Chris Collins

Tools · astronomy

Sky tonight

A star chart for your place and time — the whole sky as a dome or a zoomable view, with stars to magnitude 6.5, constellations, the Milky Way, Messier objects, the planets and the Moon's phase; a time slider, search, rise/transit/set for anything you tap, a red night mode, and a mode that shows what your phone is pointing at.

Added

How to use it

A chart of the sky as it is above you now, or at any time you choose. Whole sky is the view lying on your back with your head to the north: the circle is the horizon, the zenith is the centre, north is at the top and east is on the left — hold the screen up over your head facing the way the label says and it matches the sky. Zoom is a view toward one direction, with the ground drawn in; drag to look around, pinch or scroll (or + and −) to zoom. Double-click the whole-sky chart to zoom in there.

How to use it

  • Where: pick a City (its own time zone), type a Lat / lon with its time zone (east and north positive), or choose Here and allow the browser to use your location. The place is shared by the site’s astronomy tools, so the others open on it too.

  • When: it opens at now and keeps up. Change the date and time in the box, step an hour or a day, or drag the slider through ±12 hours. ▶ plays time forward at the speed you choose — watch the sky turn, the Moon move against the stars, or a planet rise. Now goes back.

  • Tap a star, planet, the Moon, the Sun or a deep-sky object for its magnitude, colour, distance, phase, altitude and azimuth, right ascension and declination (of date), and when it rises, is highest and sets tonight. ⌖ Centre in zoom turns the zoom view to it.

  • Search finds planets, stars by name or designation (Vega, α Lyr, alpha Lyrae, HR 7001), Messier objects (M31, Orion Nebula), a few bright NGC/IC clusters and constellations.

  • Tonight (below the chart) gives sunset, sunrise and the hours of astronomical darkness, the Moon’s phase, and for each planet its rise, transit and set and how high it gets in the dark.

  • 🔴 Night (or N) turns the whole page red on black so your eyes stay dark-adapted. Turn the screen brightness down too. ⛶ Full screen (or F) gives the chart the whole screen.

  • 📱 Point uses the phone’s compass and tilt: hold it up and the chart shows the part of the sky behind the phone, with what is near the centre named at the top. On iPhone the browser asks for permission to use motion and orientation the first time. Compasses are easily upset by metal, magnets and phone cases; if the chart is turned, type a compass correction (degrees, added to the heading; also use it for your local magnetic declination). Without sensors (most laptops) the view can be dragged instead.

The checkboxes show or hide constellation lines and names, the deep-sky objects, the Milky Way, an altitude/azimuth grid and the ecliptic (the Sun’s path, near which the planets and Moon travel). Stars to mag sets the faintest stars drawn; in twilight and daylight fainter stars are dropped automatically, as they are from the real sky. For how many stars your own sky shows, see sky darkness.

Symbols: galaxies are ellipses, globular clusters circles with a cross, open clusters dashed circles, nebulae squares. Planet colours are approximate; Saturn has its rings.

How it is calculated

Positions come from the site’s astronomy core: the planets from VSOP87 (truncated, within a few arcseconds of the full theory for 1900–2100) with light time, aberration and nutation, the Moon from Jean Meeus’s Astronomical Algorithms (chapter 47, shown topocentric — as seen from your place, not the Earth’s centre), and stars precessed from J2000 to the date. Altitudes include standard atmospheric refraction, which lifts things near the horizon by about half a degree. Rise and set times are for a level sea-level horizon. The Moon’s lit side is drawn toward the Sun as you would see it from your place.

The chart is drawn on a canvas at your screen’s pixel density with a stereographic projection (it keeps shapes right, so constellations look as they do in the sky, at the cost of stretching toward the horizon).

Data and credits

  • Stars: the Yale Bright Star Catalogue, 5th revised edition (Hoffleit & Warren, 1991; NASA/CDS catalogue V/50, public domain) — all 8,404 stars to magnitude 6.5, with their colours (B−V).

  • Star names: the IAU Working Group on Star Names list.

  • Constellation lines and names, the Messier catalogue, bright deep-sky objects and the Milky Way outline: d3-celestial by Olaf Frohn (BSD 3-clause licence), simplified for size.

All of it is in one file (sky-catalogue.json, about 350 KB) that loads with the page.

Sun and moon planner (twilight, moon phases, golden hour), sky darkness (Bortle, SQM, limiting magnitude), planet events (conjunctions, oppositions, elongations), eclipses and meteor showers, satellite passes (the ISS), telescope optics, night-sky photography, orbits.

Linking to a view

Settings travel in the share link: loc (city or typed), city (e.g. reykjavik, mauna-kea, la-palma), lat, lon, tz (an IANA zone for typed coordinates), time (local YYYY-MM-DDTHH:MM; omit for now), view (dome, zoom or point), az, alt and fov for the zoom view, mag (faintest stars), night (true for red). For example the southern Milky Way from the Atacama in June.

Privacy

Everything is calculated in your browser; the only downloads are the page’s own files. If you choose Here, your position stays in this browser (it is never put in a share link), and the compass and tilt readings are used only on the page.

Runs entirely in your browser; nothing you type leaves this page.