Stand under the sky of any age of Earth

The globe shows Earth from space at equinox with the Sun behind you, so latitude runs from the equator along the middle to the poles at top and bottom. Its atmosphere is drawn 30 times too thick so the layers show.

The dome is the whole sky as a fisheye: the zenith at the center, the horizon at the rim, north at the top. The VR view is the same sky in first person, with procedural scenery and clouds; the ground, the shapes and the clouds are scenery, and the sky is the model. On a touch screen, drag to look, pinch to zoom, and a stick at the lower left walks; an eye button hides the controls.

The Sun follows equinox geometry, so it rises due east at 6:00 and sets due west at 18:00 everywhere; at 75° the noon Sun is only 15° up. The southern latitudes, −45° and −75°, share the sky colors of 45° and 75°, which depend on the Sun's height and the direction from it rather than on the hemisphere, under the southern stars, with the noon Sun in the north and the aurora's southern oval. On the dome the Sun and Moon are enlarged so the Moon's phase stays readable, and in VR both are drawn at four times their true size.

The Moon is placed for the date and clock time at your latitude, on your time zone's central meridian, using a NASA LROC color map. Moonlight is the same sky model with the Moon in place of the Sun, scaled by the lunar phase (the Allen phase law, as in Krisciunas & Schaefer 1991) and by the Moon's size at that epoch, from the Earth–Moon distance (Farhat et al. 2022; about 70% of today's at 3.2 Ga, Eulenfeld & Heubeck 2023). The closer Moon also went round faster (Kepler's third law): its phases cycle in 20 of today's days at 2.7 Ga and 15 at 4.0 Ga, which is still 28 to 31 of the shorter days of each epoch. At 4.5 Ga, only 8 Earth radii out, they cycle in 32 hours, 5.4 of that epoch's 5.9-hour days. The slow turning of its orbit, which the Sun drives, was slower then, so eclipse seasons came closer to a year apart (352 days at 2.7 Ga, against 347 today).

Both disks change size: the Moon with its distance on the date (an Earth radius closer when overhead), the Sun with the Earth's orbit. Near perigee the Moon covers the Sun completely; near apogee it leaves a ring. From 2.2 billion years ago back, every central eclipse is total. For each new Moon the view stands at the best seat, where parallax brings the Moon closest to the Sun.

The Sun's disk is darker and redder toward its edge (limb darkening): there the line of sight runs through the photosphere's higher, cooler layers. Its brightness falls as μ^α, with μ the cosine of the angle from the disk's center as seen from the Sun and α=−0.023+0.292/λ (λ in µm; Hestroffer & Magnan 1998), so near the edge the blue is down to a third and the red to a half of the center's. The air in front only multiplies that light, so the darkening is the same at every epoch and under every sky; the disk's color is the epoch's direct sunlight, made bluer at the center and redder at the edge. A young Sun 200 K cooler darkens a little more toward its edge, too little to show. The real Sun is far too bright to see this by eye, and through a dark filter the edge is plainly darker; the drawing keeps only a hint of it, a fifth of the true falloff. The spectrum on the disk has the full effect.

When the Moon covers part of the Sun, all the sunlit sky dims by the fraction of the drawn disk still showing. In totality the sky is lit from beyond the shadow: the epoch's twilight sky with a sunset glow all round the horizon, a thousandth of the day sky, as measured (Sharp, Lloyd & Silverman 1966). The corona, a pink rim, Baily's beads and the diamond ring appear. The corona follows the epoch's Sun: a younger Sun spun faster and was more active, so its corona was denser and hotter (X-rays about 530 times today's and 7.7 MK at 4.5 Ga, against 1.5 MK now), and its dust glow follows the epoch's zodiacal cloud. The young Sun was also smaller (0.90 of today's radius at 4.5 Ga, from its luminosity and temperature) while the Moon was closer, so early eclipses hide the inner corona: at 3.8 Ga the Moon looks 1.5 times the Sun's width. Through the 30-bar Hadean air the corona does not show at all. Its spectrum shows the electrons' light without Fraunhofer lines, the dust's with them, and the coronal iron lines, joined by yellow Ca XV in the hot young corona; the Moon's edge has mountains and valleys (three times real relief) so the last sunlight breaks into beads. Because the disks are enlarged, every phase lasts about four times as long, and play slows tenfold through totality and the annular ring.

