Arcsecond.io API

Free Arcsecond.io objects API with no key: resolve an astronomical object name to coordinates in four systems, plus aliases, spectral type and physical parameters.

No API key requiredCORS enabledHTTPSFree tier

Endpoint tested and returned HTTP 200 on 21 Aug 2026

What is the Arcsecond.io API?

The Arcsecond.io objects endpoint is a free, key-free lookup that resolves an astronomical object name to a single tidy JSON record: equatorial, ecliptic, galactic and supergalactic coordinates, catalogue aliases, and physical parameters where they are known.

Arcsecond is an observatory-management platform, and this endpoint is the free public corner of it: hand it a name like `M1` and get back a flat, immediately usable object record. Underneath it queries SIMBAD — the response says so in a `source` field — but it reshapes the result into ordinary nested JSON with named keys, sparing you the TAP metadata-and-rows dance and the ADQL.

The convenience costs completeness. Fields like `parallax`, `distance`, `mass` and `effective_temperature` are frequently null even for well-studied objects, and `constellation` came back empty for the Crab Nebula in the example below. The genuinely useful parts are the four coordinate systems, all pre-converted with their J2000 epoch stated, and the `aliases` array, which for a famous object runs to dozens of survey designations. Use it for name resolution and quick coordinate lookups; go to SIMBAD or VizieR when you need measured quantities with provenance.

Quick facts

Base URL: https://api.arcsecond.io
Authentication: No API key for the public object endpoints; other parts of the Arcsecond platform require an account. The underlying data is derived from SIMBAD at CDS, whose acknowledgement conventions apply.
Rate limit: No published limit on the public endpoints. Cache resolved objects — coordinates do not change.
Pricing: Free for the public object lookup endpoints.
CORS: Enabled — callable directly from browser JavaScript
Official docs: Read the docs

How to use the Arcsecond.io API

Every request below was executed against the live API on 21 Aug 2026, and the response shown is the real body it returned — not an illustration.

1. Resolve the Crab Nebula by its Messier designation

GET https://api.arcsecond.io/objects/M1/

curl

curl 'https://api.arcsecond.io/objects/M1/' \
  -H 'Accept: application/json'

JavaScript (fetch)

const res = await fetch("https://api.arcsecond.io/objects/M1/", {
  headers: {
    "Accept": "application/json",
  },
});
if (!res.ok) throw new Error(`Request failed: ${res.status}`);
const data = await res.json();
console.log(data);

Python (requests)

import requests

headers = {
    "Accept": "application/json",
}

res = requests.get("https://api.arcsecond.io/objects/M1/", headers=headers, timeout=20)
res.raise_for_status()
print(res.json())

Response — HTTP 200 (truncated)

{
  "name": "M1",
  "source": "Simbad",
  "constellation": "",
  "equatorial_coordinates": {
    "right_ascension": 83.6324,
    "declination": 22.0174,
    "epoch": 2451545.0
  },
  "ecliptic_coordinates": {
    "longitude": 84.098,
    "latitude": -1.296,
    "epoch": 2451545.0
  },
  "galactic_coordinates": {
    "longitude": 184.559,
    "latitude": -5.785,
    "epoch": 2451545.0
  },
  "super_galactic_coordinates": {
    "longitude": 358.32,
    "latitude": -47.436,
    "epoch": 2451545.0
  },
  "spectral_type": null,
  "morphological_type": null,
  "proper_motion": null,
  "parallax": null,
  "radial_velocity": null,
  "age": null,
  "mass": null,
  "radius": null,
  "distance": null,
  "metallicity": null,
  "effective_temperature": null,
  "aliases": [
    {
      "name": "4FGL J0534.5+2201i",
      "vizier_catalogue_url": null
    },
    {
      "name": "2AGL J0534+2205",
      "vizier_catalogue_url": null
    },
    {
      "name": "eHWC J0534+220",
      "vizier_catalogue_url": null
    },
    {
      "name": "2HWC J0534+220",
      "vizier_catalogue_url": null
    },
    {
      "name": "3HWC J0534+220",
      "vizier_catalogue_url": null
    },
    {
      "name": "1RXS J053431.2+220218",
      "vizier_catalogue_url": null
    },
    {
      "name": "SRGA J053431.8+220103",
      "vizier_catalogue_url": null
    },
    {
      "name": "PCCS2 030 G184.54-05.78",
      "vizier_catalogue_url": null
    },
    {
      "name": "SNR G184.6-05.8",
      "vizier_catalogue_url": null
    },
    {
      "name": "1H 0531+219",
      "vizier_catalogue_url": null
    },

