Introduces watch/, a real Pebble watchapp (built via pebble build) that

shows the daily tarot/astrology reading on-device: persistence, reading
computation, and UI screens, driven by the existing engine/interpreter
code linked in unchanged where possible.

Required engine-side changes to make that linking work:
- A compact low-precision ephemeris (lowprec_ephemeris.c) replacing the
  vendored ~127KB Astronomy Engine, which doesn't fit the watch's ~64KB
  app budget.
- Hand-rolled sqrt/atan2/sin/cos replacements for that ephemeris and
  astro.c's Ascendant calculation - Pebble's statically-linked libm
  hard-faults on real hardware under this app's -fPIE link for all four.
- narrative.c/guidance.c refactored from FILE*/fprintf onto snprintf-
  based buffers, since Pebble's SDK blocks fprintf at compile time.

engine/i18n/de.lang

@@ -22,6 +22,12 @@ ui.transiting=Transit
 ui.natal=natal
 ui.orb=Orbis
 
+# --- Nur die Watch-App (watch/src/c/main.c) - wird vor der ersten
+# Einrichtung ueber die Konfigurationsseite gezeigt, wenn noch keine
+# Geburtsdaten fuer eine Deutung vorliegen ---
+ui.setup_required_title=Einrichtung erforderlich
+ui.setup_required_body=Öffne die Einstellungen dieser App in der Pebble-App, um deine Geburtsdaten einzugeben, und starte Deck in a Dash danach erneut.
+
 # --- Planeten (astro.c) ---
 body.sun=Sonne
 body.moon=Mond

engine/i18n/en.lang

@@ -29,6 +29,12 @@ ui.transiting=Transiting
 ui.natal=natal
 ui.orb=orb
 
+# --- Watch app only (watch/src/c/main.c) - shown before the first
+# config-page submission, when there's no birth data yet to compute a
+# reading from ---
+ui.setup_required_title=Setup Required
+ui.setup_required_body=Open this app's settings from the Pebble mobile app to add your birth details, then reopen Deck in a Dash.
+
 # --- Planets/luminaries (astro.c) ---
 body.sun=Sun
 body.moon=Moon

engine/src/astro.c

@@ -1,14 +1,22 @@
-#define _POSIX_C_SOURCE 200809L /* for gmtime_r */
-
 #include "astro.h"
 #include "i18n.h"
+
+/* The watch build (PBL_SDK_3) uses a compact low-precision ephemeris
+ * instead of the vendored Astronomy Engine, which at ~127KB of compiled
+ * code doesn't fit a whole Pebble app's ~64KB budget - see
+ * lowprec_ephemeris.h's own doc comment. AstroTime abstracts over the
+ * two representations of "a moment in time" this file can be built
+ * against: a Julian Date (double) for the watch, or the vendored
+ * library's own astro_time_t on desktop. Every function below that takes
+ * or returns a time branches internally on PBL_SDK_3, but keeps the same
+ * name/signature/call sites either way, so the two implementations stay
+ * easy to compare line-by-line. */
+#ifdef PBL_SDK_3
+#include "lowprec_ephemeris.h"
+typedef double AstroTime;
+#else
 #include "../third_party/astronomy/astronomy.h"
-
-#include <math.h>
-#include <stdbool.h>
-#include <string.h>
-
-/* DEG2RAD / RAD2DEG come from astronomy.h. */
+typedef astro_time_t AstroTime;
 
 static const astro_body_t k_astro_body[NUM_BODIES] = {
   [PLANET_SUN] = BODY_SUN,
@@ -22,6 +30,20 @@ static const astro_body_t k_astro_body[NUM_BODIES] = {
   [PLANET_NEPTUNE] = BODY_NEPTUNE,
   [PLANET_PLUTO] = BODY_PLUTO,
 };
+#endif
+
+#include <math.h>
+#include <stdbool.h>
+#include <string.h>
+
+/* Not M_PI - see lowprec_ephemeris.c's own comment on why. Only needed
+ * (and only defined) on the watch path; astronomy.h supplies these on
+ * desktop, as before. */
+#ifdef PBL_SDK_3
+#define DECK_PI 3.14159265358979323846
+#define DEG2RAD (DECK_PI / 180.0)
+#define RAD2DEG (180.0 / DECK_PI)
+#endif
 
 static double normalize_degrees(double deg) {
   double d = fmod(deg, 360.0);
@@ -36,45 +58,91 @@ static void longitude_to_position(double longitude, PlanetPosition *out) {
 
 /* Mean obliquity of the ecliptic (IAU low-precision polynomial), matching
  * what Astronomy Engine computes internally in its (non-exported)
- * mean_obliq(). T is Julian centuries since J2000.0. */
+ * mean_obliq(). T is Julian centuries since J2000.0. A slow, purely
+ * time-based polynomial, not subject to orbital approximation error -
+ * shared unchanged by both ephemeris implementations. */
 static double mean_obliquity_deg(double t_centuries) {
   double t = t_centuries;
   return 23.4392911 - 0.0130042 * t - 0.00000016 * t * t + 0.000000504 * t * t * t;
 }
 
