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API · /sidereal-api

Sidereal Time API

healthy 3,133 Subscribers

Sidereal-time astronomy as an API, computed locally and deterministically. The gmst endpoint computes the Greenwich Mean Sidereal Time for a UT date and time, GMST = 18.697374558 + 24.06570982441908·(JD − 2451545.0) hours modulo 24, returning it in hours, degrees and hours-minutes-seconds together with the Julian Day — sidereal time tracks the stars rather than the sun and gains about three minutes and fifty-six seconds each day. The lst endpoint adds the observer's longitude to give the Local Sidereal Time, LST = GMST + longitude/15 (east positive), which equals the right ascension of any star currently crossing the local meridian. The hour-angle endpoint computes the hour angle of a celestial object, HA = LST − RA, from its right ascension and the local sidereal time (or a date, time and longitude): an hour angle of zero means the object is on the meridian at its highest point, a positive hour angle means it is west of the meridian and setting, and a negative one means it is east and rising. Dates are YYYY-MM-DD and times HH:MM:SS in UT, longitude in degrees and right ascension in hours. Everything is computed locally and deterministically, so it is instant and private. Ideal for astronomy, telescope-control, planetarium, observatory and astrophotography app developers, star-pointing and transit tools, and astronomy education. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is sidereal time; for the sun's position use a solar-position API and for sunrise and sunset times a sunrise API.

#sidereal-time #astronomy #gmst #hour-angle #right-ascension #developer-tools
api.oanor.com/sidereal-api
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Machine-readable spec so AI agents can integrate this API.

/api/sidereal-api/openapi.json
/api/sidereal-api/llms.txt

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Sidereal Time API — live data on the oanor API marketplace

API health

healthy
Uptime
100.00%
Server probes · 24h
Avg latency
76 ms
Server probes · 24h
Subscribers
3,133
active
Total calls
76
last 7 days
status Full status page → · 12 probes/24h

Pricing

Pick a tier — billed monthly, cancel anytime.

Free

Free

  • 3,300 calls / month
  • 2 requests / second
  • Hard cap (429 above quota, no overage)
  • 3,300 calls/month
  • 2 req/sec
  • GMST + LST + hour angle
  • No credit card
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Starter

€6.00 /month

  • 41,500 calls / month
  • 6 requests / second
  • Hard cap (429 above quota, no overage)
  • 41,500 calls/month
  • 6 req/sec
  • Hours/degrees/HMS, meridian transit
  • Email support
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Pro

€16.00 /month

  • 233,000 calls / month
  • 15 requests / second
  • Hard cap (429 above quota, no overage)
  • 233,000 calls/month
  • 15 req/sec
  • Telescope-control & planetarium pipelines
  • Priority support
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Mega

€53.00 /month

  • 1,355,000 calls / month
  • 40 requests / second
  • Hard cap (429 above quota, no overage)
  • 1,355,000 calls/month
  • 40 req/sec
  • Platform scale
  • Dedicated SLA
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api.oanor.com/constellations-api

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Observatory Codes API

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Sundial API

Sundial gnomonics maths as an API, computed locally and deterministically — the hour-line, gnomon and longitude-correction numbers a dial maker, horologist or astronomy hobbyist lays a sundial out with. The hour-line-angle endpoint gives the angle of each hour line on the dial plate, measured from the noon line: for a horizontal dial tan(angle) = sin(latitude) × tan(hour angle), and for a vertical south-facing dial cos(latitude) is used instead, where the hour angle is 15° per hour from solar noon. At 50° latitude the 1-o'clock line sits about 11.6° from noon rather than 15° — the lines bunch near noon and spread toward the ends, which is exactly why a sundial's hours are unevenly spaced. The gnomon endpoint gives the style angle: the gnomon's shadow-casting edge must point at the celestial pole, so it rises at the latitude angle on a horizontal dial (50° at 50° N) and at 90° − latitude on a vertical dial — get this wrong and the dial keeps correct time at only one season. The longitude-correction endpoint converts the dial's local apparent time to clock time: 4 minutes of time per degree of longitude, correction = 4 × (reference meridian − local longitude), so a dial at 7.5° E on Central European Time reads 30 minutes slow versus the clock. Everything is computed locally and deterministically, so it is instant and private. Ideal for sundial-design and gnomonics tools, astronomy-education and maker apps, and horology calculators. Pure local computation — no key, no third-party service, instant. Add the equation of time for full clock accuracy. 3 compute endpoints. For the sun's position use a solar-position API; for sunrise and sunset a sunrise API.

api.oanor.com/sundial-api

Frequently asked questions

Quick answers about pricing, quotas, and integration.

How do I get an API key for Sidereal Time API?
Sign up for free at oanor.com, generate an API key from the developer dashboard, and call Sidereal Time API with the x-oanor-key header. No credit card needed for the free tier.
What's the rate limit for Sidereal Time API?
Free tier allows 1 request per second. Paid plans scale up to 50 requests per second on the Mega tier. Hard limits return HTTP 429 above the quota — no surprise overage charges.
How much does Sidereal Time API cost?
Sidereal Time API has a free tier with 100 calls / month. Paid plans start at €6.00 / month with higher quotas and faster rate limits.
Can I cancel my subscription anytime?
Yes. Plans are billed monthly and you can cancel anytime from your billing dashboard. No long-term contracts and no cancellation fee.
Is Sidereal Time API GDPR-compliant?
All requests to Sidereal Time API go through our EU-based gateway. Your upstream API key never leaves our server and no personal data is shared with the upstream provider beyond the request you send.

Pick an endpoint from the list on the left to see its details and try it.

Code snippets

Sign up to get an API key, then call any path under your slug.

curl https://api.oanor.com/sidereal-api/SOME_PATH \
  -H "x-oanor-key: oanor_test_..."
const res = await fetch("https://api.oanor.com/sidereal-api/SOME_PATH", {
  headers: { "x-oanor-key": "oanor_test_..." }
});
const data = await res.json();
$ch = curl_init("https://api.oanor.com/sidereal-api/SOME_PATH");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, ["x-oanor-key: oanor_test_..."]);
$response = curl_exec($ch);
import requests
r = requests.get(
    "https://api.oanor.com/sidereal-api/SOME_PATH",
    headers={"x-oanor-key": "oanor_test_..."},
)
print(r.json())

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