How rare a pairing is: for two targets at an instant, the most recent earlier pairing that was at least as close and the next one, searched up to ten years each way; with a location, by default only pairings seen in a dark sky with both targets up count

The first target: a body name, a named star (star:<name> or hr:<n>) or a radec: target, in the grammar of bodies above. Required.

The second target, in the same grammar as a, and different from it. Required.

The instant to evaluate. ISO 8601: a date (2026-06-21), or a date and time with an optional seconds, fraction and either Z or a numeric UTC offset (+hh:mm, no seconds). No offset and no tz reads as UTC. Or jd: followed by a Julian Day on the UT scale, read as UTC whatever tz says: jd:2461000.5 is 2025-11-21T00:00:00Z. Reliable from 1700 to 2200.

The frame the separation is measured in. apparent is the true angle between the two apparent directions, topocentric when lat and lon are given; geocentric ignores the location. No ecliptic-longitude convention is offered here.

How far the outward walk runs EACH WAY from at, in days, so the window searched is twice this. The default is the maximum, because "closest in more than ten years" is only true when the whole span was walked.

visible=any only: a separation minimum closer to at than this belongs to the pass being observed and is never reported as previous or next. Ignored, with a warning, under visible=dark.

Which pairings count. dark, the default with a location, frame=apparent and no Sun: only moments with both targets at least 3 degrees up, refracted, and the Sun at least 6 degrees down; the stretch at is in, both targets above the horizon in daylight or dark and holding one dark night (its own, or the nearer one), at most 18 hours either way, is never previous or next, and a closer moment inside it, in a dark sky, is returned as closer.before or closer.after. any, the default otherwise: every minimum of the separation, above the horizon or not. dark needs a location and frame=apparent and refuses the Sun.

Latitude in decimal degrees, north positive. No compass suffix (do not append N or S). Required together with lon, or neither.

Longitude in decimal degrees, east positive. No compass suffix (do not append E or W). Required together with lat, or neither.

Observer elevation above mean sea level, in metres. Raises the observer on a locally flat plateau; sea-horizon dip is not modeled.

A place name (place=London), a name with a country (place=Cambridge,GB), or a stable identifier from GET /v2/places (place=id:us-n35198-w111651). Supplies lat, lon and, unless tz is also sent, the time zone. Mutually exclusive with sending lat and lon directly. Withheld unless GET /v2 reports capabilities.place_lookup true.

best guesses the intended place when a name is ambiguous and reports the guess as a warning. error refuses an ambiguous name with 400 AMBIGUOUS_PLACE and the candidates in supported. Only meaningful with place.

An IANA time zone name, for example Europe/London or America/Chicago. Numeric offsets and abbreviations are not accepted. Governs how a date-only or offset-free at/start/end is read, and how local instants are rendered when time_format=iso. Before 1970 the tz database records the history of a zone for its principal location only, and some names are links to another zone: the tz_source_boundary_1970 warning names whose history was used, and tzdb_lmt_interval marks a local mean time. Neither changes an offset.

How to resolve a local time that a clock repeated, such as the return from daylight saving. error refuses with 409 AMBIGUOUS_LOCAL_TIME.

How to resolve a local time that never happened, such as inside a spring-forward gap. error refuses with 400 NONEXISTENT_LOCAL_TIME.

How instants are rendered: iso for ISO 8601 strings carrying the effective offset, or unix for integer seconds since the epoch. An ISO offset has no seconds field, so an instant in a pre-1972 local mean time era whose offset has seconds (Moscow +02:30:17 until 1916) is written in UTC with a Z; utc_offset_seconds stays exact, utc_offset_iso rounds it to the nearest minute, and the tzdb_lmt_interval warning says so.

Response representation. Each endpoint serves a subset of json, csv and txt; see that operation's description for the exact list, or GET /v2 for every endpoint's formats array. An unsupported value is 400 BAD_FORMAT with the accepted list in supported.

Comma-separated dot paths into data (for example moon.phase_name,moon.illumination) so only those fields are returned. Up to 64 paths; an array element is addressed by its object key, never by index.

nested returns the documented object tree. flat returns one object with dotted keys, format=json only.

Decimal places for numeric fields.

Whether the JSON body is indented (1) or compact (0). Has no effect on csv or txt.

full includes prose notes, frame definitions and withheld-field reasons in meta. compact keeps every value and every machine-readable name and drops the prose.