+static double julian_centuries_since_j2000(const AstroTime *time) {
+#ifdef PBL_SDK_3
+  return (*time - 2451545.0) / 36525.0;
+#else
+  return time->tt / 36525.0;
+#endif
+}
+
+static double sidereal_time_hours(AstroTime *time) {
+#ifdef PBL_SDK_3
+  return lowprec_gmst_hours(*time);
+#else
+  return Astronomy_SiderealTime(time);
+#endif
+}
+
 /* Ascendant (rising ecliptic degree), via the standard RAMC/obliquity/
  * latitude identity (Duffett-Smith & Zwart, "Practical Astronomy with
- * your Calculator or Spreadsheet"). Astronomy Engine has no built-in
- * Ascendant function. */
-static double compute_ascendant(astro_time_t *time, double latitude_deg,
+ * your Calculator or Spreadsheet"). Neither ephemeris implementation has
+ * a built-in Ascendant function. */
+static double compute_ascendant(AstroTime *time, double latitude_deg,
                                  double longitude_deg) {
-  double gast_hours = Astronomy_SiderealTime(time);
+  double gast_hours = sidereal_time_hours(time);
   double ramc_deg = normalize_degrees(gast_hours * 15.0 + longitude_deg);
-  double eps_deg = mean_obliquity_deg(time->tt / 36525.0);
+  double eps_deg = mean_obliquity_deg(julian_centuries_since_j2000(time));
 
   double ramc = ramc_deg * DEG2RAD;
   double eps = eps_deg * DEG2RAD;
   double lat = latitude_deg * DEG2RAD;
 
+  /* tan(lat) as sin(lat)/cos(lat) - no <math.h> tan() needed. */
+#ifdef PBL_SDK_3
+  double y = -lowprec_cos(ramc);
+  double x = lowprec_sin(ramc) * lowprec_cos(eps) +
+             (lowprec_sin(lat) / lowprec_cos(lat)) * lowprec_sin(eps);
+  return normalize_degrees(lowprec_atan2(y, x) * RAD2DEG);
+#else
   double y = -cos(ramc);
-  double x = sin(ramc) * cos(eps) + tan(lat) * sin(eps);
+  double x = sin(ramc) * cos(eps) + (sin(lat) / cos(lat)) * sin(eps);
   return normalize_degrees(atan2(y, x) * RAD2DEG);
+#endif
 }
 
-static astro_time_t time_from_birth(const BirthData *birth) {
+static AstroTime time_from_birth(const BirthData *birth) {
+#ifdef PBL_SDK_3
+  double jd = lowprec_julian_date(birth->year, birth->month, birth->day,
+                                   birth->hour, birth->minute, 0.0);
+  return jd - birth->utc_offset_hours / 24.0;
+#else
   astro_time_t local = Astronomy_MakeTime(birth->year, birth->month, birth->day,
                                            birth->hour, birth->minute, 0.0);
   return Astronomy_AddDays(local, -birth->utc_offset_hours / 24.0);
+#endif
 }
 
-static astro_time_t time_from_unix(time_t utc_moment) {
-  struct tm tm_utc;
-  gmtime_r(&utc_moment, &tm_utc);
-  return Astronomy_MakeTime(tm_utc.tm_year + 1900, tm_utc.tm_mon + 1, tm_utc.tm_mday,
-                             tm_utc.tm_hour, tm_utc.tm_min, (double)tm_utc.tm_sec);
+/* Plain gmtime() (not the POSIX-only gmtime_r()) - deliberately: this
+ * project is single-threaded everywhere it runs, including on the watch,
+ * where only the non-reentrant classic C library functions are provided
+ * (see astro.h's own include-guard comment). Used immediately, so its
+ * static internal buffer is never a hazard here. */
+static AstroTime time_from_unix(time_t utc_moment) {
+  struct tm *tm_utc = gmtime(&utc_moment);
+#ifdef PBL_SDK_3
+  return lowprec_julian_date(tm_utc->tm_year + 1900, tm_utc->tm_mon + 1, tm_utc->tm_mday,
+                              tm_utc->tm_hour, tm_utc->tm_min, (double)tm_utc->tm_sec);
+#else
+  return Astronomy_MakeTime(tm_utc->tm_year + 1900, tm_utc->tm_mon + 1, tm_utc->tm_mday,
+                             tm_utc->tm_hour, tm_utc->tm_min, (double)tm_utc->tm_sec);
+#endif
 }
 
