View constellations and stars for any location and date. Identify stars by magnitude, filter by constellation, and learn mythology.
Enter a location and a date and this tool draws the sky an observer standing there would see that night: constellation figures with connecting lines, stars rendered brighter or dimmer by apparent magnitude, each clickable for its name and magnitude. The same date gives different skies in Sydney and Seattle, because the chart is built from the observer's latitude, longitude, and exact time. The usual surprise is the clock: the sky rotates once a day, so 9 PM and midnight are different skies. It all runs in the browser; the location stays on your machine.
A concrete input and expected output from the current implementation.
Input
Location: New York City (40.7 N, 74.0 W). Date: 2026-01-15, 21:00 local time.
Expected output
The chart draws Orion high in the southern sky: the three belt stars in a straight row, orange Betelgeuse labeled near magnitude 0.5 at the upper left, blue-white Rigel near magnitude 0.1 at the lower right, and Sirius, the brightest star in the night sky at magnitude -1.5, low in the southeast. Stars are drawn larger and brighter as magnitude numbers get smaller; fainter stars are absent.
Orion's center sits on the celestial equator with a right ascension of about 5.5 hours, so on a January evening it is high in the south from northern latitudes, while Sirius, at declination about -17 degrees, can rise only about 33 degrees above New York's horizon, and at 9 PM it is still climbing in the southeast. The magnitude labels follow the stars' published apparent brightness.
Every star has celestial coordinates, right ascension and declination. For the chosen location and moment, the chart places each star by its hour angle, which combines those fixed coordinates with the local sidereal time, so stars rise in the east and set in the west as date and time change. Stars below the horizon are not drawn. Apparent magnitude sets dot size and brightness, and only brighter stars get labels.
The chart connects each constellation's named stars with its pattern lines, and the filter restricts the drawing to one constellation at a time. Filtering changes only lines and labels, never positions: a star stays at the same spot on the sky. Clicking a constellation opens its mythology summary; clicking a star shows its name, magnitude, and home constellation. Only catalogued stars appear, so the faintest stay absent.
The sky rotates about 15 degrees per hour, so each hour of clock time swings the sky from east to west by roughly an hour of right ascension. Stars also rise about 4 minutes earlier every night because the sidereal day is 23 hours 56 minutes, so charts for the same date at different times, or different dates at the same time, are different skies - expected, not a bug.
Yes. Enter the location and the chart is drawn for that latitude and longitude, so a southern latitude gives the southern sky: the Southern Cross (Crux) appears, and Polaris drops below the northern horizon. Remember that southern seasons are opposite, so a January evening chart from Sydney is midsummer sky, not the winter sky a northern chart would show.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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