SKY · LOADING STARS
LST —— · MAUNA KEA
FIND
PLANETS
DEEP SKY
BRIGHT STARS
MODE
INSTRUMENT
MORE
FLIGHT · LOADING
OUT  0 KM FROM EARTH
TO HD 69800 474.8 LY
LOOK 00H 00M +00° 00′
SPEED ×10⁹ c · LIGHT: 475 YEARS
HD 69800 SPANS 0.0″
SHELLS 0/4 · 0 STARS
DATA ENDS 0 LY AHEAD
LOADING STARS…
MODE
FROM EARTH · TO
FLYING
SPEED
LOOK

The sky, as catalogued

Every point on the map above is a real entry in a catalogue, drawn at its J2000 position: about 120,000 stars from the HYG database, plus the galaxies, nebulae, clusters and black holes bright enough to name. Nothing is decorative and nothing is invented — pick a telescope, and the stars that appear are the ones a telescope would add.

Zoom in past twenty degrees and 36,778,505 more arrive: Gaia DR3, down to magnitude 15, streamed in patches for the part of the sky you are actually looking at and only as deep as that zoom can resolve. They have no pages — a page needs something to say — but they are the difference between a chart of the bright stars and a chart of the sky.

[

BRIGHTEST NAMED STARS

] 24
[

DEEP-SKY LANDMARKS

] 24
[

HOW TO READ IT

] 4 DEPTHS
EYE
90° across, down to magnitude 6.5 — what a dark sky shows without help.
BINOCULARS
30° across, down to magnitude 9.5 — a pair of 10×50s on a steady night.
TELESCOPE
8° across, down to magnitude 13.0 — a modest backyard telescope.
300 MM
2° across, down to magnitude 15.0 — a 300 mm reflector, and the faint end of Gaia DR3.

Every dot is coloured and sized by something real. The colour is the star’s temperature — blue-white where the surface is around 25,000 K, yellow near the Sun’s 5,800 K, orange-red at 3,000 K — from the same blackbody ramp that colours the star on its own page. The width is not the star’s size but its brightness: how far above the limit of the chosen instrument it sits, with a halo on the brightest handful. Nothing on this chart is drawn to scale, and a wide dot is a bright star rather than a big one. The same is written out on the map itself, under the ? in its corner.

The joined-up lines are the constellation figures published by the International Astronomical Union with Sky & Telescope, drawn through the same catalogued stars everything else on the map is drawn from — turn them off under SETTINGS on the control bar, or turn on the boundaries there, which are the regions the IAU agreed in 1930 and the reason every object here belongs to exactly one constellation. There is no standard for which stars a figure joins; this is one published set, and it is the one a reader can check against a printed chart.

The Sun, the Moon and the planets are the exception to all of that. They have no catalogued position, because they are somewhere else tomorrow, so the map computes where each one is for the site and the hour you have chosen and draws it ringed in amber — the ring means “this place was calculated, not looked up”. They are the reason tonight is different from last month, and the reason they come first in the strip of objects along the bottom. That strip is in three groups — the planets, then the deep sky, then the bright stars — and each one leads with whatever is above your horizon right now; the ones that have set stay on it, marked, since the honest answer to where Andromeda is in June is that it has not risen yet. Everything on it is something to look at: a curated object with a magnitude, so a chip never points the map at an empty field.

The dashed line is the horizon for the observing site and time you pick; stars below it are drawn dim, because they have set. The faint band running across the map is the galactic equator — the plane of the Milky Way. Coordinates in the address bar are shareable: this view is /sky?ra=266.4&dec=-29.0&fov=90.0.

Where the browser supports WebGPU the map is drawn on the graphics card, and then every star the chosen instrument can reach is drawn — about two hundred thousand at once in a crowded field at 300 mm. Where it does not, the same sky is drawn by the processor instead, which can afford the brightest twenty thousand of them and no more. Nothing else differs: same positions, same colours, same catalogue.

Know what you are after? The field on the map takes a name — Pleiades, Jupiter, NGC 224 — and points the map at it, framed to fit whatever it is. Looking for a particular patch of sky? Browse all 88 constellations. Want a list rather than a map? TONIGHT on the control bar opens one, over the map rather than instead of it — so every row is somewhere to point rather than somewhere to go.

[

WHAT IS UP TONIGHT

] ABOVE 20°

The same map answers the other question people have on a clear evening, which is not “what is that” but “what should I go out and look at”. TONIGHT opens a drawer holding everything the catalogue knows that will be more than 20° above the horizon from the observing site you have chosen, in the order you will actually see them — with the state of the Sun and the Moon above it, and the next few dated events after that. Tapping anything in it moves the map to it, which is the whole reason the list is here rather than on a page of its own.

