Planetarium

Galaxy is our own Milky Way seen from outside, something no photograph can show, so it is an informed impression rather than an image: the central bar, the four major arms with their names, and the short Orion Spur that carries the Sun follow current radio and infrared mapping, while the brush strokes are procedural. The Sun sits at its true distance, about 26,700 light years from the center, roughly halfway out, and zooming always closes in on the Sun, so diving deep enough drops you back into the solar system. Drag to turn and tilt the disc, from face-on down to edge-on where the central bulge stands out of the thin disc. Here the slider means something very different: one sweep is one galactic year, the 230 million years the Sun needs for a single orbit, and the clock counts in millions of years because calendar dates are meaningless at this scale. The arms are density waves rather than fixed structures, and the time machine moves them at their approximate measured pattern speeds: the spiral pattern turns a little slower than the stars, so over one galactic year the Sun gains slowly on the arms, while the central bar spins distinctly faster and laps them. The Sun sits close to the corotation radius, the distance where stars and pattern keep pace, which is exactly why it stays near its home spur for so long.

At this scale space is black and things keep their true colors, whatever palette the charts use: blue-white Rigel and Spica, orange Betelgeuse and Arcturus, yellow Capella, the warm bar and bulge. Thirty of the bright stars and showpiece objects of our night sky are plotted at their measured directions and distances, from the Carina Nebula and the Eagle Nebula with its Pillars of Creation, eight thousand and six thousand light years out, down through the Crab, the Lagoon and Orion, to Deneb, Betelgeuse, Vega, Sirius and finally Alpha Centauri, which separates from the Sun only in the last moments of the dive. The painted arm clouds fade away as you close in, because the space between the stars really is empty. Watching the whole famous night sky collapse into a dot smaller than the width of a single arm is the honest lesson of this view.

Almost every star a naked eye can see is local, within a few hundred light years, which is why the plotted names crowd around the Sun. The farthest stars an unaided eye can pick out are rare hypergiants: Deneb, Mu Cephei, P Cygni, VY Canis Majoris, and at about eight thousand light years Rho Cassiopeiae, already out in the Perseus Arm alongside the Double Cluster. The inner arms and the far side of the disc hide behind dust, which is why no star beyond that has ever been a household name.

The zoom now connects the scales without a jump. Falling out of the galaxy lands about seventeen light years from home, and the solar system view keeps receding through the same territory: past Alpha Centauri, through the Oort cloud, past the heliopause where both Voyager probes crawl outward along their real trajectories, growing a few AU more distant every year of the time machine, then the Kuiper belt, and in to the planets. A scale bar walks the whole ladder with you, from light years through light days and light hours down to light seconds.

Solar system looks down on the planets from above Earth’s north pole, so everything runs counter-clockwise, floating in the same black space as the galaxy with the orbits drawn bright. One view holds everything; drag to pan and scroll to zoom about the cursor, or use ** on** to lock the view to a planet while you zoom and play. Orbits are to scale; planet dots are not, since at true scale even Jupiter would be smaller than a pixel. The asteroid belt circles between Mars and Jupiter, each speck advancing on the period Kepler’s third law assigns its distance. Dive inside the inner planets and the Moon appears riding with Earth; zoom into Jupiter and Io, Europa, Ganymede and Callisto resolve on their true orbits, straight from the ephemeris. The four turning points of Earth’s year are marked on Earth’s orbit and listed with their exact moments below the map. They carry the northern season names, spring and autumn for the equinoxes and summer and winter for the solstices; in the southern hemisphere the seasons are the other way around, though the moments themselves are the same for everyone.

Sky dome is the whole sky above the observer at once, drawn as if you were lying on your back with your head to the north. The rim is the horizon, the center is straight overhead, and east appears on the left, which is correct for a chart meant to be held up against the sky. Scroll to magnify the dome about the cursor, drag to slide it around, and Reset to see the whole sky again.

The globe and the sky answer each other. Moving the figure redraws the sky for that spot, and clicking anything in the sky names it on the chart and marks the place on Earth where it is straight overhead at that moment on the globe, which is exactly where you would have to stand to see it at the zenith.