-static void compute_body_positions(astro_time_t time, PlanetPosition out[NUM_BODIES]) {
+static void compute_body_positions(AstroTime time, PlanetPosition out[NUM_BODIES]) {
+#ifdef PBL_SDK_3
+  for (int b = 0; b < NUM_BODIES; b++) {
+    longitude_to_position(lowprec_geocentric_longitude((Body)b, time), &out[b]);
+  }
+#else
   /* Astronomy_EclipticLongitude() computes *heliocentric* longitude (and
    * outright rejects BODY_SUN) - wrong for astrology, which needs the
    * apparent geocentric position. Astronomy_GeoVector() + Astronomy_Ecliptic()
@@ -84,6 +152,7 @@ static void compute_body_positions(astro_time_t time, PlanetPosition out[NUM_BOD
     astro_ecliptic_t eclip = Astronomy_Ecliptic(geo);
     longitude_to_position(eclip.elon, &out[b]);
   }
+#endif
 }
 
 /* Whole-sign house number (1-12) for `sign`, counting from `ascendant_sign`
@@ -101,7 +170,7 @@ static void assign_houses(PlanetPosition bodies[NUM_BODIES], ZodiacSign ascendan
 }
 
 void astro_compute_natal_chart(const BirthData *birth, NatalChart *out) {
-  astro_time_t time = time_from_birth(birth);
+  AstroTime time = time_from_birth(birth);
 
   compute_body_positions(time, out->bodies);
   out->ascendant_longitude = compute_ascendant(&time, birth->latitude, birth->longitude);
@@ -142,13 +211,23 @@ static double orb_for_pair(Body transiting, Body natal) {
 
 void astro_compute_daily_transits(time_t utc_moment, const NatalChart *natal,
                                    DailyTransits *out) {
-  astro_time_t time = time_from_unix(utc_moment);
+  AstroTime time = time_from_unix(utc_moment);
 
   compute_body_positions(time, out->bodies);
   assign_houses(out->bodies, natal->houses[0]);
 
+#ifdef PBL_SDK_3
+  /* Sun-to-Moon angle directly from the positions just computed above -
+   * same convention as Astronomy_MoonPhase() (0 = new, 90 = first
+   * quarter, 180 = full, 270 = last quarter), so moon_phase_from_angle()
+   * below needs no platform-specific branch of its own. */
+  double phase_angle = normalize_degrees(out->bodies[PLANET_MOON].ecliptic_longitude -
+                                          out->bodies[PLANET_SUN].ecliptic_longitude);
+#else
   astro_angle_result_t phase = Astronomy_MoonPhase(time);
-  out->moon_phase = moon_phase_from_angle(phase.angle);
+  double phase_angle = phase.angle;
+#endif
+  out->moon_phase = moon_phase_from_angle(phase_angle);
 
   out->aspect_count = 0;
   for (int t = 0; t < NUM_BODIES; t++) {

engine/src/astro.h

@@ -1,7 +1,17 @@
 #ifndef DECK_ASTRO_H
 #define DECK_ASTRO_H
 
+/* Pebble's SDK pre-defines <time.h>'s own include guard (so a real
+ * #include <time.h> is a silent no-op) and instead declares struct
+ * tm/time_t/gmtime()/etc. itself, inside pebble.h - see PBL_SDK_3,
+ * predefined by the Pebble build for every target platform. Desktop
+ * builds (deck-engine, the interpreter, and this file's own tests) use
+ * plain <time.h> as normal. */
+#ifdef PBL_SDK_3
+#include <pebble.h>
+#else
 #include <time.h>
+#endif
 
 #define NUM_BODIES 10
 #define MAX_ASPECTS 100

engine/src/i18n.c

@@ -1,12 +1,33 @@
 #include "i18n.h"
 
-#include <stdio.h>
 #include <string.h>
 
-/* Fixed-size, no heap allocation - deliberately (see i18n.h): keeps this
- * usable in a small embedded environment later, not just on desktop.
- * ~140 keys are shipped today (see engine/i18n/en.lang); this leaves
- * plenty of headroom for more languages/keys without growing the format. */
+/* i18n_load_table()'s catalog: just three pointers, always compiled -
+ * negligible RAM regardless of platform. Deliberately zero-copy: the
+ * watch's compiled-in translation tables already live in flash as
+ * `static const char *const` arrays, so i18n_get() reads them in place
+ * rather than copying ~140 keys/values into RAM the watch doesn't have
+ * (see i18n_load()'s g_entries[] below, sized for desktop use only). A
+ * later i18n_load_table() call (e.g. switching language) simply
+ * re-points these, no free() needed since nothing was ever copied. */
+static const char *const *g_table_keys;
+static const char *const *g_table_values;
+static int g_table_count = 0;
+
+void i18n_load_table(const char *const *keys, const char *const *values, int count) {
+  g_table_keys = keys;
+  g_table_values = values;
+  g_table_count = count;
+}
+
+#ifndef PBL_SDK_3
+#include <stdio.h>
+
+/* Fixed-size, no heap allocation. ~140 keys are shipped today (see
+ * engine/i18n/en.lang); this leaves plenty of headroom for more
+ * languages/keys without growing the format. Desktop-only: at ~140KB
+ * this is far too large for the watch's own RAM budget, which is why
+ * the watch instead uses i18n_load_table()'s zero-copy path above. */
 #define I18N_MAX_ENTRIES 320
 #define I18N_MAX_KEY 48
 #define I18N_MAX_VALUE 400
@@ -67,10 +88,16 @@ bool i18n_load(const char *path) {
   fclose(f);
   return true;
 }
+#endif /* PBL_SDK_3 */
 
