Will a target be observable from the National Astronomical Observatory Rozhen (41.693° N, 24.739° E, 1750 m) on a given night — and what does the night itself look like: sunset and twilights, the Moon, the target's altitude hour by hour, and the cloud forecast over the Rhodopes.
Try: M31 · M42 · V1057 Cyg · HD 189733 · NGC 7000 · Vega · 3C 273 · ← previous night · next night → · plan a whole night with several targets → · best nights for this target over the next 30 →
Bands: daylight → civil → nautical → astronomical twilight → night. Dashed grey: the Moon. Dashed lines: 20° (airmass ~2.9) and 30° (airmass 2.0). Brown: the terrain horizon under the target's path.
Mean cloud cover during astronomical night: 9%
ERA5 reanalysis — a historical reconstruction from a global model, not a measurement at the observatory.
| local | sky | target alt | Moon alt | cloud % | low / mid / high | RH % | wind km/h | rain mm | °C | quality |
|---|---|---|---|---|---|---|---|---|---|---|
| 21:00 | nautical twilight | 55° | 19 | 10 | 0 / 1 / 9 | 65 | 3.2 | 0.0 | 17.4 | poor 0.1 |
| 22:00 | night | 65° | 18 | 2 | 0 / 1 / 1 | 63 | 2.4 | 0.0 | 17.2 | good 0.77 |
| 23:00 | night | 76° | 15 | 2 | 0 / 1 / 2 | 62 | 3.1 | 0.0 | 17.0 | good 0.78 |
| 00:00 | night | 86° | 10 | 38 | 0 / 3 / 37 | 64 | 5.0 | 0.0 | 16.4 | marginal 0.47 |
| 01:00 | night | 82° | 3 | 15 | 0 / 0 / 15 | 66 | 5.8 | 0.0 | 16.2 | good 0.69 |
| 02:00 | night | 71° | down | 4 | 0 / 2 / 2 | 68 | 4.9 | 0.0 | 15.8 | good 0.96 |
| 03:00 | night | 60° | down | 0 | 0 / 0 / 0 | 73 | 4.8 | 0.0 | 14.5 | good 1.0 |
| 04:00 | night | 50° | down | 0 | 0 / 0 / 0 | 73 | 5.0 | 0.0 | 14.4 | good 0.93 |
| 05:00 | astronomical twilight | 40° | down | 0 | 0 / 0 / 0 | 75 | 5.0 | 0.0 | 13.8 | marginal 0.43 |
| 06:00 | nautical twilight | 30° | down | 0 | 0 / 0 / 0 | 76 | 4.2 | 0.0 | 13.6 | poor 0.12 |
Weather: ERA5 reanalysis — what it was. RH above ~90 % with little wind is the fog signature on this ridge. Download: hourly CSV · 5-minute track CSV · JSON
The Rhodope ridges as seen from the dome: highest 1.34° (azimuth 26°, 5.2 km away), mean 0.0°, from SRTM 30 m with refraction. Dots: the target's path below 15°. A target behind the ridge counts as not visible, whatever its altitude. JSON
The 20°/30° limits are the usual photometric ones (airmass ~2.9 / 2.0), not the mechanical limits of any instrument — those, the dome, and the observing schedule are the observatory's to confirm.
2026-10-10.1.| source | kind of data | when |
|---|---|---|
| Open-Meteo forecast | model prediction, hourly, refreshed every 30 min | the last week to 4 days ahead |
| ERA5 reanalysis | historical reconstruction from a global model and observations — not a measurement at the observatory | 1940 → about 5 days ago |
| none | astronomy only; overall mark Unknown | further ahead, or Open-Meteo down |
All times are local (Europe/Sofia). The night runs from local noon to the next local noon.
curl -sS "https://devops.majbase.com/rozhen-planner/api?target=M31&date=2026-08-22"
curl -sS "https://devops.majbase.com/rozhen-planner/api?target=83.63+22.01" # RA Dec in degrees
curl -sS "https://devops.majbase.com/rozhen-planner/api" # just the night (Sun, Moon, clouds){
"night_date": "2026-10-09",
"site": {"name": "NAO Rozhen", "lat": 41.693, "lon": 24.739, "elev_m": 1750},
"night": {"sunset": "…Z", "sunrise": "…Z", "twilight": {"civil": {…}, "nautical": {…}, "astronomical": {…}},
"dark_from": "…Z", "dark_to": "…Z", "dark_hours": 9.55,
"moonrise": "…Z", "moonset": "…Z", "moon": {"illumination": 0.007, "phase": "new moon", "waxing": false},
"moon_free_dark_hours": 9.55},
"target": {"ra": 10.68, "dec": 41.27, "ra_of_date": 11.03, "dec_of_date": 41.41,
"transit": {"t": "…Z", "alt": 89.6, "az": 187.9, "airmass": 1.0},
"good_hours": 9.58, "usable_hours": 9.58, "moon_separation": 144.4,
"verdict": "good", "summary": "…", "resolved": {"name": "M31", "common_name": "Andromeda Galaxy", "type": "AGN"},
"track": [{"t": "…Z", "alt": 12.3, "az": 41.0, "airmass": 4.6}, …]},
"assessment": {"score": 84, "verdict": "good", "headline": "Worth it",
"score_astronomy": 92, "verdict_astronomy": "good", "score_weather": 71, "weather_known": true,
"quality_hours": 3.4,
"best_window": {"label": "good", "start_local": "22:30", "end_local": "02:15", "hours": 3.75, "mean_quality": 0.82},
"windows": [{"label": "marginal", "start_local": "20:30", "end_local": "22:30", …}, …],
"hourly": [{"time": "21:00", "sky": "night", "alt": 45, "moon_alt": -12, "cloud": 20, "low": 5, "mid": 0, "high": 20,
"rh": 71, "wind": 9.4, "precip": 0, "temp": 6.1, "q": 0.78, "label": "good"}, …],
"reasons": ["cloud cover cost 2.1 quality-hours"], "hard_limit": null, "cost": {…}},
"forecast": [...hourly rows...], "forecast_source": "forecast", "forecast_updated_at": "…Z",
"calculation_version": "2026-10-10.1", "warnings": []
}Errors are never silent: an unreadable date is HTTP 400 {"error": "…", "code": "invalid_date"}, never a quiet fallback to tonight. Three marks: score_astronomy (darkness × altitude × Moon, always known), score_weather (clouds over the dark hours; null without a forecast or history) and score, the overall — null with verdict: "unknown" when the weather is not known, never a geometry-only number dressed up as a promise. Errors: {"error": "…", "code": "target_unresolved"} with HTTP 400 when a target is given but cannot be resolved; date is the evening the night starts on, as 2026-10-09, 2026.10.09 or 09.10.2026; omitted = tonight. The response carries night (sunset/sunrise, twilights, dark window, Moon), target (coordinates, transit, dark hours above the limits, Moon separation, verdict of good / marginal / poor / none / never, and the 5-minute track of altitude, azimuth and airmass) and forecast (hourly rows for the night). Times are ISO 8601 UTC.