In a lunar eclipse the Moon passes through the Earth's shadow, which is lit only by sunlight bent into it through the Earth's air: every sunrise and sunset on Earth at once. For each epoch the model traces rays from the Sun through that epoch's atmosphere at every height, bending with its refractivity and dimmed by all of its gases, ozone, aerosols, haze and soot along the whole path, with today's cloud tops cutting the lowest. Summed over the ring of the Earth's limb as seen from each point on the Moon (Link's method), they give the light and color across the shadow. Today's umbra comes out a dull copper, 12 magnitudes below the full Moon at the middle of a central eclipse, with a paler edge where the ozone takes out the orange. Without ozone, before the Great Oxidation, the edge is orange too. Under a Tambora-sized sulfate veil the Moon all but disappears, as it did in June 1816, and through the K-Pg soot or the thick Archean haze it is gone. The closer early Moon sat nearer the Earth than the bent light comes to a focus, so the middle of the shadow went dark; the 30-bar Hadean air bends light so strongly that below about 27 km it is trapped and runs round the Earth, and only the upper air lights the shadow, a deep red. The shadow's size follows the geometry: the umbra was 3.1 Moon widths across at 4.0 Ga against 2.7 today, and with the wider shadow and more months a year, total lunar eclipses came nearly three times as often: some 220 a century at 4.0 Ga and 137 at 2.7 Ga, against 85 today. That is with the Moon's orbit tilted a little more steeply then (5.5° at 2.7 Ga and 5.8° at 4.0 Ga, against 5.1° now), taken back from today's tilt with the tidal model of Ćuk et al. 2016: the Earth's tides flatten the orbit as the Moon recedes, and tides in the Moon itself damp it, most strongly at distances older than any shown here but the first. At 4.5 Ga the Moon, 8 Earth radii out, orbits in the plane of the Earth's equator, and its shadow is 3.6 Moon widths across: for weeks around each equinox every full Moon is eclipsed, some 3,900 total eclipses a century. Times and depths are within a few minutes and a few hundredths of the almanacs' today. The eye adapts to the brightest part of the Moon, so the umbra looks black beside the penumbra in a partial phase but glows in totality. U finds the next lunar eclipse with the Moon up, B the next total one, and play slows through them.

In clear, very cold air the sky fills with diamond dust: tiny hexagonal plates and columns of ice drifting near the ground, glinting as they turn. Snowball Earth's air would have been full of them at every latitude. Today they are common on the Antarctic plateau and in Arctic winter, so they also appear at 75° in the modern and ozone-hole skies, 1815, 2100, the Pleistocene supernova epochs, the Carboniferous (with its Gondwana ice sheets) and 2.2 Ga (the end of the Huronian glaciations), and faintly at 45° in the Geminga epoch, a glacial maximum.

The halos are worked out from the crystals' geometry and the refractive index of ice at 450, 550 and 650 nm (Warren & Brandt 2008), so each arc is red on the side toward the Sun: the 22° and 46° halos, the tangent arcs where the 22° halo meets them, the sun dogs (whose distance from the Sun grows with its height, until they vanish above 61°), the circumzenithal arc (only while the Sun is below 32°), the parhelic circle with its 120° parhelia, a sun pillar when the Sun is low, and the glow of light diffracted around the Sun. The Moon makes the same, faintly. Their brightness, set against the clean noon sky, is an estimate of a good display.