The compass under the globe is the observer seen from above, with the wedge showing where they are looking, and the protractor below it is the same person from the side, showing how far their head is tipped back from the horizon. Drag either one and the dome highlights that patch of sky, labeled with its direction and altitude; in the horizon view the two widgets are the view direction itself, so turning or tipping them moves the whole picture, and looking around in the sky moves them back.

Horizon is the standing view. Drag to look around, scroll to zoom, and read the compass letters along the horizon line.

identifies things. Planets and the Moon carry their own colors, galaxies are violet, nebulae red, open clusters blue, globular clusters amber, and the Milky Way is the warm gold band, the color the core’s star clouds actually glow in photographs. Galaxies draw as ellipses, nebulae as squares, open clusters as dashed circles, and globulars as crossed circles, which are the standard atlas glyphs. The red palette overrides all of it, because preserving night vision is that mode’s entire point.

The slider spans control sets how much time the slider and the Play button sweep, from a single day up to a whole millennium, starting at the chosen date. Watch a day to see the sky rotate; longer spans switch the sky views into sidereal steps. The clock then advances in whole sidereal days, which holds Earth’s rotation angle fixed and freezes the nightly spin, so the only motion left on screen is the slow kind: the Moon and planets wandering through a steady star field over a month or a year, and over a century or a millennium the precession of Earth’s axis sliding the whole star field past the fixed pole point while the stars also creep along their own proper motions. The same lock serves the southern sky identically, so an observer in the south watches their pole drift through Octans and Carina the same way the north watches it leave Polaris.

The year box accepts -3000 to 5000, in astronomical numbering where year 0 is 1 BCE. Set the year to -2700 and the pole star is Thuban, the star the pyramid builders aimed their shafts at. Today the job belongs to Polaris; by the year 5000 the pole is drifting on toward Alderamin. The stars themselves also creep along their measured proper motions, so over millennia the familiar figures slowly deform.

Solar system, lunar, and planetary positions come from Astronomy Engine, implementing VSOP87 and ELP2000, with precession and nutation applied for the chosen date. Stars come from the HYG database version 4.1, cut at magnitude 5.5, roughly the naked-eye limit under a dark sky. Constellation figures are the modern set shipped with Stellarium. The deep sky markers are a hand-picked set of the objects people actually photograph, and the Milky Way band is a simplified outline of the galactic plane, not a survey image.

The galaxy view is illustrative rather than catalogued: no one has photographed the Milky Way from outside, and the arm layout follows the current best mapping while the individual dabs of light are generated. The orientations, however, are real, and no two norths agree. Earth’s axis leans 23.4 degrees off the ecliptic pole, and the ecliptic pole in turn stands 60.2 degrees from the galactic pole, so the solar system rolls through the galaxy tilted nearly on edge, like a coin. The zoom now draws that geometry: approaching the crossing, the galactic disc eases into exactly the 60 degree inclination it truly has when seen from above the ecliptic, which is the direction the solar system view looks from, and the nearby stars in the deep solar system view sit at their true ecliptic bearings. Stars far above or below the galactic plane, such as Arcturus, Vega or Fomalhaut, shift position between the two views at the hand-off; that shift is genuine projection geometry rather than an error, because each flat map must flatten a three dimensional neighbourhood into its own plane. A corner panel draws the three norths with their numbers, 60.2 degrees from the galactic pole to the ecliptic pole and 23.4 more to Earth’s axis, which lands 62.9 degrees from the galactic pole because the two tilts lean different ways. The panel is a real projection rather than a picture, sharing the camera of the view it sits in, so turning the galaxy turns the fan of poles and flattens its arcs, and a pole aimed at the reader collapses to the dot in a circle that means exactly that. It rides along through the whole dive and fades out only once the planets fill the view. Everything else on the page is computed.

Far from the present the picture is honest but not arcsecond-perfect. The precession model is built for a window of a few thousand years around today, linear proper motion ignores radial velocity, Pluto’s model covers a narrower span than VSOP87 and drops out rather than being shown wrong, and the rotation angle of the ancient Earth carries the accumulated uncertainty of Delta T. For naked-eye work across this range, none of it moves anything visibly out of place.

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