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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 →

0/100
V1057 Cyg — Not tonight
No usable window tonight. · highest 87.3° at 22:00 · 0.1 quality-hours, peak 0.1 (100 = four hours at the best conditions, which must themselves be perfect)
⚠ Overcast (≥80 % cloud) for 10.0 of 11.0 dark hours — the sky is closed whatever the geometry says.
What held it back: cloud cover cost 3.9 quality-hours; moonlight cost 3.7 quality-hours; the target being low or below 20° cost 1.9 quality-hours; daylight and twilight cost 1.3 quality-hours.
poor 22:00–23:00 (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 Friday, 25 September 2026

Sun
  • Sunset 19:13 · sunrise 07:12
  • Civil twilight ends 19:41 · begins 06:44
  • Nautical ends 20:13 · begins 06:12
  • Astronomical night 20:46 → 05:39 (8.89 h)
Moon — full moon, 99% lit
  • Moonrise 18:36 · moonset 06:40
  • Moon-free dark time: 0.0 h of 8.89 h
Clouds over Rozhen

Mean cloud cover during astronomical night: 95% · 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 68° 15 100 99 / 95 / 18 63 6.1 0.0 6.3 unusable 0.0
21:00 night 78° 25 99 16 / 98 / 74 65 7.6 0.0 5.7 unusable 0.0
22:00 night 87° 34 72 7 / 60 / 41 69 4.1 0.0 5.4 poor 0.1
23:00 night 79° 42 89 1 / 85 / 28 71 4.2 0.0 4.9 unusable 0.0
00:00 night 68° 46 92 1 / 91 / 28 73 4.7 0.0 4.3 unusable 0.0
01:00 night 58° 46 100 0 / 99 / 85 76 4.0 0.0 4.2 unusable 0.0
02:00 night 47° 43 100 11 / 100 / 83 80 3.2 0.0 4.1 unusable 0.0
03:00 night 37° 36 100 19 / 99 / 86 87 2.1 0.0 4.0 unusable 0.0
04:00 night 28° 28 100 15 / 100 / 79 89 4.2 0.0 3.9 unusable 0.0
05:00 night 19° 18 100 4 / 100 / 95 90 5.8 0.1 3.9 unusable 0.0
06:00 astronomical twilight 12° 7 100 5 / 100 / 96 94 7.0 0.1 3.6 unusable 0.0
07:00 civil twilight 5° down 100 60 / 100 / 99 97 7.0 0.1 3.6 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=2026-09-25"
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.