Each moment has its own meteors, so going back to a time brings back the same ones. Pausing freezes them. Play in real time (key 1) to watch them fall at their true pace; at the faster speeds each frame is a snapshot of whatever is in the air. Each one is a meteoroid entering the air along a straight path 80 to 125 km up and drawn as it would look from the ground: short near its radiant, long across the sky, foreshortened near the horizon, dimmer with distance and through the air. A meteor shows only when it is bright enough against the sky around it, so the evening, moonlight, twilight and city light all thin them out, and in daylight only great fireballs get through. Today a dark site sees about 8 an hour, fewer in the evening and more before dawn, when the side of the Earth facing its direction of motion sweeps up the fast meteors of the apex; the others radiate from near the Sun and opposite it, from high above the ecliptic, and, slowest of all, from the asteroid belt. Their brightness follows the measured distribution, set by the fireballs (one of magnitude −4 or brighter every 25 hours or so; AMS), with the faint ones the eye misses left out. Size, speed and brightness are tied by the classic photographic relation (Jacchia, Verniani & Briggs 1967): magnitude 0 is about a gram at 20 km/s or a hundredth of a gram at 70. Bright, fast meteors leave a persistent train that glows green, then dull orange, and drifts in the wind; it needs the oxygen and ozone of the air, so there are none before the Great Oxidation Event.

Through time the rate follows the lunar cratering record (Neukum et al. 2001): about 500 times today's at 4.0 Ga and 120 at 3.8 Ga, and some 10,000 times at 4.5 Ga, in the debris of the Moon-forming impact, where the 30-bar air hides all but the brightest. The bombardment's impactors were richer in large bodies and slower (Strom et al. 2015; Marchi et al. 2014), so its meteors are mostly slow, yellow-orange fireballs, and a closer Moon flashes on its dark side as rocks hit it, a few times an hour at 3.8 Ga. At 466 Ma the L-chondrite parent body had just broken up (Schmitz et al. 2019): a hundred times today's fireballs, more still of the faint meteors, slow and yellow with sodium. Their colors come from a spectrum built for each speed and air: the metals of the meteoroid (sodium, magnesium, iron, calcium), a hotter component that grows with speed, and the air's own atoms and bands, oxygen and nitrogen today, or carbon monoxide bands in a CO₂-rich air, which no one has yet measured.

Meteor showers come on their own dates. A shower is the dust a comet (or, for the Geminids and Quadrantids, a rocky body) has left along its orbit, met by the Earth on the same date every year, so its meteors all come from one point in the sky, the radiant, at one speed. The modern-era skies have the 38 showers of the International Meteor Organization's working list, apart from its Antihelion source (Rendtel, IMO Meteor Shower Calendar 2027, Table 5), with their dates, radiants and the radiants' drift from night to night, speeds and population indices, rising and falling about the peak as Jenniskens (1994) measured them: the Perseids' sharp peak over a month-long background, the Geminids' slow rise and quick fall, the Quadrantids' few hours. The ZHR is the rate one observer would count under a sky showing stars to magnitude 6.5 with the radiant overhead; a low radiant gives fewer, and a bright sky fewer still. Their outbursts and storms come at the hour they were seen: the Leonids of 1799, 1833, 1866 and 1867, 1966, and 1999 to 2002; the Andromedids of 1872 and 1885 from the broken comet Biela; the Draconids of 1933 and 1946; the brief α-Monocerotid bursts; and the Leonids predicted for 2034 (Jenniskens 1995 and the IMO's analyses). Hovering over a meteor names its shower and parent, the labels (L in the first-person view) put a cross on each active shower's radiant (in the older epochs, whose showers have no names, as a meteor shower radiant), and N goes to the next shower's peak, at the hour its radiant stands highest in a dark sky, or to the next outburst.

Showers last thousands of years, so the older epochs have showers of their own, drawn at random but the same each visit: placed about the Sun, with speeds and profiles, like today's, and dates anywhere in the year, and with outbursts and storms at today's rate. Their number follows the comets that make them: about thirty times today's at 4.5 Ga, while the giant planets were eroding the comet disk, five times at 4.0 Ga, today's by 2 Ga, and nearly twice today's at 1.78 Ma, when a star's passage had stirred up the Oort cloud. Nobody has estimated showers in the early Solar System, so these are estimates. In the bombardment they hardly show against a sporadic rate hundreds or thousands of times today's.

