henge

The sky and the calendar

The ephemeris stack — Established

Everything the app computes about the sky, and the published work it is computed from. Each is pinned by fixtures a reader can check against the printed page.

Piece	Source
Solar position	Meeus, Astronomical Algorithms 2nd ed., ch. 25 (abridged VSOP87)
Obliquity	Laskar’s polynomial, Meeus eq. 22.3
ΔT (TT−UT)	Espenak & Meeus, NASA/TP-2006-214141
Calendar ↔ Julian Day	Meeus ch. 7, astronomical years, Julian/Gregorian reform honoured
Moon	truncated ELP-2000 (position, distance, parallax); phases by elongation root-find
Planets	truncated VSOP87D (CDS VI/81), magnitudes from Astronomical Almanac formulae
Precession	series fitted to the modern era; the 2800 BC pole costs about half a degree and StarFieldTests says so out loud
Stars	Hipparcos ESA SP-1200 (CDS I/239), V ≤ 6.5, proper motion applied
Star names	IAU-CSN register (E. Mamajek), regenerated whole by scripts/generate_star_names.py
Refraction & rise	standard −0.833° sunrise definition throughout
Sky model	Preetham et al. 1999, closed form
Moon face	NASA SVS CGI Moon Kit (LRO), public domain

The wheel of the year — Established arithmetic, Debated meaning

• Eight stations solved from apparent solar longitude — the quarters (equinoxes, solstices) and cross-quarters (Imbolc, Beltane, Lughnasadh, Samhain) at exact 45° steps of the sun.
• The honest number the app shows rather than hides: the customary festival dates run three to seven days from the sun’s own cross-quarter moments. Two calendars, two questions.
• That the quarters mattered at Stonehenge is Established (the axis is the plan); that the cross-quarters were observed there is tradition read backwards — the lore panel tiers it.

The constellation figures — our own drawings

29 figures, 188 segments, authored in-repo against the vendored catalogue (scripts/generate_constellation_lines.py). Membership is ancient common knowledge (Ptolemy’s Almagest); the stick figures are ours, which is what keeps the layer licence-free. Thuban rides in Draco because it held the pole for the builders — the deep-time sky’s load-bearing fact, pinned in StarFieldTests.

The zodiac — Established positions, labelled as constellations

Twelve IAU constellation centroids (Delporte 1930 / Roman 1987, VizieR VI/42), riding precession with their stars. Constellations, not signs: the two have drifted a full sign apart since Babylon, and an app about precession keeps the distinction.

The lunar machinery — Debated

Hawkins 1963/1965 and Hoyle 1966 read the 56 Aubrey holes as an eclipse computer. research/lunar-markers.md holds the audit; the app’s gold-marker mode enacts Hoyle’s scheme, badged on screen every time, scored honestly against the ephemeris.