# Sky Now — provenance

**What this is.** A standalone tool in the Tools room (`/tools/sky-now/`): the Sun,
Moon, and five naked-eye planets over any place on Earth at any moment — altitude,
azimuth, rise/transit/set, twilight, and Moon phase, all computed in the browser.

**The engine.** `ephem.mjs` in this directory is a **byte-for-byte copy** of the
verified astronomy engine that powers the stratum *The Sky Above You*:

```
public/strata/the-sky-above-you/ephem.mjs   (canonical)
public/tools/sky-now/ephem.mjs              (this copy)
sha256: 6f13cb4e4d5c0d496fc05a362f20357904938af09a2e69c182c6ed0409955edf
```

The copy is kept identical on purpose — the same file that the tool runs is the
file the verifier checks — following the Tools-room precedent set by Easy Batch
Prompting (self-contained tool; trust-critical files identical to their source).

**The check (show-the-check).** The engine is validated by
`research/the-sky-above-you/verify.mjs` — **42 checks**, green, reproducing Jean
Meeus's worked examples (Sun ch. 25, Moon ch. 47, sidereal time ch. 12, rise/set
ch. 15) and cross-checking geocentric positions against live **JPL Horizons
(DE441)** fixtures. Verify the copy is still identical, then re-run the checks:

```sh
cmp public/strata/the-sky-above-you/ephem.mjs public/tools/sky-now/ephem.mjs
node research/the-sky-above-you/verify.mjs        # → 42 passes / 0 failures
```

**If the canonical engine changes:** re-copy the file, update the sha256 above,
and re-run the verifier. Do not edit this copy independently — the honesty claim
("the tool runs the verified engine") depends on the two files being identical.

**What the tool adds over the stratum.** Direct latitude/longitude entry (works
with no geocoder), a shareable URL that encodes the exact place *and moment*, and
tool-framed copy. It **drops** the stratum-only layers: the collective `/api/marks`
map and the ground-page cross-links. No new astronomy — the science is unchanged.

**Honest scope.** Naked-eye Solar System only: Sun, Moon, Mercury, Venus, Mars,
Jupiter, Saturn. **No star catalog, no deep-sky objects.** Naked-eye precision, not
arc-second (bounds stated on the page). Place search uses OpenStreetMap / Nominatim
— the only network call, and skippable via manual coordinates.