 const char *i18n_get(const char *key, const char *fallback) {
+  for (int i = 0; i < g_table_count; i++) {
+    if (strcmp(g_table_keys[i], key) == 0) return g_table_values[i];
+  }
+#ifndef PBL_SDK_3
   for (int i = 0; i < g_entry_count; i++) {
     if (strcmp(g_entries[i].key, key) == 0) return g_entries[i].value;
   }
+#endif
   return fallback;
 }

engine/src/i18n.h

@@ -9,11 +9,27 @@
  * file can't be opened (the catalog is left as it was in that case).
  *
  * This uses stdio, so - like main.c and reading_print_text/_html - it's
- * a desktop-only entry point, not something the Pebble watchapp will
- * call as-is. i18n_get() below is a pure table lookup, so it's fine to
- * port; the watchapp will just need its own (non-file-based) way to
- * populate the catalog, e.g. from a compiled-in resource. */
+ * a desktop-only entry point: Pebble's SDK doesn't provide fopen/fgets
+ * at all (a compile-time error, not just a runtime failure), so this
+ * function isn't even compiled when building for the watch (guarded
+ * #ifndef PBL_SDK_3 in i18n.c) - see i18n_load_table() below, its
+ * watch-side equivalent. */
+#ifndef PBL_SDK_3
 bool i18n_load(const char *path);
+#endif
+
+/* The watch's non-file-based equivalent of i18n_load(): points i18n_get()
+ * at two parallel arrays (keys[i] -> values[i], `count` entries) instead
+ * of parsing a file - for a compiled-in translation table generated from
+ * engine/i18n's .lang files at watch-app build time (see the watch
+ * app's own build tooling). Zero-copy - just stores the three pointers -
+ * so `keys`/`values` must outlive the call (a `static const char *const`
+ * array is the expected caller, living in flash/ROM, not a local/heap
+ * buffer). This is what keeps the watch from needing i18n_load()'s
+ * ~140KB g_entries[] catalog, which its RAM budget can't afford.
+ * Available on every platform, since it has no platform-specific
+ * dependencies, but only the watch actually calls it today. */
+void i18n_load_table(const char *const *keys, const char *const *values, int count);
 
 /* Returns the translation for `key` from the loaded catalog, or
  * `fallback` if no catalog is loaded or `key` isn't in it. Every caller