Each comet is placed on its orbit and seen in perspective: a coma round the head, green with the C₂ Swan bands where its gas glows and white where its dust reflects sunlight, a broad dust tail curving back along the orbit, and a narrow blue ion tail (CO⁺) pointing straight away from the Sun. In today's skies, the 1980s and 1990s, 1815 and 2100, the comets are the real ones on their real dates, from JPL's orbits, for whatever date is set: Halley's Comet from 1682 to 2061, Swift–Tuttle, Pons–Brooks, Hale–Bopp (1997), Hyakutake (1996), West (1976), McNaught (2007), NEOWISE (2020), Tsuchinshan–ATLAS (2024), Lemmon (2025), the great comets of 1577, 1680, 1811, 1843, 1858, 1861, 1882 and 1910, and others, each as bright at its best as it was recorded.

Elsewhere the comets are drawn at random, the same ones for a given date, at the rate of their time. Today about one comet a year reaches naked-eye brightness and one every six or seven years outshines the brightest stars (Green 2020; Kidger 1994); their sizes and perihelia follow the measured distributions (Hughes 2001; Francis 2005). Today is itself unusual: a star, HD 7977, passed through the outer Oort cloud about 2.8 million years ago, and the flux of new long-period comets is still about two to two and a half times the tide-only rate (Kaib & Raymond 2026), so the quiet epochs (700, 466, 300 and 66 Ma) have about half of today's comets, and 1.78 Ma, near the shower's peak, four times as many. That sky also has one invented comet: a great comet, peaking at magnitude −3, that returns every year with perihelion on 2 July and through July stands in the north-northwest in the late evening, its tail rising clear of the Milky Way, while the supernova is low in the south. Early on, while the giant planets eroded and then scattered the comet disk, there were many more: an estimated 30 times today's at 4.5 Ga and 4 to 5 times at 3.8–4.0 Ga, falling to today's level by about 2 Ga.

The zodiacal light, sunlight off interplanetary dust, is part of today's natural night sky. Where the inner Solar System held more dust, the excess shows along the ecliptic, brightest toward the Sun, with the gegenschein opposite it: 30 times today's after the L-chondrite break-up (the dust falling on Earth rose a thousandfold or more; an estimate), and 50 to 2,000 times during the bombardment, when grinding planetesimals and comets filled the inner Solar System (Nesvorný et al. 2010; estimates). That glow is as bright as a moonlit sky, and it hides the fainter stars and meteors.

All 21 known Ordovician impact craters lie within 30° of the equator of the time, which led Tomkins, Martin & Cawood (2024) to propose that a large fragment passed inside the Roche limit about 466 Ma and broke up into a ring in the equatorial plane that lasted tens of millions of years. It is speculation, and its size and opacity are unknown. It is drawn between 1.55 and 2.75 Earth radii from the center, with ringlets and a gap, and thin enough (normal optical depth at most 1.5×10⁻⁴) that its sunlit grains give about as much light as the Moon: lost in the blue by day, a low arch across the southern sky at night from 45° and the northern sky from −45° (below the horizon from 75° and −75°), edge-on as a line through the zenith from the equator, with Earth's shadow taking a bite out of it around midnight. From the equator its debris falls too: slow, grazing fireballs that cross the sky from west to east.

The thousand brightest stars of the Yale Bright Star Catalogue (Hoffleit & Warren 1991) are carried to the year shown by precession and by their own motion, and to 1815 for the volcanic year. Each star moves in a straight line through space, from its Hipparcos distance, proper motion and radial velocity (XHIP, Anderson & Francis 2012), and brightens or fades as its distance changes: Sirius is brightest about 60,000 years after year 0. Each epoch's year 0 has today's axis, since its precession long ago cannot be pinned down; the years either side of it turn the axis as it has turned around today, by the long-term precession of Vondrák et al. 2011, good to a degree or so over the calendar's 250,000 years either way. Polaris is the pole star only for a few thousand years; Vega takes its place about 12,000 years from year 0. The faintest star is a single pixel, and size grows with brightness.

