• The camera glides to a body without changing your zoom or viewing angle (it only backs off if it would end up inside the body). Flying in close keeps your viewing angle too; if you are looking at the night side, the Night sides setting can light it.
• Lock (in the settings) keeps the camera travelling with the selected body. Unlock it to let the body drift away, and lock again to catch up with it. Right-drag, shift-drag, the arrow keys or a two-finger drag move the view sideways; locked, the camera keeps following the body from there.
• The speed slider runs from real time (1×) at its left end to about five years per second at its right. Up to 60× the speed is shown as a multiple of real time, faster as simulated time per second (“5 min/s”, “2 d/s”, “1 yr/s”). Play and Pause keep the chosen speed: paused, it is shown dimmed. Running backward keeps the speed too, and marks it with a −.
• The calendar takes dates in the time the clock shows (local or UT). On a phone it opens the phone’s own date picker; on a computer type or pick a date and press Set (or Enter).
• The button above the clock switches between Local time and Scientific, which shows Universal Time (UT, Earth’s rotation, UT1) and adds TT (the uniform time the planets move by), ΔT = TT − UT and the Julian Date. The model takes UTC to be UT1; the two never differ by more than 0.9 s.
• Night sides (in the settings) adds a light from where you are looking, so night sides and the dark of an eclipse show their surface; the further right, the brighter. It is not real light: the shadows themselves stay exactly where they are. At Real, night sides are black as a camera exposed for daylight sees them; your eye, adapted to the dark, would see the Moon’s dark side faintly lit by Earth (earthshine). Sun glare turns off the glare around the Sun, which helps to see what lies close to it, such as a planet crossing the Sun’s disc.
• Up close, a card beside the body describes it. It hangs beside the body, growing and shrinking with it as you zoom and swinging a little behind as you turn the view, and fades away as you zoom out (or far in), and the view moves aside to make room for it where the screen is small. × closes it until you choose a body again. Tap the card anywhere else (More data) to open the information panel with the body’s figures, and the panel’s – goes back to the card.
• Choose Panel under Body info in the settings to have the information panel instead, always there in the corner: it starts folded down to the body’s name; tap it (or +) to open it, and – folds it again. Tap an open panel to show all of its data (More data), and again to hide it.

At real scale the Solar System is almost entirely empty: if the Sun were a grapefruit, Earth would be a grain of sand 13 m away and Neptune 390 m away. This is how the model first opens. The overview shrinks distances and enlarges bodies so everything fits on one screen, and the Scale slider in the settings morphs smoothly between the two, from real scale on the left to the overview on the right. Directions are never distorted, so alignments such as eclipses and conjunctions look right at every setting. Bodies too small to see are marked by their labels; hovering over one circles it.

Shadows are computed for every point on a surface from the true sizes and distances of the Sun and of the body in the way. They are therefore correct at every scale setting, even when the Moon is drawn closer than it really is. The Sun’s limb darkening is included (the light left in a deep partial shadow comes from the Sun’s redder edge, which tints it brown), and so is the light’s travel time: the sunlight hitting Earth passed the Moon 1.3 s earlier, which moves a solar eclipse 35 km along its track.

During a lunar eclipse the Moon turns copper-red, because the only light reaching it has been bent and reddened by Earth’s atmosphere. Saturn’s rings cast shadows on the planet and Saturn casts one on its rings. Jupiter’s moons cast theirs on Jupiter and disappear into its shadow, whose flattened shape is taken into account (it lengthens Io’s eclipses by three minutes). Pluto and Charon eclipse each other only in two seasons of each 248-year orbit, the last in 1985–1990.

One liberty: where a moon’s shadow falls on its planet, the partial shadow is drawn darker than a light meter would record. On a screen a surface at half light looks almost fully lit, and the Moon’s partial shadow, thousands of km across, would be only a faint smudge; it is deepened a little, and the full shadow, where the eclipse is total, stays the one black core. Where every shadow begins and ends, and the full shadow itself, are exact. Earth’s shadow on the Moon needs no such help and is drawn as measured.

The positions were compared with NASA JPL’s DE440 ephemeris (via Horizons) over 1800–2050:

Outside 1800–2050 an Extrapolated tag appears next to the clock. The planets stay good to about an arcminute for centuries; the moons of Mars, Saturn, Neptune and Pluto are extrapolated fits. Uranus’s moons are fitted to JPL’s ephemeris over 1600–2400, all it covers; beyond that Miranda’s and Ariel’s places along their orbits grow uncertain, by tens of degrees by the years 1000 and 3000.

Earth’s rotation depends on ΔT, the slowly growing lag of Earth’s rotation behind atomic time. The model uses the measured values (telescopic timings since 1657, atomic clocks since 1955) and the official prediction to 2033. Before 1657 ΔT is estimated from historical eclipse records and uncertain by minutes; beyond the 2030s it cannot be predicted well, so eclipse tracks can shift east–west by tens of km.

Dates before 15 October 1582 are in the Julian calendar, as in historical records and NASA’s eclipse canon. The Gregorian reform then skipped ten days.

