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Illustration of a telescope dome on a mountain ridge under a night sky with a crescent Moon and a star's arc across the sky

Rozhen Observing Planner

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 →

31/100
HD 189733 — Barely
Best window 20:50–02:25 (5.58 h, marginal) · highest 71.1° at 21:15 · 2.55 quality-hours, peak 0.49 (100 = four hours at the best conditions, which must themselves be perfect)
What held it back: the target being low or below 20° cost 3.9 quality-hours; moonlight cost 2.7 quality-hours; daylight and twilight cost 1.2 quality-hours; cloud cover cost 0.4 quality-hours.
poor 19:45–20:50marginal 20:50–02:25poor 02:25–02:45 (good ≥ 0.6, marginal ≥ 0.3, poor > 0 — the product of darkness × altitude × Moon × clouds)
0°30°60°90°12:0014:0016:0018:0020:0022:0000:0002:0004:0006:0008:0010:0012:00local time (Europe/Sofia)Moontarget

Bands: daylight → civil → nautical → astronomical twilight → night. Dashed grey: the Moon. Dashed lines: 20° (airmass ~2.9) and 30° (airmass 2.0).

Night of Wednesday, 23 September 2026

Sun
  • Sunset 19:16 · sunrise 07:10
  • Civil twilight ends 19:44 · begins 06:42
  • Nautical ends 20:16 · begins 06:10
  • Astronomical night 20:49 → 05:37 (8.78 h)
Moon — waxing gibbous, 92% lit
  • Moonrise 17:53 · moonset 04:24
  • Moon-free dark time: 1.2 h of 8.78 h
Clouds over Rozhen

Mean cloud cover during astronomical night: 7% · what it was: ERA5 reanalysis

19:00cloud cover % per hour (dark = night)08:00

Hour by hour

localsky target alt Moon alt cloud %low / mid / high RH %wind km/h rain mm °Cquality
20:00 nautical twilight 66° 20 34 9 / 31 / 1 78 7.4 0.0 2.4 poor 0.05
21:00 night 71° 27 33 20 / 30 / 5 81 8.8 0.0 1.9 marginal 0.34
22:00 night 69° 32 5 5 / 2 / 0 79 8.1 0.0 2.0 marginal 0.49
23:00 night 61° 34 4 4 / 0 / 0 84 8.8 0.0 1.2 marginal 0.48
00:00 night 50° 33 6 6 / 0 / 0 86 9.8 0.0 0.9 marginal 0.45
01:00 night 39° 29 7 3 / 4 / 0 89 8.5 0.0 0.4 marginal 0.42
02:00 night 28° 22 6 1 / 5 / 0 89 6.3 0.0 -0.1 marginal 0.38
03:00 night 17° 14 2 2 / 0 / 0 90 7.1 0.0 -0.4 unusable 0.0
04:00 night 6° 4 1 1 / 0 / 0 89 6.7 0.0 -0.6 unusable 0.0
05:00 night -3° down 1 1 / 0 / 0 90 7.1 0.0 -0.9 unusable 0.0
06:00 astronomical twilight -12° down 1 1 / 0 / 0 90 6.9 0.0 -1.1 unusable 0.0
07:00 civil twilight -19° down 4 4 / 0 / 0 89 6.7 0.0 -1.1 unusable 0.0

Weather: ERA5 reanalysis — what it was. RH above ~90 % with little wind is the fog signature on this ridge.

Telescopes at Rozhen

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.

ℹ️ How it is computed, and how far to trust it
  • Sun and Moon from Meeus' Astronomical Algorithms: low-precision solar position (~1′) and a truncated lunar series (~0.3°). Rise/set use the standard horizons (−0.833° Sun, +0.125° Moon); times are good to a minute or two, which is all a plan needs.
  • Target altitude is plain spherical trigonometry from the coordinates precessed to the date (Meeus 21.3) and local sidereal time, every 5 minutes; airmass is Kasten & Young. Nutation, refraction at the target and proper motion are left out: together under 3′ for anything above 20°, nothing on a chart whose fields are degrees wide. Altitudes shown are geometric.
  • The score multiplies four factors per 5 minutes — darkness, target altitude, moonlight (illumination × closeness × height), cloud cover — and integrates them over the night, then scales by the night's peak quality; 100 = four hours at perfect conditions, and ten marginal hours cannot pass for a perfect night. Cloud ≥ 80 % zeroes a sample outright; the Moon only dims. The reasons line says which factor cost the most. Calculation version 2026-10-10.1.
  • Names go to CDS Sesame (SIMBAD / NED / VizieR). Solar-system bodies are not resolved — give coordinates for the night from JPL Horizons.
  • Weather is Open-Meteo: the hourly model forecast at the observatory's coordinates and altitude for the coming 4 days (refreshed every 30 minutes), and for past nights the ERA5 reanalysis — hourly cloud cover reconstructed from observations, 1940 onwards, about 5 days behind real time. Mountain weather is local; neither knows about the Rhodope fog in the valley below.

All times are local (Europe/Sofia). The night runs from local noon to the next local noon.

💡 Show API usage examples (cURL) 
curl -sS "https://devops.majbase.com/rozhen-planner/api?target=M31&date=2026-09-23"
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)

Response shape

{
  "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", "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: {"error": "…", "code": "target_unresolved"} with HTTP 400 when a target is given but cannot be resolved; an unparseable date is not an error — tonight is used and a warning is added. 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.