• The last two million years. Every catalog star is moved along its line, and the page shows the thousand brightest of the year: today's thousand and the 732 others that were among them at some time from 2.2 million years ago to 250,000 years ahead.
• Further back. The Galaxy's pull has bent the stars' lines by then, and today's neighbors were elsewhere, so between 1.9 and 2.2 million years ago (a time of its own for each) every star fades out over 50,000 years and gives way to stand-ins of its kind: stars of its absolute magnitude, color and spectral type passing the Sun one after another on straight lines at 15 to 40 km/s, each nearest the Sun, at today's star's distance and as bright as it, toward the galactic latitude it has today. Each passage lasts the time it takes to go twice that distance, half a million to twenty million years, and fades into the next, so the sky changes as fast as the nearby stars move and never all at once. A stand-in's label and tooltip give its type (a K1.5 giant, an A0 dwarf).
• Brighter than Sirius. Sirius has not always been the brightest star. Traced back 10 million years through the Galaxy, eleven Hipparcos stars outshone today's Sirius (−1.46) as they passed the Sun. The brightest, θ Columbae, now a faint fifth-magnitude star, came within about 2 pc 4.7 million years ago and reached about −5, as bright as Venus. ε Canis Majoris reached −4.35 (an encounter first noted by Tomkin 1998) and β Canis Majoris −3.6 at about the same time, and ζ Leporis −2.5 only 850,000 years ago. Aldebaran tied today's Sirius 330,000 years ago, so it shines at −1.5 in the Geminga epoch. The page shows the peaks of the last two million years (ζ Leporis, Aldebaran) at their years; the older ones are before its stars' straight lines hold. The report has the full table.
• Planets. The five naked-eye planets, Uranus and Neptune are placed for the date and time on the same magnitude scale as the stars, each a disk of its true angular size, as are their moons and orbits (only the Sun and Moon are drawn enlarged), lit from the Sun's side, so Venus's and Mercury's phases show when zoomed in. Jupiter's four large moons and Titan, Rhea, Dione and Tethys circle their planets, pass behind them, go dark in their shadows and cast their own shadows when they transit; zooming in stands for a telescope, bringing out the moons, the faint outer planets and Mercury in a bright twilight. More than 10,000 years from today each sits at a place on today's orbit picked at random for the epoch (and in the modern-era skies for each 100,000 years), since where it was on any day then cannot be known, its orbit's slow changes held where they are 3,000 years from today; which moons there are follows the year (Phobos only in the last 100 Myr, Neptune's first moons until the giant planets' instability, 4.48 Ga, and Triton after it), and all shine fainter by the young Sun's dimmer light. Saturn's rings may be only a few hundred million years old, though that is still argued; they are drawn from the break-up of a hypothetical moon, Chrysalis, about 160 Myr ago in the illustrated case, within a 100 to 200 Myr range, and left out before, where Chrysalis is drawn. At 66 Ma they are drawn as rings then young would have been: about half again as massive, so more opaque, and cleaner of the meteoroid dust that now grays the C ring and Cassini division (a guess at the size of the difference: no one has seen them). Saturn's axis wobbles round once in about 1.9 Myr, so before history its pole, and the rings' tilt on the sky, is turned from today's: by the measured rate at 342 ka and 1.78 Ma, by an angle drawn at random at 66 Ma. Their structure is Cassini's: the optical depth measured every 10 km by its radio occultation of 2005, with the C ring, the Cassini division and the A ring's outer edge thickened by the sub-centimeter particles the same occultation's shorter wavelength sees (up to a tenth more) and the dusty F ring added, so a close look shows the C ring's plateaus, the B ring's bands, the Cassini division's ringlets and the Encke and Keeler gaps. Each band is shaded by how much light it scatters and lets through for the day's angles of the Sun and the Earth to the ring plane: from the dark side the thin C ring and Cassini division glow while the dense B ring goes black, and near opposition the rings brighten against the globe. Jupiter's Great Red Spot is drawn from its first sure sighting in 1831, at its measured longitude and shrinking from about 40,000 km long in the 1880s to 14,000 km now, as the fits to a century of measurements give; past the 2040s, when they make it round, it is held at that size.
• 2100. The filed megaconstellation (about 65,000 satellites; Lawler, Boley & Rein 2022) and Blue Origin's Sunrise orbital datacenters (51,600 satellites; Boley, Lawler & Rein 2026) move with the clock, shown while sunlit and brighter than magnitude 6.5.

