Star Chart for a Specific Date and Location

Date

Day Month Year

Time

Town

Words on your poster (optional) Show my sky

Opens the generator with these filled in, as one round chart of the whole sky. The time is the clock time in that town; its time zone and summer time are applied for you.

Not a decorative approximation: this chart is computed star-by-star for the date, time and coordinates you enter, with the math open to inspection.

Generate your star chart
Free, in your browser, with no account and nothing to buy. The SVG, the 300 dpi PNG and the PDF (a real page of the size you pick, from 21 × 29.7 cm to 70 × 100 cm) are each the complete poster.

The pipeline is classical positional astronomy:

1. Local time → UTC → Julian Date (Meeus, Astronomical Algorithms, ch. 7; a date before 15 October 1582 is read in the Julian calendar unless you switch it to Gregorian).
2. Julian Date → Greenwich sidereal time → local sidereal time (IAU 1982 polynomial).
3. J2000 → your date: every catalogue position is rotated from the year-2000 equator and equinox to those of your date by long-term precession (Vondrák, Capitaine & Wallace 2011, as in the IAU’s SOFA library).
4. Per star: that right ascension/declination + your latitude + sidereal time → altitude and azimuth.
5. Projection: everything above the horizon is mapped stereographically onto a disc — zenith at centre, horizon at the rim.

The chart includes the 1,630 stars of the Yale Bright Star Catalogue down to magnitude 5.0 with brightness-scaled, colour-tinted dots, 89 IAU constellation figures clipped at the horizon, the Milky Way band, compass points, an optional altitude grid, and name labels for the brightest stars visible from your spot.

Print an observing chart for tonight

Most free printable sky charts are monthly sheets drawn for one latitude (40° north is the usual choice), so from Copenhagen or Sydney their horizon is in the wrong place. Pick Layout → Observing chart, or open the generator with it already chosen, on tonight’s date, and you get two A4 or US Letter pages for your own town and hour, in black ink on white so they print well on a mono printer:

• Page 1, the sky to hold overhead: the horizon at the rim, a cross at the zenith, all eight compass points (east on the left, as when you look up), a magnitude key and a symbol key for stars, planets, the Moon and five kinds of deep-sky object.
• Page 2, “Tonight from …”: the Moon’s phase, rise and set; every planet that is up, with its magnitude, direction and height now and the times it rises, stands highest and sets; 8–12 deep-sky highlights at least 25° up at your chosen time, each marked naked eye, binoculars or small telescope; and the brightest stars to start from.

A worked example: Copenhagen, 24 December 2024 at 21:00. The sheet lists Jupiter rising at 14:17 (NASA/JPL Horizons: 14:17), highest at 22:45 and 56° up in the south (Horizons: 22:46, 56.2°); the Moon as a 33%-lit waning crescent that rises at 02:45; and the Pleiades, the Andromeda Galaxy and the Double Cluster as naked-eye targets. Its last line reads “Good from 20:00 to 22:00 tonight, about 4 minutes earlier each night: 19:32 to 21:32 by 31 Dec.” That drift is real: the Earth turns once against the stars in 23 h 56 min, so the same sky returns 3 min 56 s earlier every night, half an hour a week.

Its limits: rise and set times assume a flat, open horizon (hills or houses delay a rising by minutes); the deep-sky pick comes from a list of 133 objects (the Messier catalogue plus 23 showpieces), brightest first, and a total magnitude flatters big, faint galaxies; “naked eye” means from a dark site, not under street lights; and the sheet’s words are in English whatever the poster language.

Accuracy, stated plainly

• Precession is applied, once, to everything on the chart: 0.36° by 2026 and 13° by 1066, so without it a medieval sky would be badly wrong and even a modern one a third of a degree off. Nutation and aberration (each under 21 arcseconds) are not, and do not show at any print size.
• Proper motion is not modelled: the catalogue keeps each star at its year-2000 place. Over 2,000 years that leaves Alpha Centauri 2.1°, Arcturus 1.3° and Sirius and Procyon 0.7° each from where they really stood (about 1°, 0.6° and 0.3° for a chart of 1066), and every other star of magnitude 1.5 or brighter under 0.4°. For a date within a century of 2000 it stays under 0.14° even for the fastest-moving star in the catalogue, the faint Epsilon Indi.
• Atmospheric refraction is ignored (it lifts stars near the horizon by up to ~0.5°).
• The Moon is drawn at its true altitude, azimuth and illuminated phase, from the standard truncated ELP-2000/82 series (~0.003°, checked against almanac values). It is placed where you would actually see it: an almanac's Moon position is measured from the centre of the Earth, and you are standing an Earth radius off that line, which moves the Moon by up to ~1° near the horizon — enough to decide whether it is up at all. That correction (diurnal parallax) is applied for your latitude and longitude. What is left is the ~0.003° series error plus the refraction noted above.
• Mercury, Venus, Mars, Jupiter and Saturn are drawn from the Keplerian elements JPL publishes for them (E. M. Standish), carried to the Earth and through the same precession to your date as the stars. Checked against NASA/JPL Horizons at six dates from 1900 to 2049, the worst planet is 0.16° off (Saturn, whose elements are the roughest) and the rest within 0.1°; at 1700, 1750 and 2150, outside the elements' best range, the worst is 0.27°. Each is sized by its brightness that night (Mallama & Hilton, 2018, within 0.1 magnitude of Horizons), though Venus and Jupiter are capped at the size of the brightest star dot rather than drawn larger. Outside 3000 BC–3000 AD, where the elements are not valid, no planet is drawn.
• The Sun is drawn whenever it was above your horizon (Meeus's solar series, carried through the same precession as the stars; within 0.01° of NASA/JPL Horizons), and the line under the chart names the light at that moment from the Sun's real altitude: daylight above −0.833°, civil twilight to −6°, nautical to −12°, astronomical to −18°, full dark below. The stars are always drawn where they stood, so this is what tells you daylight or twilight hid them.
• The Milky Way is drawn as a five-step band whose centre line is the IAU galactic equator, carried through the same precession and horizon clip as the stars, so it runs within a degree of where the real band's midline does. Its outline and star clouds (Sagittarius, Scutum, Aquila, Cygnus, Carina, Norma) are a hand-drawn sketch of the naked-eye band, not an isophote traced from a survey, so its edges can be several degrees off.
• Satellites and faint comets are not drawn. The 21 great naked-eye comet apparitions since 1900, Hale-Bopp and NEOWISE among them, are drawn when you tick Great comets (Hale-Bopp over London on 1 April 1997), the tail in its true anti-solar direction but its length illustrative. Uranus, Neptune and 133 deep-sky objects (all 110 Messier objects plus 23 southern and binocular showpieces) appear when you tick Deep-sky objects (open it ticked on a winter evening, London on 24 December 2025 at 21:00, where the Orion Nebula (M42) is marked in Orion’s sword and the Pleiades (M45) and the Andromeda Galaxy (M31) are higher up), and the Observing chart below always carries its own pick of them. A solar or lunar eclipse is drawn when your moment falls inside one seen from your place: the Moon over the Sun to scale, or the Moon in the Earth’s shadow.
• The engine passes 44 automated checks against textbook values (J2000 epoch, GMST, pole altitude = latitude, zenith culmination, projection orientation).

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