diff --git a/docs/BUG_HISTORY.md b/docs/BUG_HISTORY.md
index c6f1b977..5c35c066 100644
--- a/docs/BUG_HISTORY.md
+++ b/docs/BUG_HISTORY.md
@@ -36503,3 +36503,18 @@
 - 相关记录：BUG-1268。
 - 复发自：无。
 - 修复版本：分支 `codex/upstream-sync8-20261017`。上游提交 `8643bc9a`。
+
+## BUG-1381 | 月亮正好在星宿起点时大运余额变成 0
+
+- 状态：resolved
+- 首次发现 / 最近更新：2026-10-17 / 2026-10-18
+- 来源：issue #80，任务书 T2。上游 `3c2dad95`、`8643bc9a`、`9e04f0db`（只取星宿定位）。检索「lon_to_nakshatra」「int(moon_lon」，没有更早的同类记录。
+- 现象：月亮经度恰好是某星宿起点时，第一大运余额变成 0，整段大运被跳过。27 个起点里有 16 个会这样。
+- 触发条件：星宿序号用 `int(经度/跨度)`，已用比例又用 `经度 % 跨度`，两个浮点结果在起点上对不上。
+- 根因：序号和进度不是从同一个量算出来的。起点被算成「上一宿几乎走完」。
+- 修复：起点用同一把分数网格：这一点属于刚开始的那一宿，已用比例为 0。`cmd_dasha`、`lon_to_nakshatra`、`find_nakshatra` 和星盘星宿字段都走这个函数。不是起点的经度仍用原来的取模，结果不变。
+- 验证：27 个起点的第一大运年数等于完整年数，第一段小运主星等于大运主星。起点左右相邻浮点数分别落在前一宿和这一宿。5000 个非起点随机经度与旧算法一致。
+- 防复发：`tests/test_nakshatra_native_dasha_clock.py`。
+- 相关记录：BUG-1268、BUG-1380。
+- 复发自：无。
+- 修复版本：分支 `codex/upstream-sync8-20261017`。上游提交 `3c2dad95`、`8643bc9a`、`9e04f0db`。
diff --git a/scripts/dasha_analyzer.py b/scripts/dasha_analyzer.py
index 479e4f8f..cfe92a90 100644
--- a/scripts/dasha_analyzer.py
+++ b/scripts/dasha_analyzer.py
@@ -68,12 +68,18 @@ def fuzzy_match_nakshatra(name):
     return None
 
 def lon_to_nakshatra(moon_lon):
-    """月亮黄经（0-360）→ Nakshatra + Pada"""
-    nak_span = 360 / 27
-    pada_span = nak_span / 4
-    idx = int(moon_lon / nak_span)
-    progress = (moon_lon % nak_span) / nak_span
-    pada = int((moon_lon % nak_span) / pada_span) + 1
+    """月亮黄经（0-360）→ Nakshatra + Pada。序号与进度来自同一把尺子。"""
+    try:
+        from nakshatra_advanced import nakshatra_index_and_progress
+    except ImportError:
+        from scripts.nakshatra_advanced import nakshatra_index_and_progress
+    idx, progress = nakshatra_index_and_progress(moon_lon)
+    if progress == 0.0:
+        pada = 1
+    else:
+        nak_span = 360 / 27
+        pada_span = nak_span / 4
+        pada = int((float(moon_lon) % nak_span) / pada_span) + 1
     return NAKSHATRA_LIST[idx], progress, pada
 
 def build_dasha_timeline(birth_date_str, nak_info, nak_progress):
diff --git a/scripts/jyotish_engine.py b/scripts/jyotish_engine.py
index f66aa24a..d5c8daf1 100644
--- a/scripts/jyotish_engine.py
+++ b/scripts/jyotish_engine.py
@@ -12023,10 +12023,16 @@ def _load_strict_evidence_collector():
 # ============================================================================
 def _nakshatra_fields(sidereal_lon):
     """Nakshatra, pada and lord of a sidereal longitude — planets and the ascendant alike."""
-    nak_span = 360.0 / 27
+    from nakshatra_advanced import nakshatra_index_and_progress
     lon = sidereal_lon % 360
-    pada = int((lon % nak_span) / (nak_span / 4)) + 1
-    name, lord, _ = NAKSHATRA_LIST[int(lon / nak_span) % 27]
+    idx, progress = nakshatra_index_and_progress(lon)
+    if progress == 0.0:
+        pada = 1
+    else:
+        nak_span = 360.0 / 27
+        pada = int((lon % nak_span) / (nak_span / 4)) + 1
+        idx = int(lon / nak_span) % 27
+    name, lord, _ = NAKSHATRA_LIST[idx]
     return {"nakshatra": name, "nakshatra_pada": pada, "nakshatra_lord": lord}
 
