Every point of light is a catalogued star with celestial coordinates (α, δ) — right ascension and declination — and a visual magnitude m, defined on Pogson's logarithmic scale:

A star's color is not arbitrary: it comes from its B−V color index, which encodes surface temperature. Blue stars (B−V < 0) are young and hot; orange and red ones (B−V > 1) are cool, old giants. The data come from the AT-HYG v4.0 catalog, which combines measurements from Hipparcos, Tycho-2 and Gaia: 865,000 real stars, down to magnitude 11.

The positions of the planets — Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune — are computed with the VSOP87 planetary theory (Variations Séculaires des Orbites Planétaires), developed by P. Bretagnon and G. Francou at the Bureau des Longitudes in Paris.

VSOP87 breaks each planet's motion into hundreds of periodic terms that describe the gravitational pull of every body in the solar system on the others. Each heliocentric position comes from evaluating trigonometric series of the form:

where τ is Julian millennia since J2000.0. We also apply light-time correction (we see the planets where they were, not where they are), IAU 1980 nutation (63 terms), annual aberration (~20″), precession and conversion to the FK5 frame. Accuracy: better than 1 arcminute for the inner planets and 2 for the outer ones.

References: Bretagnon & Francou — Astronomy & Astrophysics 202, 309 (1988); Jean Meeus — Astronomical Algorithms, 2nd ed. (Willmann-Bell, 1998).
Jean Meeus (1928–2024), a Belgian astronomer, spent decades turning centuries of celestial mechanics into precise, accessible algorithms. Without his work, this map would not exist.

The Moon's position is computed with the ELP2000/82 theory by M. Chapront-Touzé and J. Chapront, which models its motion with 120 periodic terms in longitude, latitude and distance. Accuracy is better than 10 arcseconds in longitude.

Unlike the planets, the Moon is so close that where you stand on Earth matters: topocentric parallax can shift its apparent position by up to a full degree (twice its apparent diameter). We correct for it using the geometry of the Earth's ellipsoid.

Its phase, illuminated fraction and the tilt of its bright limb follow Meeus's equations (ch. 48), including the parallactic angle, so the shadow is rotated correctly for your location. The lunar seas are drawn at their real selenographic coordinates, corrected for optical libration.

Reference: Chapront-Touzé & Chapront — Astronomy & Astrophysics 124, 50 (1983); Meeus, ch. 47–48.

The sky is a sphere — the map is a circle. The hour angle H turns equatorial coordinates (α, δ) into altitude a and azimuth A for an observer at latitude φ:

where H = θ_L − α, and θ_L is the Local Sidereal Time derived from the Julian Day of the observation. Finally, an azimuthal equidistant projection places every star on the flat disc:

The zenith sits at the center, North at the top and East on the left — the classic planisphere layout that lets you hold the map up against the real sky.

Reference: Jean Meeus — Astronomical Algorithms, ch. 12 (Sidereal Time) and 13 (Coordinate Transformations).

The map includes the 88 constellations recognized by the International Astronomical Union (IAU), drawn with their official figures. They are projected together with the rest of the sky — part of the same calculation, not a separate overlay.

Names appear in Latin (Orion, Crux, Scorpius, Ursa Major…), the IAU's international standard. StarMap was born in Chile 🇨🇱, so you can also switch them to Spanish.

Each constellation label sits at the geometric center of its visible stars. When a constellation is near the horizon or partly hidden, its label is left out — respecting the real geometry of the sky for that date, time and place.

The Untangle labels option runs an iterative separation algorithm over the labels' bounding boxes, reducing collisions until the map is as legible as possible. The result is different on every map — because every sky is unique.

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