Parameters

Parameter	Type	Required	Description
`object_name`	path segment	Required	Object name or catalogue designation. Messier, NGC, HD and many survey identifiers resolve. `M1`
`(trailing slash)`	path	Required	The endpoint requires a trailing slash; without it you get a redirect. `/objects/M1/`

Response fields

namestring: The name as resolved, echoing what you asked for.
sourcestring: Where the record came from — `Simbad` for most objects, which tells you the real authority behind the answer.
equatorial_coordinatesobject: Right ascension and declination in degrees with the epoch as a Julian date (2451545.0 is J2000).
ecliptic_coordinates / galactic_coordinates / super_galactic_coordinatesobject: The same position pre-converted into three other systems, which is this API's main convenience.
aliases[]array: Every catalogue designation known for the object, each with an optional VizieR catalogue link.
spectral_type / parallax / distance / mass / radiusvaries: Physical parameters where known. Frequently null — absence here does not mean the value is unmeasured, only that this API does not carry it.

What you can build with the Arcsecond.io API

Resolve an object name to coordinates for a telescope pointing script
Convert a position between equatorial, galactic and ecliptic frames
Look up alternative catalogue designations for an object
Validate user-entered object names in an observing planner
Build a simple astronomy lookup without learning ADQL

Common errors and how to fix them

404

The name did not resolve.

Fix: Try a canonical designation — `M1`, `NGC 1952` or `HD 1` rather than a colloquial name. Solar system bodies are not covered at all.

301 redirect

The trailing slash was omitted.

Fix: The path is `/objects/{name}/` with the trailing slash. Follow redirects or include it.

Null physical parameters

The record simply does not carry them.

Fix: Query SIMBAD or VizieR directly when you need measured quantities with uncertainties and references.

Empty constellation

The field is not populated for all object types.

Fix: Derive the constellation from the coordinates if you need it, using the IAU boundary definitions.

Arcsecond.io API — frequently asked questions

Is the Arcsecond.io API free?

The public object endpoints are free and need no key. Arcsecond is a commercial observatory-management platform, so other parts of its API require an account, but object resolution does not.

Where does the data come from?

Mostly SIMBAD at CDS, which the response states explicitly in its `source` field. Arcsecond reshapes SIMBAD's output into flat JSON with pre-converted coordinate systems.

Why are so many fields null?

The endpoint returns a fixed schema and leaves fields empty when it does not carry a value, which is common even for well-studied objects. A null here means this API lacks the value, not that the quantity is unknown to astronomy.

Does it cover planets and asteroids?

No. It resolves objects outside the solar system. For planets, moons, asteroids and comets you need an ephemeris service such as JPL Horizons or the Small-Body Database.

Tools that pair with this API

JSON Formatter

Format, beautify and minify JSON online with 2-space, 4-space or tab indentation. Sort keys alphabetically and catch syntax errors instantly — free and private.

Coordinate Converter

Convert GPS coordinates between decimal degrees, degrees-minutes-seconds and decimal minutes instantly. Paste any format like 40°26'46"N and copy the result.

Unit Converter

Convert between units of length, weight, temperature, area, volume, speed, time and data storage instantly, with a swap button and common conversion tables.

Alternatives to Arcsecond.io API

If Arcsecond.io does not fit, these science & space apis cover similar ground. Every one was called before publishing, and each links to its own reference page with a working curl command and the response it returned.

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NCI CACTUS Resolver	Free NCI CACTUS Chemical Identifier Resolver with no key: convert any chemical name, CAS number, InChI or SMILES into any other representation. One URL, plain text response.	No key	Yes	21 Aug 2026

All Science & Space APIs

Arcsecond.io is an independent third-party service and is not affiliated with ByteTools or ByteVancer. Details on this page were verified on 21 Aug 2026; always check the official documentation before relying on this API in production, as terms and limits can change.