The hour. One clock for the drawer and the chart: whatever the TIME control under SETTINGS says, which starts at the middle of the coming night when the drawer is opened by a link and at the present moment otherwise. Drag it and the horizon, the planets and the list all move together, because they are three readings of the same instant.

The order. Not the catalogue’s magnitudes but the magnitudes you will see: each one has the extinction of the air between you and it added, 0.2 magnitudes per air mass, measured against the zenith rather than against space so the limits are not charged for the same air twice. Four tenths of a magnitude goes at 20°, a hundredth at 70°.

The cut. Each piece of equipment carries the limiting magnitude the chart uses for the same depth — 7 of them, from a dark-adapted eye at 6.5 to a 300 mm reflector at 15.0 — so the list and the stars behind it cannot disagree about how faint you can see. Positions of stars are carried forward from J2000 by their own proper motion and precessed to tonight; the Sun, Moon and planets come from an ephemeris, so their magnitudes are the ones they have this week.

GET /api/v1/visible?lat=19.75&lon=-155.5&min_alt=20

The same arithmetic as the drawer, same order, no key required — the API docs have the parameters.

[

LEAVE EARTH AND GO AND LOOK AT IT

] FLY

A constellation is a coincidence of direction. The three stars of Orion’s belt are not neighbours and never were. Every chart above throws that away on purpose, because a map of the sky is a picture taken from one fixed point. Press FLY on the map — or FLY TO IT in the panel a click opens — and the same catalogue keeps its distances instead: each star sits where Gaia measured it, the camera leaves Earth, and the thing you flew to is drawn at the size its own catalogue entry says it is. Escape brings the map back, pointed exactly where you left it.

The star field is a sphere 554 light-years in radius holding about half a million stars. The flights run from a few light-minutes to a few hundred light-years — nine orders of magnitude — so the speed control is in multiples of the speed of light, and the ladder is built from the route. Fly to Mars at ×1 and it takes exactly as long as light does.

The model of the destination is not an illustration. Its size is the radius the catalogue holds — in solar radii for a star and in kilometres for a planet. Its colour, its bands, its craters and its corona are the same ones its own page draws, from the same code. An object whose size this catalogue does not hold is flown to and stays a point of light, because a sphere of invented size is a claim a reader would measure.

The wall of the star field is the far end of every flight. Past it the camera is pointed away from every star this site holds and the sky is black, so the flight is not offered rather than sold and then delivered empty. Galaxies, nebulae and clusters are left out for the other half of the same reason: what this catalogue knows about their shape is an inclination, a position angle and an angular size, which is what their own pages draw them from and is not a sphere — so a fly-through has nothing to put on the screen when it gets there.

The distances are inverted parallaxes, and here that is safe. Inverting a parallax is the thing every Gaia paper warns about, because at twentieth magnitude the uncertainty is half a milliarcsecond and the answer is biased rather than merely fuzzy. This catalogue was cut at magnitude 15 for an unrelated reason, and across that whole range Gaia DR3’s parallax uncertainty is 0.02 to 0.03 milliarcseconds — half a percent at the far wall, and better nearer. What is not corrected is the DR3 zero point, about −0.017 milliarcseconds, which makes every distance here roughly three parts in a thousand too small at the edge; the published correction needs inputs this catalogue does not carry, so it is stated rather than half-applied.

It thins outward, and that is a selection effect, not emptiness. A magnitude-15 cut admits everything down to absolute magnitude 13.5 at twenty parsecs and nothing fainter than absolute 8.9 at the far wall, so the further out you fly the more the crowd is made only of stars brighter than the Sun. Red dwarfs are the most common thing in the galaxy and almost none of them are out there in this map.

Asteroids and comets have no flight. They carry a semi-major axis, an eccentricity and an inclination — enough to describe the orbit, and not enough to say where on it the body is now. Placing one would mean picking a point and letting a reader believe it. The Sun, the Moon and the eight planets do have a real position for the current instant, from the same ephemeris that backs /api/v1/visible.

Past the wall there is nothing, because the data stops. The stars arrive in 4 concentric shells, nearest first, each fetched before the camera reaches it. A flight to something further than 554 light-years leaves the volume and keeps going, and the readout says so rather than drawing a backdrop that would claim the distant sky never changes.

Where the browser supports WebGPU every resident star is drawn and its brightness recomputed from the camera each frame, and the destination is ray traced against an analytic sphere. Where it does not, the processor draws a few thousand of the brightest stars and the destination as a shaded disc: the same positions, the same colours and the same size, and a much smaller crowd.

[

NAMED STARS IN RANGE

] 22

Everything inside the star field with a name, and how far light takes to cross to it. The map is a canvas, so these are links too: the name opens the star’s own page, and fly aims the camera at it. An object page carries the same link wherever a flight to it would arrive somewhere worth looking at.