Planet Sunsets

About this tool

Three layers make up the picture, and the label on it names them. The sky’s colours come from tables sampled from NASA’s simulated video (NASA Goddard SVS 13645, by Geronimo Villanueva and James Tralie, made with the Planetary Spectrum Generator). The ground on Mars, Venus and Titan is an AI-imagined picture, generated from a text description: it is not a photograph and shows no real place. The Sun, its glow, the sea and Uranus’s clouds are this page’s own drawing. No spacecraft photograph is shown on this page.

Why are Martian sunsets blue? From JPL’s caption of Curiosity’s photograph: fine dust lets blue light through more efficiently, so blue stays close to the Sun while yellow and red scatter more widely, and the effect is strongest at sunset because the light path is longest. The colour table taken from NASA’s video runs from tan to a white that clips near the Sun, so on Mars, from mid-dusk on, this page changes the hue: the brightness is still the simulation’s, but the colour follows the pattern measured in that photograph, blue-white near the Sun and fading to grey-blue. This is an adjustment made here, not NASA’s output; the link above shows the real thing.

Not every world is hazy, so the air over the land is set per world. Mars: about 0.006 bar, 95% carbon dioxide, thin air with fine dust, so distant ridges stay clear. Venus: 92 bar, 96.5% carbon dioxide, with sulfuric-acid cloud hiding the Sun from far overhead, not fog on the ground; but air that dense scatters strongly by itself (roughly a kilometre’s mean free path in green light), so distant land fades there too. Titan: about 1.5 bar, about 95% nitrogen and 5% methane, in orange haze that softens distant land (Huygens’s data imply the air near the ground was clear for tens of kilometres). Earth’s three skies show clear air, haze and overcast over a drawn sea. Uranus is 82.5% hydrogen and 15% helium, with no ground at all. Distance sets the Sun’s size, from 1.38× Earth’s at Venus to 0.052× at Uranus.

What up and down means: NASA’s simulation never states where the Sun is, so dragging up and down moves through its frames, and the percentage is how far the Sun has gone towards this page’s estimate of that world’s sunset, not degrees of elevation. The Sun’s height and the picture’s brightness are schematic: Titan’s real surface in particular is far darker than this, and Venus’s is dimmer and more orange. On Mars the glow is also moved to surround the Sun, which the table does not do. Venus and Titan show no crisp Sun, only a diffuse glow, and Earth under overcast shows the same. Uranus’s clouds are schematic, drawn from the table’s own colours.

Dragging left or right moves along the distance from the Sun. Between two stops the page fades one world into the other under a single Sun whose size follows the distance, though only Earth, Mars and Uranus show one to see, since Venus and Titan have only a glow. The blend is a transition, not a simulated atmosphere, and the label says so while it lasts. What travels with you, and what the strip and the percentage show, is how far the Sun has gone towards setting on each world, so a set Sun stays set and coming back returns the same moment. Each world’s sunset point is this page’s estimate from its colour table (where the glow at the horizon starts to fall), not a figure from NASA. The shared clock runs every Sun through sinking its own diameter, the one span this page has a real time for; NASA’s simulation has no time axis, so the rest of the dusk has none.

How the numbers are computed, in one function: the Sun’s apparent diameter is 0.5332° divided by the mean distance in AU (Venus 0.723, Earth 1, Mars 1.524, Titan 9.58, Uranus 19.19); the time to sink its own diameter is that diameter ÷ 360° × the length of the solar day, at the equator at equinox (Venus 116.75 days, Earth 24 h, Mars 24.66 h, Titan 15.97 days, Uranus 17.24 h). Distances are means, so the real figure varies through the year (about ±9% on Mars). Venus rotates backwards, so the Sun sets in the east there. Uranus rotates backwards on its side, so where the Sun sets changes with its seasons, and the 4.8 s holds only at equinox: about 35 s at solstice, and at high latitudes the Sun does not set for years. The Sun is drawn at true relative size between worlds, Earth = 1 (minimum 2 px, so distant Suns can be enlarged), but not to scale with the land in the picture.

Sources. Skies: NASA Goddard SVS 13645, Geronimo Villanueva / James Tralie, Planetary Spectrum Generator. Atmospheres: NASA planetary fact sheets and NASA’s Titan facts. Mars’s colour pattern: measured from Curiosity PIA19400 (NASA/JPL-Caltech/MSSS/Texas A&M Univ.), which is linked, not reproduced here. Ground: AI-generated images. This page was not made by NASA and NASA does not endorse it; no NASA logo is used, and NASA’s video and its soundtrack are not used.