 
@@ -12165,8 +12171,9 @@ def _build_pratyantar_timeline(md_lord, ad_lord, ad_start, ad_end, reference_dt=
 def cmd_dasha(args):
     nak_info = None; progress = 0.5
     if args.moon_lon is not None:
-        ns = 360.0 / 27; idx = int(args.moon_lon / ns); progress = (args.moon_lon % ns) / ns
-        nak_info = NAKSHATRA_LIST[idx % 27]
+        from nakshatra_advanced import nakshatra_index_and_progress
+        idx, progress = nakshatra_index_and_progress(args.moon_lon)
+        nak_info = NAKSHATRA_LIST[idx]
     elif args.nakshatra:
         nl = args.nakshatra.lower().replace(" ", "").replace("-", "")
         for n in NAKSHATRA_LIST:
@@ -12183,8 +12190,9 @@ def cmd_dasha(args):
         moon_lon = moon.get("degree_raw")
         if moon_lon is None:
             return {"error": "无法计算Moon位置"}
-        ns = 360.0 / 27; idx = int(moon_lon / ns); progress = (moon_lon % ns) / ns
-        nak_info = NAKSHATRA_LIST[idx % 27]
+        from nakshatra_advanced import nakshatra_index_and_progress
+        idx, progress = nakshatra_index_and_progress(moon_lon)
+        nak_info = NAKSHATRA_LIST[idx]
 
     nak_name, start_lord, start_years = nak_info
     birthdate = args.birthdate or f"{args.year}-{args.month:02d}-{args.day:02d}"
@@ -15425,10 +15433,15 @@ def cmd_full_reading(args):
     try:
         moon_data = planets.get('Moon', {})
         moon_lon = moon_data.get('degree_raw', moon_data.get('degree', 0))
-        nak_idx = int(moon_lon / (360 / 27)) % 27
+        from nakshatra_advanced import nakshatra_index_and_progress
+        nak_idx, nak_progress = nakshatra_index_and_progress(moon_lon)
+        if nak_progress != 0.0:
+            nak_idx = int(moon_lon / (360 / 27)) % 27
+            pada = int((moon_lon % (360/27)) / (360/108)) + 1
+        else:
+            pada = 1
         nak_name = NAKSHATRA_LIST[nak_idx][0]
         nak_lord = NAKSHATRA_LIST[nak_idx][1]
-        pada = int((moon_lon % (360/27)) / (360/108)) + 1
 
         birthdate = f"{args.year}-{args.month:02d}-{args.day:02d}"
         today_str = getattr(args, 'transit_date', None) or getattr(args, 'today', None) or datetime.now().strftime('%Y-%m-%d')
@@ -16459,8 +16472,13 @@ def cmd_full_reading(args):
         jaimini_data = report['modules'].get('jaimini', {})
         chara_dasha_data = jaimini_data.get('chara_dasha', {}) if isinstance(jaimini_data, dict) else {}
         moon_lon = planet_lons.get('Moon', 0)
-        moon_nakshatra_index = int(moon_lon / (360 / 27)) % 27
-        moon_pada = int((moon_lon % (360 / 27)) / (360 / 108)) + 1
+        from nakshatra_advanced import nakshatra_index_and_progress
+        moon_nakshatra_index, moon_nak_progress = nakshatra_index_and_progress(moon_lon)
+        if moon_nak_progress != 0.0:
+            moon_nakshatra_index = int(moon_lon / (360 / 27)) % 27
+            moon_pada = int((moon_lon % (360 / 27)) / (360 / 108)) + 1
+        else:
+            moon_pada = 1
         tithi_number = int(((moon_lon - planet_lons.get('Sun', 0)) % 360) / 12) + 1
         birth_dt = _birth_datetime_from_args(args)
         ll_name = SIGN_LORDS[SIGNS[asc_idx]]
diff --git a/scripts/nakshatra_advanced.py b/scripts/nakshatra_advanced.py
index f33574c4..fe33e3fd 100644
--- a/scripts/nakshatra_advanced.py
+++ b/scripts/nakshatra_advanced.py
@@ -16,6 +16,14 @@ Nakshatra深度分析系统
 from typing import Dict, List, Tuple, Optional
 import math
 from datetime import datetime, timedelta
+from bisect import bisect_right
+from fractions import Fraction
+
+# Exact star openings. float(Fraction) is the grid cmd_dasha must share with
+# lon_to_nakshatra; 360/27 and modulo disagree at several of these points.
+_NAK_SPAN = Fraction(40, 3)
+_NAK_STARTS = tuple(float(_NAK_SPAN * index) for index in range(27))
+_NAK_SPAN_FLOAT = 360.0 / 27.0
 