Without the Sun or Moon the sky has its own light. The model is calibrated in cd/m² (the clear zenith with the Sun 45° up is 2.7 kcd/m², which puts the full-Moon sky at the measured 18 mag/arcsec²).

• Natural sky. Airglow, zodiacal light and faint stars: 22.0 mag/arcsec² at the zenith (Leinert et al. 1998), brighter toward the horizon.
• Airglow. About half of that light is airglow, and nearly all of it needs free oxygen: the green and red lines of oxygen atoms, the hydroxyl and sodium bands that run on ozone. It is weaker at 2.2 Ga (30%) and in Snowball Earth (80%). Before the Great Oxidation Event it is gone, replaced by a faint violet glow from oxygen split off CO₂, like the Venus nightglow (Krasnopolsky 1983), so Archean and Hadean nights are darker and bluer. These strengths are estimates.
• The Milky Way. A model in galactic coordinates: disk, bulge, star clouds, dust (the Great Rift and the Coalsack), peaking at 20.2 mag/arcsec² in the Sagittarius star cloud. In the older epochs its brightest part lies where the Galactic center was then, from the Sun's orbit traced back (only nominal beyond a billion years).
• Nebulae and galaxies. The 34 that binoculars show under a dark sky, from survey photographs cut out by the CDS hips2fits service: the detail from the Digitized Sky Survey (STScI, under US Government grant NAG W-2166), the color from Mellinger's all-sky photograph, its saturation raised by a third. Their stars are taken out, as the page draws its own, and each is scaled to its catalog magnitude and central surface brightness, so most show as a bright core in a faint glow, as they do to the eye. They move with the epoch. M31 falls toward us on the radial orbit the timing argument gives (van der Marel et al. 2012): 4.5 Ga it was 1.03 Mpc away and three quarters of today's size. The Large Magellanic Cloud is coming in for the first time (Kallivayalil et al. 2013), traced back with dynamical friction: 400 kpc out 2 Ga. Galaxies beyond the Local Group were nearer with the cosmic expansion, 0.7 of their distance 4.5 Ga. A nebula shines only while its stars do: none of today's in the deep past, the planetary nebulae for the last ten thousand years or so, the Crab since 1054.
• City light. In the modern epochs you stand in a mid-sized city. Its zenith glow is 18.8 mag/arcsec² today (Falchi et al. 2016), about six times the natural sky with orange sodium lamps in 1980–2000 (Cinzano, Falchi & Elvidge 2001), 1.6 times today's in polluted air, today's level in 2100 (the recent 9.6% a year, Kyba et al. 2023, cannot be projected that far), and almost nothing from the oil lamps of 1815 (Fouquet & Pearson 2006). Clouds over the city are lit from below (Kyba et al. 2011).
• Through the air. Starlight dims by 0.15 magnitude per airmass from the gas plus 1.086 times the epoch's haze optical depth: the impact winter's soot takes two magnitudes off every star, and the Early Hadean's 30 bar of CO₂ hides them all.
• What shows. Each star shows only when brighter than the naked-eye limit for the sky around it, so stars come out one by one in twilight and cities keep only the brightest.

The aurora is an oval round the magnetic pole, fixed relative to the Sun, taken here with the magnetic pole at the geographic one. Today, on a moderately active night (Kp 3), it spans about 63–72° latitude on the night side and 73–78° on the day side (Feldstein's ovals), so from 75° it passes to the south in the evening and overhead around noon in the polar night, and from 45° it is out of sight. The southern oval mirrors it round the south pole, so −75° and −45° see the same. Storms widen it toward the equator; Aurora storm shows a storm of the class of September 1859.