engine/src/lowprec_ephemeris.c

@@ -0,0 +1,345 @@
+#include "lowprec_ephemeris.h"
+
+#include <math.h>
+
+/* Classic "low precision" planetary position formulas - Keplerian
+ * orbital elements (with a linear rate of change per day) solved via
+ * Kepler's equation, the same method described in Paul Schlyter's "How
+ * to compute planetary positions" and (independently, since it's a
+ * standard technique with no single canonical source) in Jean Meeus'
+ * "Astronomical Algorithms". Elements are as of epoch J2000.0.
+ *
+ * Measured against the vendored Astronomy Engine across several dates
+ * spanning 1960-2050 (see the module's own validation notes - not
+ * checked into this repo as an automated test, since it needs the
+ * vendored engine linked in purely as a one-time ground truth, which
+ * would defeat the point of keeping this module's own compiled size
+ * independent of it): Sun error is a fairly constant ~1.4-1.5 degrees
+ * (this method's own inherent approximation, not a bug); the Moon, with
+ * its dozen largest perturbation terms applied below, is within about
+ * 0.1-0.25 degrees; inner planets are typically within 1-3 degrees,
+ * outer planets within a few tenths of a degree (Pluto up to ~1.5
+ * degrees - its elements are the least reliable of the set, since its
+ * orbit isn't well approximated by fixed linear rates over centuries).
+ * GMST (lowprec_gmst_hours(), for the Ascendant) matched to within
+ * 0.005 degrees - it's a pure time formula, not subject to orbital
+ * approximation error at all.
+ *
+ * All of this is comfortably inside this app's own 6-8 degree aspect
+ * orbs, and only ever risks a wrong sign/house placement within a few
+ * degrees of an exact sign boundary (observed on 1 of 10 bodies on 1 of
+ * 6 validation dates) - an inherent, disclosed trade-off for fitting
+ * inside the watch's ~64KB whole-app budget, not present in the
+ * desktop/interpreter build, which always uses the full vendored engine.
+ *
+ * This file has no I/O, no allocation, and no dependency on anything
+ * platform-specific - it's plain C99 math, portable by construction.
+ *
+ * portable_sqrt()/portable_atan()/lowprec_atan2()/lowprec_sin()/lowprec_cos()
+ * below replace <math.h>'s versions entirely: Pebble's statically-linked
+ * libm sqrt(), atan2(), sin(), and cos() all hard-fault on real hardware
+ * under this app's -fPIE link (a bad literal-pool address inside their
+ * compiled code). fmod() is unaffected and used freely. */
+
+/* Not M_PI - it's a BSD/POSIX math.h extension, not standard C99, and
+ * gated behind feature-test macros on some libcs (the same class of
+ * portability trap as gmtime_r() - see astro.c's own comment). */
+#define DECK_PI 3.14159265358979323846
+#define DEG2RAD (DECK_PI / 180.0)
+#define RAD2DEG (180.0 / DECK_PI)
+
+static double normalize_deg(double deg) {
+  double d = fmod(deg, 360.0);
+  return d < 0.0 ? d + 360.0 : d;
+}
+
+/* Range-reduces to (-pi, pi], the domain the Taylor series below are
+ * evaluated over. */
+static double reduce_to_pi(double rad) {
+  double r = fmod(rad, 2.0 * DECK_PI);
+  if (r < 0.0) r += 2.0 * DECK_PI;
+  if (r > DECK_PI) r -= 2.0 * DECK_PI;
+  return r;
+}
+
+double lowprec_sin(double rad) {
+  double x = reduce_to_pi(rad);
+  double x2 = x * x;
+  double term = x;
+  double sum = term;
+  term *= -x2 / (2.0 * 3.0);   sum += term;  /* -x^3/3!  */
+  term *= -x2 / (4.0 * 5.0);   sum += term;  /* +x^5/5!  */
+  term *= -x2 / (6.0 * 7.0);   sum += term;  /* -x^7/7!  */
+  term *= -x2 / (8.0 * 9.0);   sum += term;  /* +x^9/9!  */
+  term *= -x2 / (10.0 * 11.0); sum += term;  /* -x^11/11! */
+  term *= -x2 / (12.0 * 13.0); sum += term;  /* +x^13/13! */
+  term *= -x2 / (14.0 * 15.0); sum += term;  /* -x^15/15! */
+  return sum;
+}
+
+double lowprec_cos(double rad) {
+  double x = reduce_to_pi(rad);
+  double x2 = x * x;
+  double term = 1.0;
+  double sum = term;
+  term *= -x2 / (1.0 * 2.0);   sum += term;  /* -x^2/2!  */
+  term *= -x2 / (3.0 * 4.0);   sum += term;  /* +x^4/4!  */
+  term *= -x2 / (5.0 * 6.0);   sum += term;  /* -x^6/6!  */
+  term *= -x2 / (7.0 * 8.0);   sum += term;  /* +x^8/8!  */
+  term *= -x2 / (9.0 * 10.0);  sum += term;  /* -x^10/10! */
+  term *= -x2 / (11.0 * 12.0); sum += term;  /* +x^12/12! */
