Site Sky Ephemerides Calculator

The Bortle scale (2001) rates sky darkness from 1 (truly dark) to 9 (inner city). Class 1: Zodiacal light casts faint shadows; M33 visible naked eye; limiting magnitude 7.6-8.0. Class 5: Rural transition; limiting magnitude 5.6-6.0. Class 8-9: City; only bright planets and a few stars visible; limiting magnitude under 4.0. Knowing your Bortle class sets realistic expectations for what objects are accessible.

Higher elevation reduces the column of atmosphere above the observer. With less air, there is less extinction (light loss), less turbulence (better seeing on high dry mountains), and reduced water vapor (less IR absorption, important for near-IR astronomy). At 3000 m, the atmospheric pressure is about 70% of sea level, reducing extinction by 30%. Eliminating the lowest, densest, most turbulent atmospheric layers dramatically improves image quality.

Transparency refers to the clarity of the atmosphere — how well it transmits light. Good transparency requires low humidity, no thin cirrus clouds, and clean air (low aerosol loading). Good transparency allows seeing fainter objects. Transparency is separate from seeing (image sharpness). You can have good transparency and bad seeing (star images blurred but sky dark) or vice versa.

Objects near the horizon traverse more atmosphere than objects at zenith. At altitude 10 degrees, airmass is about 5.6. At altitude 5 degrees, airmass exceeds 10. The denser, turbulent lower atmosphere causes blurring, extinction, and color dispersion. Observers prefer targets more than 20-30 degrees above the horizon for best results.

Tools include the Bortle map (light pollution atlas), Clear Outside app, Light Pollution Map (lightpollutionmap.info), and the International Dark-Sky Association's list of Dark Sky Places. Drive at least 50-100 km from city centers, find areas with low artificial sky glow (check satellite night-sky images), and verify with an SQM meter on site.

Astronomical twilight ends when the Sun drops 18 degrees below the horizon. Before astronomical twilight ends (evening) or after it begins (morning), the sky is not fully dark for faint object observing. At latitudes above about 50 degrees in summer, astronomical twilight never fully ends around the summer solstice (continuous civil or nautical twilight). This limits the dark observing window to winter months at high latitudes.

Absolutely. Even the best sites have cloudy nights. Weather is the primary limiting factor for ground-based astronomy. Sites are chosen partly for their fraction of clear nights per year (photometric nights). Mauna Kea has about 300 clear nights per year; La Palma about 280. Humidity, dust (Saharan dust events affect Canary Islands) and local orographic clouds all reduce the usable observing time.

Light pollution is artificial sky brightening from cities and towns. It scatters off aerosols and molecules in the atmosphere, raising the sky background above its natural level. Measured as sky brightness in mag/arcsec^2, light pollution can reduce limiting magnitude by 2-4 magnitudes compared to a pristine dark site. Blue-rich LED lighting is particularly harmful because it scatters more strongly in the atmosphere.

The full Moon raises sky background dramatically, limiting faint object observing. A full Moon in a clear sky can produce SQM readings of 17-18 even at a normally dark site. Observers typically schedule deep-sky observing around new Moon (within a week of new Moon). Planets, double stars, and bright clusters are less affected and can be observed in any phase.