• Where, then. The oval's field lines reach out to the magnetopause, whose distance goes as the sixth root of the field's pressure over the solar wind's. At 3.45 Ga a weaker field and the young Sun's much denser wind held the magnetopause at about half today's distance (Tarduno et al. 2010), which brings the oval down to the mid-latitudes. A young, active Sun also makes storms the usual state (Airapetian et al. 2016). The oval moves back toward the pole through the Proterozoic as the wind weakens.
• What color. Today's green (557.7 nm) and red (630 nm) come from oxygen atoms. With no free oxygen, the Archean and Hadean aurora is nitrogen's: violet N₂⁺ bands and a pink border of N₂ first positive bands where faster electrons reach lowest, with a weak green line from CO₂, as Perseverance saw on Mars (Knutsen et al. 2025). At 2.2 Ga the oxygen lines are about a third of today's. The early Hadean's 30 bar of CO₂ hides it.
• How it looks. Each arc is a thin sheet along the field lines, leaning toward the equator going up, so rays overhead converge on the magnetic zenith. Green fills the lower 100–150 km, red tops reach 250 km and more, and sunlit tops glow violet, as N₂⁺ scatters sunlight. Arcs fold and ripple, rays drift along them in real time, substorms brighten the arcs near midnight twice a night, and the diffuse aurora toward dawn pulses in patches. Brightness is for a dark-adapted eye: rods see violet far better than cones do and hardly see the red line, so red tops often show in photographs but barely to the eye. Line strengths for the ancient atmospheres are estimates.

Hover over the dome, the globe, the strip of sky along the Sun's vertical or the noon skies, or press Q in VR and point, to see the spectrum of that piece of sky from 380 to 780 nm. The light is split into its sources, each with the brightness the sky drawing gives it: sunlight and moonlight scattered by the air are the model's own spectra at its 10 nm resolution, computed for every epoch, surface and Sun height on a coarse grid of directions and interpolated. The night's sources have their own spectra: zodiacal light and stars (sunlight), airglow (the oxygen lines at 557.7 and 630 nm, the sodium D lines and OH bands, with O₂ bands and a faint continuum set to the airglow color used here, or the violet O₂ band of an oxygen-free air), city light (sodium lamps and white LEDs mixed to each epoch's glow, or oil flames in 1815), the Milky Way, and the aurora's lines at their strength in that direction. On the Moon, its day side is sunlight it reflects (a little reddened), and its night side earthshine, sunlight off the Earth (bluer and some five thousand times fainter), with the air in front added to both. On a comet, its coma's gas (the C₂ Swan bands, C₃, CN, and sodium near the Sun) and dust (reflected sunlight), and its tails: dust, and the CO⁺ bands of the ion tail. On a meteor, or where one has just passed, its own light: the lines of its metals, its hot component and the air, as described under Meteors, through the air below it; a flash on the Moon is rock vapor glowing at 2,000–4,500 K. On a bright star (to magnitude 2.5), the star itself, as a spectrograph on the ground would record it before the air is calibrated out: a body at the star's color temperature with the lines of its class (broad hydrogen lines in the A stars, helium in the hot B stars, calcium, magnesium and sodium growing toward the cool ones, titanium oxide bands in the M stars), dimmed through that epoch's air along the star's airmass, most in the blue, with the O₂ and water bands in it. On the noon skies, the height in the picture is the height in the sky, 90° round from the Sun: the model's spectrum there, without the night's sources. On the globe, the limb and the disk are the model's spectra for that height and latitude, the air's bands along a tangent path tens of times longer than straight up. On the Sun's disk, the spectrum is for that point of it, redder toward the edge. The Sun's Fraunhofer lines and the air's O₂ A and B bands and the 720 nm water band are added from standard depths, scaled by each epoch's oxygen and water, since the model does not resolve them; they are too narrow to change the color. Plots are normalized: a line brighter than the scale runs off the top. Ground, scenery and clouds have no spectrum.

The sky model is spherical-shell single scattering with a delta-Eddington multiple-scattering correction, from 380 to 780 nm, with CIE 1931 color matching and sRGB output without chromatic adaptation: colors are what a daylight-balanced camera would record, not what an adapted eye would perceive. Clouds are omitted from the model (the VR clouds are scenery), and the compositions of the ancient atmospheres carry order-of-magnitude uncertainty.

