A real sky, drawn from real catalogues.

The star field is ESA's Gaia Data Release 3, down to G = 20: 1,060,293,402 stars, each with its position, parallax, proper motion and colour. The count isn't estimated — it matches the Gaia archive's own count for G < 20, star for star. Click any of them and the inspector gives you its Gaia identifier, G magnitude, BP−RP colour, distance and proper motion.

Four projections, configurable time and observer, live coordinates under the cursor, and a frame counter that keeps the frame cost honest as catalogues grow. Click a star or a deep sky object to select it and read it in the inspector; search finds it by name.

The deep sky is two layers, drawn as chart symbols: OpenNGC's Messier, NGC and IC objects, and HYPERLEDA's galaxies below them. Conspicuous nebulae also show their traced shape — 238 closed outlines over 171 objects from OpenNGC, drawn where a ring is at least twelve pixels across, because the catalogue ellipse that suits a galaxy is a circle around a filament for the Veil.

Named on the chart and readable in the inspector with their magnitude, elongation, phase angle and light time. DE440s covers 1849 to 2150; outside that interval Sky42 draws no bodies rather than extrapolating.

The 88 constellation boundaries are drawn from Delporte's B1875 definition, sampled as the curves they are. The naked eye stars come from the Bright Star Catalogue with their proper names and Bayer designations — Gaia does not have the bright stars, twelve of the fourteen brightest in the sky are absent from it — and the constellation figures join those stars rather than tracing coordinates.

The chart representation covers astrometric ICRS, apparent CIRS of date, and observed place including atmospheric refraction. A separate readout selector lets the cursor and inspector use a different system — for example, an observed chart with catalogue-comparable ICRS right ascension and declination. Apparent CIRS remains the chart default; ICRS remains the numeric default.

Minor planets use MPCORB elements, conservative light-time/parallax screening, H-G photometry and an inspector showing orbital epoch and quality. The default --asteroid-dynamics auto uses fast two-body propagation on wide charts and switches to perturbed ASSIST/IAS15 trajectories when the whole viewport falls below one square degree; kepler and assist remain explicit overrides.

Comets come from the MPC's CometEls with the same observed-place chain, but a solver and a photometry of their own: one universal-variable conic covers the ellipses, the 244 near-parabolic orbits, and the hyperbolic ones. Brightness follows the MPC's M1/K1 total-magnitude model rather than the asteroids' H-G phase law. A comet with no published photometry is drawn and says its magnitude is unknown, instead of silently disappearing.

The deep sky, the naked eye stars by proper name, Bayer, Flamsteed, HR, HD and HIP, every minor planet by number, name or provisional designation, and the comets. When a name belongs to more than one object — Halley is a minor planet and a comet — the matches are listed instead of counted. Whatever is selected is marked on the chart by four ticks around it, even when it is too faint for the field to draw it: the ticks say the thing is here, without putting a light where there is none.

Orbital elements can be refreshed from the toolbar or the command line: Sky42 asks the Minor Planet Center whether its files are newer than yours, downloads and rebuilds on a worker thread, and replaces each pack atomically — a cancelled or failed update leaves the previous catalogue byte for byte as it was. The ⋯ menu holds what isn't a layer: the ephemeris table, About Sky42, and Check for Updates, which asks the Sky42 site whether a newer version has been published. Sky42 downloads and installs nothing — the button opens the download page. The same check runs silently at most once a week and speaks only when there's something to say.