Observing planner

Each object is rated easy, moderate, hard, very hard or not visible for that sky and aperture at its best moment, then ranked by how high it gets in the dark hours, that rating, and the Moon. Stars and clusters are judged by their brightness against the telescope’s limit; galaxies and nebulae by their surface brightness (magnitude spread over their area) against the sky’s, with a bonus for aperture. The Moon brightens the sky according to its phase, height and distance from the object — near a full moon most galaxies drop out and the clusters and doubles rise to the top. It is a rough guide, made to put the likely things first; a nebula filter, your eyes and the transparency on the night matter as much.

★ adds an object to My list (kept in this browser, sorted by when each is highest; download it as JSON). Log keeps a session log — the object, the time, seen or not, telescope, eyepiece, magnification, seeing and transparency (1–5), the sky and notes — and downloads it as CSV or JSON (and imports a JSON log back). Point a GoTo mount at it hands the object to the telescope mount page.

Forecast asks Open-Meteo for the hours of the night: total, low, mid and high cloud, humidity, temperature, dew point, wind and gusts, with a verdict. High cloud and humidity spoil transparency even when the sky looks clear; when the temperature comes within 2 °C of the dew point your optics will dew up without a heater or a dew shield. Seeing (how steady the air is) is not forecast.

Messier marathon finds the best nights of a spring for seeing all 110 Messier objects in one night — the nights round the new moon of March or early April, each scored by how many objects a running order can catch from your place — and gives that order with times and altitudes: at each moment the object that will be lost soonest, so the galaxies setting in the west at dusk come first (M74, M77) and the globulars rising into the dawn last (M72, M73, M2, M30). Set the minutes per object, the lowest altitude and how dark the sky must be (the Sun 12° down is usual). From much north of 35° some objects are out of reach.

Works in any current browser, on a phone too.

Positions (J2000) are from SIMBAD (CDS, Strasbourg); magnitudes, sizes and names are the values commonly quoted in the Messier and Caldwell lists (public facts; the compilation is mine). Sun and Moon from the site’s astronomy core (Meeus, Astronomical Algorithms, and VSOP87). Surface brightness is the object’s magnitude spread over the ellipse of its quoted size — an average: many galaxies have a bright core and faint arms. The naked-eye limit from a sky reading uses the usual fit to Schaefer’s model; Bortle classes are mapped to typical SQM readings (class 1: 21.95, 4: 21.0, 5: 20.1, 7: 18.7, 9: 17.6).

The place, list and log stay in this browser (localStorage). Nothing leaves it until you press Get the forecast: that sends the place’s latitude and longitude, rounded to 0.001°, to Open-Meteo (free, no key, CORS) and nothing else. Your location from the browser is never put in a link.

Related: sky tonight (a star chart), sky darkness, planet events, eclipses, sun and moon times, planet details, telescope mount, telescope optics, night-sky photography, satellite passes.