Planet of the Day (public, no key)

Calendar date YYYY-MM-DD (UTC). Defaults to today.

Language for this call, overriding Accept-Language. The canonical per-call form: a page that renders in one language passes it here rather than reconfiguring its client. Unknown or inactive codes fall back to uk.

Comma-separated dotted paths, relative to data, to keep in the response. A path is applied to every element of an array, so planets.name means the name of each planet. Paths that match nothing come back in _fields_unmatched; a list where nothing matches at all is a 400, because an empty object would be a confident wrong answer.

Round every non-integer number in the response to this many decimal places. Integers are left alone, because an id is not a measurement. Ecliptic longitudes ship with fourteen decimals by default; 2 is about a tenth of an arcminute.

BCP-47 language (e.g. uk, en, de). Set it once on your HTTP client as its default; for a single call, ?lang= overrides it. Unknown or inactive codes fall back to uk.