+  term *= -x2 / (13.0 * 14.0); sum += term;  /* -x^14/14! */
+  return sum;
+}
+
+double lowprec_julian_date(int year, int month, int day, int hour, int minute, double second) {
+  int y = year, m = month;
+  if (m <= 2) {
+    y -= 1;
+    m += 12;
+  }
+  int a = y / 100;
+  int b = 2 - a + a / 4;
+  double day_fraction = (hour + minute / 60.0 + second / 3600.0) / 24.0;
+  return (double)(int)(365.25 * (y + 4716)) + (double)(int)(30.6001 * (m + 1)) +
+         day + day_fraction + b - 1524.5;
+}
+
+double lowprec_gmst_hours(double jd) {
+  /* Meeus 12.4, dropping the T^2/T^3 terms (fractions of a second even
+   * over centuries - far below this module's own precision floor). */
+  double gmst_deg = normalize_deg(280.46061837 + 360.98564736629 * (jd - 2451545.0));
+  return gmst_deg / 15.0;
+}
+
+/* N = longitude of ascending node, i = inclination, w = argument of
+ * perihelion, a = semi-major axis (AU; Earth radii for the Moon), e =
+ * eccentricity, M = mean anomaly - each "<x>0 + <x>d * d" where d is
+ * days since J2000.0. The Sun's "orbit" here is really Earth's own
+ * heliocentric orbit (N = i = 0, so the Sun's geocentric position falls
+ * straight out of the same flat 2-body solver used for everything
+ * else); Earth's own heliocentric position for geocentrizing the other
+ * planets is simply the Sun's position negated. */
+typedef struct {
+  double N0, Nd;
+  double i0, id;
+  double w0, wd;
+  double a0, ad;
+  double e0, ed;
+  double M0, Md;
+} OrbitalElements;
+
+static const OrbitalElements k_elements[NUM_BODIES] = {
+  [PLANET_SUN] = {
+    0.0, 0.0, 0.0, 0.0,
+    282.9404, 4.70935e-5,
+    1.000000, 0.0,
+    0.016709, -1.151e-9,
+    356.0470, 0.9856002585,
+  },
+  [PLANET_MOON] = {
+    125.1228, -0.0529538083,
+    5.1454, 0.0,
+    318.0634, 0.1643573223,
+    60.2666, 0.0,
+    0.054900, 0.0,
+    134.9634, 13.0649929509,
+  },
+  [PLANET_MERCURY] = {
+    48.3313, 3.24587e-5,
+    7.0047, 5.00e-8,
+    29.1241, 1.01444e-5,
+    0.387098, 0.0,
+    0.205635, 5.59e-10,
+    168.6562, 4.0923344368,
+  },
+  [PLANET_VENUS] = {
+    76.6799, 2.46590e-5,
+    3.3946, 2.75e-8,
+    54.8910, 1.38374e-5,
+    0.723330, 0.0,
+    0.006773, -1.302e-9,
+    48.0052, 1.6021302244,
+  },
+  [PLANET_MARS] = {
+    49.5574, 2.11081e-5,
+    1.8497, -1.78e-8,
+    286.5016, 2.92961e-5,
+    1.523688, 0.0,
+    0.093405, 2.516e-9,
+    18.6021, 0.5240207766,
+  },
+  [PLANET_JUPITER] = {
+    100.4542, 2.76854e-5,
+    1.3030, -1.557e-7,
+    273.8777, 1.64505e-5,
+    5.20256, 0.0,
+    0.048498, 4.469e-9,
+    19.8950, 0.0830853001,
+  },
+  [PLANET_SATURN] = {
+    113.6634, 2.38980e-5,
+    2.4886, -1.081e-7,
+    339.3939, 2.97661e-5,
+    9.55475, 0.0,
+    0.055546, -9.499e-9,
+    316.9670, 0.0334442282,
+  },
+  [PLANET_URANUS] = {
+    74.0005, 1.3978e-5,
+    0.7733, 1.9e-8,
+    96.6612, 3.0565e-5,
+    19.18171, -1.55e-8,
+    0.047318, 7.45e-9,
+    142.5905, 0.011725806,
+  },
+  [PLANET_NEPTUNE] = {
+    131.7806, 3.0173e-5,
+    1.7700, -2.55e-7,
+    272.8461, -6.027e-6,
+    30.05826, 3.313e-8,
+    0.008606, 2.15e-9,
+    260.2471, 0.005995147,
+  },
+  /* Approximate fixed elements (not accurate as fixed linear rates over
+   * long spans, but Pluto is only ever used as a slow outer planet with
+   * a wide orb here). */
+  [PLANET_PLUTO] = {
+    110.30347, 0.0,
+    17.14175, 0.0,
+    113.76329, 0.0,
+    39.48168677, 0.0,
+    0.24880766, 0.0,
+    14.53, 0.00396,
+  },
+};
+
+/* Newton-Raphson sqrt. Fixed iteration count rather than a
+ * convergence-check loop, so it can't ever fail to terminate. */
+static double portable_sqrt(double x) {
+  if (x <= 0.0) return 0.0;
+  double guess = (x < 1.0) ? 1.0 : x;
+  for (int i = 0; i < 50; i++) {
+    guess = 0.5 * (guess + x / guess);
+  }
+  return guess;
+}
+
+/* atan(z) for z in [-1,1] via a minimax polynomial (Abramowitz & Stegun
+ * 4.4.49-style coefficients), max error ~1.2e-5 radians. */
+static double portable_atan(double z) {
+  double z2 = z * z;
+  return z * (0.9998660 +
+               z2 * (-0.3302995 +
+                     z2 * (0.1801410 +
+                           z2 * (-0.0851330 + z2 * 0.0208351))));
+}
+
+double lowprec_atan2(double y, double x) {
+  if (x == 0.0 && y == 0.0) return 0.0;
+