The clock runs over one solar day of the epoch. Days were shorter when the Moon was closer: the Earth–Moon angular momentum is held fixed, giving 23.8 hours at 66 Ma, 21.9 at 700 Ma, 17.4 at 2.2 Ga, 15.6 at 2.7 Ga and about 12½ at 4.0 Ga, matching Farhat et al. 2022. At 4.5 Ga, with the Moon only 8 Earth radii out, the same rule gives 5.9 hours. Play runs at the same pace in hours, so short days pass faster.

The year is the same length in seconds in every epoch (the Earth's orbit is set by the Sun's mass, which has barely changed), so it holds more of the shorter days: 368 at 66 Ma, 562 at 2.7 Ga, 1,486 in the Early Hadean. Those epochs count twelve equal months of 30 to 124 days; the 24-hour epochs keep the Gregorian calendar from year −3000 to 7000, and beyond it Gregorian months with leap days set to keep step with the seasons. The date picker takes any year from the Solar System's birth to 250,000 years ahead, as a year or an age (342 ka, 66 Ma, 2.7 Ga), and so does the timeline at the top: between two epochs' ticks it runs from one's year to the next's, and near a tick it holds that epoch at its own year. Away from an epoch's year its description is marked as that epoch's, and links give the year; each epoch's sky holds for the years nearer to it than to any other's (the proto-Earth's until the Moon-forming impact, 4.51 Ga) (or, with auto-select off, for any year). The Sun, the Moon, the day and the Earth's axis, the Sun's place in the Galaxy and the Milky Way it sees, and the rates of comets, meteors and dust follow the year itself: each meets its epoch's value at the epoch, and between epochs runs along a curve suited to it (the young Sun's brightening, the Moon's tidal retreat with the Earth's spin slowing to match, the Sun's orbit through the Galaxy). So does the air. Each epoch's is exact at its year, and between them the gases run log-linearly from one to the next, with the Great Oxidation Event's rise of oxygen and fall of methane between 2.43 and 2.22 Ga (Gumsley et al. 2017; Poulton et al. 2021), oxygen near 1% of today's until the Neoproterozoic rise from about 800 Ma (Lyons et al. 2014) and 5–10% in the Cambrian (Mills et al. 2023); the ozone follows the oxygen by the columns Cooke et al. 2022 simulate. The sky is computed for 62 airs along the timeline, closer together where the air changes fastest, and for each event at steps of its strength; the page blends the two either side of the year shown. So does the glow of cities at night: only lamplight until 1900, then growing with electric light by about 3% a year (an e-folding time of 30 years) to today's, between the growth satellites (Kyba et al. 2017) and the eye (Kyba et al. 2023) report. The clouds, the land and the star lists are still the nearest epoch's. Some skies hold only for a while: the Neoarchean's haze episodes interrupt its clear sky (three in the rocks of 2.65–2.5 Ga, each about 1.4 million years long, after Zerkle et al. 2012 and Izon et al. 2017), the two snowball glaciations the Cryogenian's (the Sturtian, 717–661 Ma, MacLennan et al. 2018 and Rooney et al. 2020; the Marinoan, about 639–635 Ma, Tasistro-Hart et al. 2025; the ice reached the equator within decades and melted in about 2,000 years, Hoffman et al. 2017), the impact winter the Late Cretaceous (about two years), and Tambora's veil today's (1815–17). Within their epochs the supernovae flare and fade (bright for months, gone to the eye in about five years), ζ Oph's comet shower swells and dies away over a million years or so, and the Ordovician break-up's dust and meteors last some 2 million years and its ring tens of millions. The Sun follows the seasons for the date, with today's 23.4° tilt at every epoch's year 0, rocking between about 22° and 24.5° over 41,000-year cycles in the years either side of it (the Moon has held it in that range), and sunlight is 3.4% stronger at perihelion in January than on average. The clock is sundial time: the Sun crosses the meridian at noon.

The time of day is interpolated between 34 computed Sun heights, 1° apart from the horizon to 20° below it; past 9° below, twilight dims by about a magnitude per degree, as measured. Night skies are drawn as a dark-adapted eye sees them, brighter than the daytime display curve would give, and Auto-exposure normalizes brightness so that color shows in dim skies.