The Sun is the only light, so night sides are black, as a camera exposed for daylight sees them (the Night sides slider in the settings lights them); Earth’s night side shows its city lights. Distant worlds are brightened so they stay visible (Neptune receives 1/900 of the sunlight Earth does).

The Sun’s disc is drawn as it looks in white light through a filter: featureless, darker and redder toward its edge. Around it is the glare it would cause in an eye or a camera exposed for the planets, which drowns the nearby stars; it fades as the disc grows large, as it does in a close or telescope view. Its fine radial streaks are light scattered inside the eye’s lens, and like the real ones they change as you look around. Behind the stars lies the faint glow of the Milky Way, with the Magellanic Clouds and the Andromeda galaxy.

Earth’s atmosphere is a thin blue haze toward the limb, lit only where sunlight reaches it. The Moon and other dusty airless worlds scatter light back toward the Sun, so a full Moon is evenly bright to its edge instead of shading like a ball. Every moon turns according to the IAU rotation models, including the slight rocking (libration) that keeps the Moon from showing exactly the same face.

Each orbit line is the ellipse the body would follow if nothing but its primary pulled on it, fitted to its motion at this instant. Earth’s is the ellipse of the Earth–Moon pair’s centre of mass (4,700 km from Earth’s centre, inside the Earth), drawn through Earth’s centre: Earth itself weaves a few thousand km to either side of it every month. From Saturn outward the ellipses are drawn around the Solar System’s centre of mass, which the Sun itself circles because of Jupiter’s pull.

Some things are representative rather than exact:

• Cloud patterns on Earth, Venus, Jupiter and the other giants are snapshots (Venus’s clouds do circle the planet every four days, as the real ones do), and sunspots are not shown.
• The fine structure of Saturn’s rings comes from an image whose opacity is scaled to measured values.
• Phobos and Deimos are drawn as ellipsoids of their true dimensions.
• Parts of Charon and Triton, and the northern halves of Uranus’s five large moons, have never been seen, because they were in polar night when New Horizons and Voyager 2 flew past; they are filled in plain grey. Voyager saw Triton’s colours only through orange, violet and ultraviolet filters, so its map is shown in grey, tinted with the moon’s overall colour.

• Ephemerides: Astronomy Engine (Don Cross, MIT): VSOP87, Brown’s lunar theory (Improved Lunar Ephemeris), Lainey L1.2; satellite fits to JPL Horizons.
• Rotation: IAU WGCCRE 2015 (Archinal et al. 2018), for the moons via NASA NAIF’s pck00011 kernel. ΔT: US Naval Observatory and IERS.
• Physics of the picture: lunar–Lambert photometry (McEwen 1991); Saturn’s ring optical depths (Colwell et al. 2009); Danjon’s rule for Earth’s shadow.
• Stars: Yale Bright Star Catalogue, 5th ed. (Hoffleit & Warren, via CDS). Milky Way: Deep Star Maps 2020, NASA/Goddard Space Flight Center Scientific Visualization Studio, from Hipparcos-2, Tycho-2 and Gaia DR2 (ESA/Gaia/DPAC).
• Venus, Jupiter, Uranus, Saturn’s rings, Earth’s clouds and night lights: Solar System Scope (CC BY 4.0), based on NASA data; their Neptune too, recoloured to its true pale greenish blue (Irwin et al. 2024).
• Earth: NASA Blue Marble Next Generation (Reto Stöckli, NASA Earth Observatory), one map per month, with sea ice from the NSIDC Sea Ice Index. Moon: NASA SVS CGI Moon Kit (LRO). Mars: USGS Viking MDIM 2.1 mosaic, coloured after Hubble’s true-colour Mars. Mercury: MESSENGER MDIS basemap (NASA / JHUAPL / Carnegie, USGS). Saturn: Hubble OPAL program (NASA / ESA / STScI, PI Simon), August 2025. Colours calibrated to the planets’ measured albedos (Mallama et al. 2017).
• Io, Ganymede, Callisto, Europa, Phobos: USGS Astrogeology / NASA / JPL Voyager and Galileo global mosaics (public domain); Callisto’s patchwork look is the uneven coverage of those missions. Deimos: Philip Stooke’s Viking, Mariner 9 and HiRISE mosaic (NASA PDS). Pluto: NASA / JHUAPL / SwRI, New Horizons (public domain). Charon: USGS Astrogeology New Horizons LORRI/MVIC global mosaic; Triton: USGS Astrogeology Voyager 2 global color mosaic; Miranda, Ariel, Umbriel, Titania, Oberon: USGS Astrogeology (Tammy Becker) / NASA / JPL Voyager 2 mosaics (all public domain). Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus: Cassini global color maps, 2014 (NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute), shown in their brightness only, tinted with each moon’s overall colour, as their colours were enhanced.
• Rendering: three.js (MIT). Fonts: EB Garamond and IBM Plex Mono (OFL).

Made by Luca Janken. The source code is on GitHub.