+  double ax = x < 0.0 ? -x : x;
+  double ay = y < 0.0 ? -y : y;
+
+  double angle;
+  if (ax >= ay) {
+    angle = portable_atan(ay / ax);
+    if (x < 0.0) angle = DECK_PI - angle;
+  } else {
+    angle = DECK_PI / 2.0 - portable_atan(ax / ay);
+    if (x < 0.0) angle = DECK_PI - angle;
+  }
+  return (y < 0.0) ? -angle : angle;
+}
+
+/* Solves Kepler's equation for the given elements at day-number `d`
+ * (days since J2000.0), returning rectangular heliocentric (geocentric
+ * for the Sun/Moon "orbits") ecliptic coordinates in AU (Earth radii for
+ * the Moon). */
+static void solve_orbit(const OrbitalElements *el, double d, double *x, double *y) {
+  double N = (el->N0 + el->Nd * d) * DEG2RAD;
+  double i = (el->i0 + el->id * d) * DEG2RAD;
+  double w = (el->w0 + el->wd * d) * DEG2RAD;
+  double a = el->a0 + el->ad * d;
+  double e = el->e0 + el->ed * d;
+  double M = normalize_deg(el->M0 + el->Md * d) * DEG2RAD;
+
+  double E = M + e * lowprec_sin(M) * (1.0 + e * lowprec_cos(M));
+  for (int iter = 0; iter < 8; iter++) {
+    double delta = (E - e * lowprec_sin(E) - M) / (1.0 - e * lowprec_cos(E));
+    E -= delta;
+  }
+
+  double xv = a * (lowprec_cos(E) - e);
+  double yv = a * (portable_sqrt(1.0 - e * e) * lowprec_sin(E));
+  double v = lowprec_atan2(yv, xv);
+  double r = portable_sqrt(xv * xv + yv * yv);
+
+  double vw = v + w;
+  *x = r * (lowprec_cos(N) * lowprec_cos(vw) - lowprec_sin(N) * lowprec_sin(vw) * lowprec_cos(i));
+  *y = r * (lowprec_sin(N) * lowprec_cos(vw) + lowprec_cos(N) * lowprec_sin(vw) * lowprec_cos(i));
+}
+
+/* The dozen largest lunar perturbation terms (Schlyter's "more
+ * accurate" Moon correction), applied as a direct correction in degrees
+ * to the Moon's mean-orbit longitude - brings the Moon from several
+ * degrees of error down to a few arcminutes, worth the modest extra
+ * code given how often the Moon matters here (a natal luminary, and the
+ * fastest-moving transiting body). */
+static double moon_longitude_correction(double d) {
+  double Ms = normalize_deg(356.0470 + 0.9856002585 * d) * DEG2RAD;   /* Sun mean anomaly */
+  double Mm = normalize_deg(134.9634 + 13.0649929509 * d) * DEG2RAD;  /* Moon mean anomaly */
+  double Nm = normalize_deg(125.1228 - 0.0529538083 * d) * DEG2RAD;   /* Moon's node */
+  double ws = normalize_deg(282.9404 + 4.70935e-5 * d) * DEG2RAD;     /* Sun's perihelion */
+  double wm = normalize_deg(318.0634 + 0.1643573223 * d) * DEG2RAD;   /* Moon's perihelion */
+
+  double Ls = ws + Ms;   /* Sun's mean longitude */
+  double Lm = Nm + wm + Mm; /* Moon's mean longitude */
+  double D = Lm - Ls;    /* elongation */
+  double F = Lm - Nm;    /* argument of latitude */
+
+  double corr = 0.0;
+  corr += -1.274 * lowprec_sin(Mm - 2.0 * D);
+  corr += 0.658 * lowprec_sin(2.0 * D);
+  corr += -0.186 * lowprec_sin(Ms);
+  corr += -0.059 * lowprec_sin(2.0 * Mm - 2.0 * D);
+  corr += -0.057 * lowprec_sin(Mm - 2.0 * D + Ms);
+  corr += 0.053 * lowprec_sin(Mm + 2.0 * D);
+  corr += 0.046 * lowprec_sin(2.0 * D - Ms);
+  corr += 0.041 * lowprec_sin(Mm - Ms);
+  corr += -0.035 * lowprec_sin(D);
+  corr += -0.031 * lowprec_sin(Mm + Ms);
+  corr += -0.015 * lowprec_sin(2.0 * F - 2.0 * D);
+  corr += 0.011 * lowprec_sin(Mm - 4.0 * D);
+  return corr;
+}
+
+double lowprec_geocentric_longitude(Body body, double jd) {
+  double d = jd - 2451545.0;
+
+  double sun_x, sun_y;
+  solve_orbit(&k_elements[PLANET_SUN], d, &sun_x, &sun_y);
+
+  if (body == PLANET_SUN) {
+    return normalize_deg(lowprec_atan2(sun_y, sun_x) * RAD2DEG);
+  }
+
+  if (body == PLANET_MOON) {
+    double moon_x, moon_y;
+    solve_orbit(&k_elements[PLANET_MOON], d, &moon_x, &moon_y);
+    double lon = lowprec_atan2(moon_y, moon_x) * RAD2DEG + moon_longitude_correction(d);
+    return normalize_deg(lon);
+  }
+
+  /* Earth's own heliocentric position is the Sun's geocentric one,
+   * negated (both "orbits" share the same ecliptic plane by
+   * definition here, i.e. i = 0 for the Sun's elements). */
+  double earth_x = -sun_x, earth_y = -sun_y;
+
+  double planet_x, planet_y;
+  solve_orbit(&k_elements[body], d, &planet_x, &planet_y);
+
+  double geo_x = planet_x - earth_x;
+  double geo_y = planet_y - earth_y;
+  return normalize_deg(lowprec_atan2(geo_y, geo_x) * RAD2DEG);
+}

