Perigee

What every answer rests on: data sources and when they were fetched, the models and their typical accuracy, catalogue sizes, frame, attribution and the educational-use disclaimer.

The closest distance between two objects in space over a period (default 1 day, at most 1): when, how close (timed to a fraction of a second) and, for two Earth satellites, how fast they pass each other. For education only: public orbits are not precise enough for collision avoidance, and the answer says so.

Student-level definitions (secondary school) of the terms shown in Perigee: NORAD number, COSPAR id, apogee and perigee, inclination, orbital period, element epoch, ephemeris window, phase… Without a term, lists them all.

Turns a Perigee link or embed URL (from perigee_view or a shared link) into words: a one-sentence alternative text for the image or iframe, and a description for lesson notes (which view, the selected object, where it is and how reliable its position is, the date and speed of time, the frame, the filters and what is hidden). The camera framing is not in the link.

Centre-to-centre distance between two objects at a date (km and astronomical units) and the light travel time, with the provenance of both positions. Example: from "earth" to "mars".

Distances between two objects sampled evenly over a period (for a table or a chart), with the closest and farthest samples. The step limits their precision: use more steps for a precise closest approach.

Events over a period (default: the next 30 days, at most 31): Moon phases (super and micro full Moons), lunar perigee and apogee, lunar and solar eclipses (with a place: how they look from there), equinoxes and solstices, the Earth's perihelion and aphelion, planet oppositions, conjunctions and greatest elongations, and the next key events of the missions (flybys, arrivals, launches) from Perigee's catalogue.

Looks up objects by name or identifier (English or French, accents optional): planets, moons, missions beyond Earth orbit, featured science satellites, any active Earth satellite (by name, NORAD or COSPAR id), landing and launch sites, named stars. Returns ids for the other tools.

The path on the ground below an Earth satellite over one or two orbits: a list of points (time, latitude, longitude, height) and a GeoJSON MultiLineString split at the antimeridian, ready for a map. Up to 360 points.

Facts about an object: name, agency, country, launch date, status, mission goals and notes, identifiers (NORAD, COSPAR, Horizons), official website, sources and verification date, a catalogue image with credit and licence, and links to its view in Perigee. include_image also returns the image itself.

Orbit of an object: perigee and apogee altitudes (periapsis/apoapsis), period, inclination, eccentricity, semi-major axis. Earth satellites: the mean elements of their published element set, with the orbit regime (LEO, MEO, GEO, HEO); other objects: instantaneous two-body elements around their centre at a date (default: the body they orbit; inclination on its equator, on the ecliptic around the Sun).

When an Earth satellite (ISS = 25544, Tiangong = 48274, Hubble = 20580, or any NORAD number) passes over a place: rise, highest point and set times with direction (azimuth, compass point) and elevation, and whether it can be seen with the naked eye (sunlit satellite, dark sky) and when. Give the place's latitude and longitude (degrees, east positive) and, for local times, its IANA time zone. Up to 3 days ahead.

Whether an object is in sunlight, in the penumbra or in the umbra of the body it orbits (Earth satellites: the Earth's shadow; lunar and Martian orbiters: the Moon's or Mars'), and when it next enters and leaves the shadow (default: over two orbits, at most 12 hours). Explains why a satellite crossing the night sky suddenly fades out.

What is in the sky from a place at a date (default: now): day, twilight or night; the Sun with its next rise, set and twilights; the Moon with its phase, illuminated share, rise and set; each planet with its altitude, direction, brightness (magnitude), constellation and whether it can be seen with the naked eye; and the bright named stars above the horizon. For "what can I see tonight", give the evening time.

Apparent close pairings seen from a place over a period (default 7 days, at most 7): the Moon and the planets with each other and with the bright stars near the ecliptic (Regulus, Spica, Antares, Aldebaran, Pollux…), at their closest, with where to look and whether the pair is visible (above the horizon, dark sky). With `satellite` (a NORAD number), that satellite passing near them during its passes over one day.

Position (km, ICRF/J2000 equatorial axes) and velocity (km/s) of an object at a date, relative to a centre (default: the body it orbits or stands on), with distance, altitude above the centre, speed, light time, whether it is in sunlight, and provenance. Around the Earth, the Moon or Mars it also gives the ground point below it (latitude, longitude; on the Earth the country or sea, e.g. "over Brazil", "over the Indian Ocean").

Counts of the active satellites: total, by owner (country or organisation, e.g. US, PRC, CIS, ESA), by operator (SpaceX, OneWeb, Amazon Kuiper…), by CelesTrak group (constellations, navigation, weather…), by orbit regime (LEO, MEO, GEO, HEO), object type, launch year, recent launches and low-orbit altitude bands. owner, operator or group narrow a list to matching entries (code or name).

What there is to see tonight from a place (the coming night, or the current one): the night's start and end, the Moon (phase, illuminated share, rise and set), the planets visible to the naked eye, the visible passes of the ISS, Tiangong and Hubble, the close pairings of the Moon and planets, and the sky's events of the night, with short highlights ready to read. Give the place's latitude and longitude and, for local times, its IANA time zone.

Link to Perigee showing an object (optionally at a date, paused or accelerated, followed), and an iframe snippet for a lesson page: the live 3D view without bars or panels. The embedding page can then drive the view and take PNG snapshots with postMessage (protocol perigee:v1, documented in `embedApi`). For a star, star_map opens its 3D map with the Sun instead (every named star at its real distance, the Sun–star distance drawn; compare adds a second star), alone in the iframe.