 2200

Cost is roughly 42 KB per year of coverage. The window is widened to
the enclosing Chebyshev record boundary, so the requested span is
always fully covered.

## Options

  * `--source` — path to an existing full kernel. Downloads one to a
    temporary file if omitted.

  * `--dest` — output path. Defaults to `priv/de440s-astro.bsp`.

  * `--from` — first year to cover. Defaults to `1950`.

  * `--to` — last year to cover. Defaults to `2100`.

  * `--keep-earth` — retain the Earth→EMB segment rather than
    reconstructing it at runtime. Roughly doubles the output size.
    See `Astro.Ephemeris.Subset` for why it is redundant.

  * `--force` — overwrite the output file without prompting.

---

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