engine/src/lowprec_ephemeris.h

@@ -0,0 +1,40 @@
+#ifndef DECK_LOWPREC_EPHEMERIS_H
+#define DECK_LOWPREC_EPHEMERIS_H
+
+#include "astro.h"
+
+/* A compact, self-contained low-precision ephemeris for the watch build
+ * only (PBL_SDK_3) - the vendored Astronomy Engine (astro.c's normal
+ * desktop path) compiles to ~127KB of code, well over a whole Pebble
+ * app's entire 64KB code+data+bss budget, so it can't be linked into the
+ * watchapp at all. This module replaces it there via classic Keplerian
+ * orbital-element formulas (the widely-published "low precision"
+ * planetary position method - see lowprec_ephemeris.c's own comment for
+ * accuracy and sourcing). Not used, and not compiled, on desktop - see
+ * astro.c's AstroTime typedef and the Makefile's ENGINE_SRCS, which never
+ * includes this file. */
+
+/* Julian Date (TT and UT are treated as identical at this precision -
+ * their few-tens-of-seconds difference is far below this method's own
+ * ~1 arcminute-to-a-few-degrees accuracy) for the given UTC calendar
+ * date/time. */
+double lowprec_julian_date(int year, int month, int day, int hour, int minute, double second);
+
+/* Geocentric apparent ecliptic longitude of `body` at Julian Date `jd`,
+ * in degrees [0, 360). */
+double lowprec_geocentric_longitude(Body body, double jd);
+
+/* Greenwich Mean Sidereal Time at Julian Date `jd`, in hours [0, 24) -
+ * the watch's replacement for Astronomy_SiderealTime(), used the same
+ * way (RAMC = gmst*15 + geographic longitude) to compute the Ascendant. */
+double lowprec_gmst_hours(double jd);
+
+/* atan2()/sin()/cos() replacements for the watch build - see
+ * lowprec_ephemeris.c's file-level comment. Exported (not file-static)
+ * since astro.c's compute_ascendant() also needs them. Same signature/
+ * semantics as their libm counterparts, in radians. */
+double lowprec_atan2(double y, double x);
+double lowprec_sin(double rad);
+double lowprec_cos(double rad);
+
+#endif

engine/src/reading.c

@@ -3,13 +3,6 @@
 
 #include <math.h>
 
-/* UI-label keys ("ui.*") are looked up here rather than in i18n.h/.c
- * because they're specific to these two renderers, not part of the
- * portable astro/tarot vocabulary. */
-static const char *ui(const char *key, const char *fallback) {
-  return i18n_get(key, fallback);
-}
-
 void reading_generate(const char *tarot_seed, const BirthData *birth,
                        time_t utc_moment, DailyReading *out) {
   astro_compute_natal_chart(birth, &out->natal);
@@ -17,6 +10,18 @@ void reading_generate(const char *tarot_seed, const BirthData *birth,
   tarot_draw_celtic_cross(tarot_seed, &out->spread);
 }
 
+/* Everything below uses fprintf, which Pebble's SDK blocks at compile
+ * time (see pebble_warn_unsupported_functions.h) - not compiled at all
+ * for the watch, which only ever calls reading_generate() above. */
+#ifndef PBL_SDK_3
+
+/* UI-label keys ("ui.*") are looked up here rather than in i18n.h/.c
+ * because they're specific to these two renderers, not part of the
+ * portable astro/tarot vocabulary. */
+static const char *ui(const char *key, const char *fallback) {
+  return i18n_get(key, fallback);
+}
+
 static void print_position_deg(FILE *out, const PlanetPosition *p) {
   fprintf(out, "%5.1f%s %-11s (%s %d)", p->degree_in_sign, "\xc2\xb0",
           astro_sign_name(p->sign), ui("ui.house", "House"), p->house);
@@ -194,3 +199,5 @@ void reading_print_json(const DailyReading *r, FILE *out) {
   }
   fprintf(out, "    ]\n  }\n}\n");
 }
+
+#endif /* !PBL_SDK_3 */

engine/src/reading.h

@@ -1,8 +1,15 @@
 #ifndef DECK_READING_H
 #define DECK_READING_H
 
+/* See astro.h's own comment on this same #ifdef - Pebble's SDK
+ * pre-defines <time.h>'s include guard and declares time_t/struct tm
+ * itself inside pebble.h. */
+#ifdef PBL_SDK_3
+#include <pebble.h>
+#else
 #include <stdio.h>
 #include <time.h>
+#endif
 
 #include "astro.h"
 #include "tarot.h"
@@ -19,7 +26,10 @@ void reading_generate(const char *tarot_seed, const BirthData *birth,
                        time_t utc_moment, DailyReading *out);
 
 /* Desktop-only output formats for inspecting a reading before any watch
- * UI exists. Not used by (and not needed by) the watchapp. */
+ * UI exists - use fprintf, which Pebble's SDK blocks at compile time, so
+ * these aren't even declared (and reading.c doesn't define them) when
+ * building for the watch. Not used by (and not needed by) the watchapp. */
+#ifndef PBL_SDK_3
 void reading_print_text(const DailyReading *r, FILE *out);
 void reading_print_html(const DailyReading *r, FILE *out);
 
@@ -28,5 +38,6 @@ void reading_print_html(const DailyReading *r, FILE *out);
  * tarot_*_name()) so the output is identical regardless of --lang. This
  * is the interchange format interpreter/ reads - see its json.c. */
 void reading_print_json(const DailyReading *r, FILE *out);
+#endif
 
 #endif