Body-fixed frame
The Mars-fixed reference frame is built from IAU 2009 constants (Archinal et al. 2011): pole direction α₀ = 317.68143° − 0.1061°·T, δ₀ = +52.88650° − 0.0609°·T (T = centuries from J2000), and prime-meridian phase W(t) = 176.630° + 350.89198226°·d, where W₀ is the value tied to the crater Airy-0 by the Mars Geodesy/Cartography Working Group (Duxbury et al. 2002). The later IAU 2015 model (Archinal et al. 2018) restates these as α₀ = 317.269202°, δ₀ = +54.432516°, W₀ = 176.049863° with nutation-precession terms of amplitude up to 1.591274°; the two are equivalent only when those periodic terms are carried, so the 2009 constants are used here unmixed. Mars-fixed basis vectors in ICRS: p̂_z = pole (in Cygnus, ~9° from Deneb — there is no bright Mars pole star), p̂_x = ascending node of Mars's equator on the ICRS equator rotated by W(t), and p̂_y = p̂_z × p̂_x. Site (φ, λ) on a sphere of mean radius R = 3389.5 km (Smith et al. 2001 MOLA) gives the local horizon basis (east, north, up), then transformed to ICRS via the body-fixed basis.
Sun and planets
Heliocentric state vectors come from Astronomy Engine 2.1 (Cross 2017–) in EQJ2000. Sun apparent magnitude follows m_V = −26.74 + 5 log₁₀(d_AU), giving ≈ −25.6 to −26.0 V across Mars's heliocentric range (1.38–1.67 AU). Solar angular diameter ~21′ from Mars — about ⅔ Earth's full-Moon size, still a resolved disc. Earth appears as a planet from Mars: at opposition it reaches m_V ≈ −2.5 (sometimes brighter than Sirius), unresolved to the naked eye but distinctly bluish; the Earth-Moon system can resolve as a double "star" with a small telescope. The other inner planets cluster around the Sun; Jupiter and Saturn are bright; Pluto remains telescopic.
Sky color and atmosphere
Mars's atmosphere is ~6.36 hPa at mean datum — 0.6% of Earth's surface pressure. CO₂ dominates (95.32%), with traces of N₂, Ar, O₂. Suspended dust (typical optical depth τ ≈ 0.3–0.6, much higher in storm seasons) Mie-scatters short wavelengths preferentially out of the line of sight, leaving the daytime sky a butterscotch tan; near-Sun and twilight skies pick up a famous bluish glow from the same dust forward-scattering blue light. We approximate this in v1 with a simple sky-color interpolation: as the Sun's altitude crosses the horizon, the SVG background blends from a tan (#c89970) day color down through a deep mahogany twilight to near-black at deep night. Stars are kept rendered regardless — useful pedagogically, though in reality only the brightest are naked-eye-visible during the day.
Phobos and Deimos
Both moons via Jacobson 2010 elements, treated as circular Keplerian in Mars's equatorial plane (inclinations 1.075° and 1.788° to that plane — small enough to neglect at planispheric resolution). Phobos: a = 9376 km, P = 7ʰ 39ᵐ 13ˢ, mean radius 11.27 km, angular diameter ~12′ from the surface. Deimos: a = 23463 km, P = 30ʰ 18ᵐ 43ˢ, mean radius 6.20 km, angular diameter ~2′. Because Phobos's orbital period (7ʰ 39ᵐ) is shorter than Mars's solar day (24ʰ 39ᵐ), Phobos appears to rise in the west and set in the east roughly every 11 hours of Mars time — twice per sol. Deimos has period only slightly longer than the sol, so it crawls eastward against the rotating sky and takes ~2.7 sols between rises.
Stars and constellations
Catalog identical to the Earth and Pluto Sky Observation tools — HYG (Hipparcos / Yale BSC merge) plus Stellarium IAU asterism polylines. Star positions are ICRS J2000; no parallax correction applied (Mars's 1.5 AU baseline shifts the nearest star by ~1.1″ — well below planispheric resolution). Constellation lines connect the same physical stars as on Earth, projected through Mars's frame; Polaris is no longer near the pole, and the celestial-pole region instead falls in Cygnus near Deneb (about 9° offset from the bright star).