Henge

The instrument

The sky is computed, and every claim about the stones can be checked.

Henge is a real-time 3D simulation of Stonehenge for iPhone and Mac, built on a hand-rolled Metal 3 renderer — no game engine, no third-party code. Choose any date, from the monument's own era to tonight, and stand inside a sky that is computed rather than painted.

The sky is computed, never faked

Sun, moon, planets and 8,870 Hipparcos stars from the published ephemerides — Meeus, VSOP87, Laskar's obliquity — with ΔT and atmospheric refraction carried honestly back to 3000 BC. No hardcoded azimuths. The solstice bearing is derived, and the fact that it misses the built axis by a fraction of a degree is a finding, not a bug.

The shadows are the calendar

The renderer's shadows are measured against an analytic solution that lives outside the renderer, and the suite fails if they disagree by more than 0.28 m on the ground — a tolerance set by the shadow map's texel, not by taste. Visual plausibility is never accepted as agreement.

III

The stones are surveyed

Every dimension traces to cited survey data — Cleal, Walker & Montague's Stonehenge in its Landscape, Petrie's numbering, the measured station rectangle. Every statement the app makes about the monument carries a tier and a citation, enforced by the type system.

True scale, on real ground

One unit is one metre, everywhere — the stones, the grass, the sun's angular diameter, and Salisbury Plain itself from Environment Agency LiDAR. The sun rises over the actual skyline ridge, which delays it, which moves its bearing. That is the kind of detail the whole app is made of.

The numbers

Accuracy is measured by the test suite, not asserted by the marketing.

These tolerances are pinned by tests against published worked examples — loosening any of them requires the owner's sign-off, by charter.

0.001°

Solar apparent longitude

vs. Meeus's worked examples

0.002°

Right ascension & declination

vs. published ephemeris values

0.0001°

Obliquity of the ecliptic

Laskar — what keeps 2500 BC honest

0.28 m

Rendered shadow vs. analytic solution

measured on the ground, every run

What the engine establishes, computed rather than assumed — sunrise and sunset bearings against the monument's built axis of 49.9°.
Bearing	2500 BC	2026
Midsummer sunrise, flat horizon	47.71°	48.77°
Midsummer sunrise, 0.6° skyline	49.08°	50.11°
Midwinter sunset, 0.6° skyline	230.10°	231.12°

Against the built axis, midwinter sunset is the tighter fit — two tenths of a degree in the builders' own era. The monument may face the dying year, not the growing one. The app shows you both and lets the sky argue.

Four thousand years

Watch the monument rise — or fall — while the calendar walks between the eras.

Two surveyed states, never blended: the completed Stage-2 monument of about 2200 BC, and the ruin as it stands. Switching between them plays the passage — the ditch silts, the bank settles, stones arrive one at a time in construction order and leave in gravity's own — while the sun arcs overhead in timelapse and the year counter spins through four millennia. Every frame of that sky is a real computation for a real date.

AS IT WASc. 2200 BC · the whole monument · fresh chalk banks

AS IT STANDStoday · the ruin as surveyed · the silted ring

The research

Everything the app claims, with its sources, in the open.

The knowledge base behind the build — assembled so a reader can check what we made against where it came from. The code carries the same citations at the point of use.

The earthwork and the ground
The enclosure ditch and bank — which Stonehenge wears backwards — the turf, and the reference imagery. READ → The stones and their journeys
Sarsens from the Marlborough Downs, bluestones from Wales, and the Altar Stone's seven-hundred-kilometre Scottish provenance. READ → Geometry and alignments
Surveyed dimensions, the axis, the station rectangle that is only a rectangle at this latitude — and the speculative constructions, labelled as such. READ → The sky and the calendar
The ephemeris stack, the eightfold wheel of the year, and the lunar hypotheses from Hawkins and Hoyle. READ → Data provenance
Every vendored dataset — LiDAR terrain, Hipparcos, the lunar photograph — its licence and its file. READ →

ESTABLISHED DEBATED MODERN TRADITION

Every claim carries one of three tiers, the same register the app enforces on screen. Druidry is presented as living inheritance, never as the builders' religion — the stones are two millennia older than the druids, and the app says so plainly.