 NAK_LIST = [
     ("Ashwini","Ketu",7),("Bharani","Venus",20),("Krittika","Sun",6),
@@ -53,15 +61,42 @@ NAK_ELEMENT = {
 }
 
 
+def nakshatra_index_and_progress(longitude: float) -> Tuple[int, float]:
+    """星宿序号和已用比例，由同一把尺子量出。
+
+    经度恰好等于某星宿起点时，这一点属于刚开始的那一宿，已用比例为 0。
+    其余经度保持原来的取模结果，避免改动非边界点。
+    """
+    lon = float(longitude) % 360.0
+    idx = bisect_right(_NAK_STARTS, lon) - 1
+    if idx < 0:
+        idx = 0
+    elif idx > 26:
+        idx = 26
+    if lon == _NAK_STARTS[idx]:
+        return idx, 0.0
+    legacy_idx = int(lon / _NAK_SPAN_FLOAT) % 27
+    legacy_progress = (lon % _NAK_SPAN_FLOAT) / _NAK_SPAN_FLOAT
+    return legacy_idx, legacy_progress
+
+
 def find_nakshatra(longitude: float) -> Dict:
     """从恒星黄道经度精确计算Nakshatra"""
-    nak_span = 360.0 / 27  # 每个星宿13.333...°
-    nak_idx = int(longitude / nak_span) % 27
-    nak_start = nak_idx * nak_span
-    deg_in_nak = longitude - nak_start
-    pada_span = nak_span / 4  # 每个Pada 3.333...°
-    pada = int(deg_in_nak / pada_span) + 1
-    deg_in_pada = deg_in_nak - (pada - 1) * pada_span
+    idx, _progress = nakshatra_index_and_progress(longitude)
+    lon = float(longitude) % 360.0
+    if lon == _NAK_STARTS[idx]:
+        nak_idx = idx
+        deg_in_nak = 0.0
+        pada = 1
+        deg_in_pada = 0.0
+    else:
+        nak_span = 360.0 / 27  # 每个星宿13.333...°
+        nak_idx = int(longitude / nak_span) % 27
+        nak_start = nak_idx * nak_span
+        deg_in_nak = longitude - nak_start
+        pada_span = nak_span / 4  # 每个Pada 3.333...°
+        pada = int(deg_in_nak / pada_span) + 1
+        deg_in_pada = deg_in_nak - (pada - 1) * pada_span
 
     return {
         'nakshatra': NAK_NAMES[nak_idx],
@@ -126,11 +161,13 @@ def calc_vimshottari_start(moon_longitude: float) -> Dict:
     nak_lord = NAK_LORDS[nak_idx]
     total_years = NAK_YEARS[nak_idx]
 
-    # 已用比例
-    nak_span = 360.0 / 27
-    nak_start = nak_idx * nak_span
-    deg_in_nak = moon_longitude - nak_start
-    used_ratio = deg_in_nak / nak_span
+    # 已用比例与星宿序号来自同一把尺子。起点处必须是 0，不能是「几乎走完」。
+    _, used_ratio = nakshatra_index_and_progress(moon_longitude)
+    if float(moon_longitude) % 360.0 != _NAK_STARTS[nak_idx]:
+        nak_span = 360.0 / 27
+        nak_start = nak_idx * nak_span
+        deg_in_nak = moon_longitude - nak_start
+        used_ratio = deg_in_nak / nak_span
     remaining_ratio = 1 - used_ratio
 
     # 第一个Mahadasha的剩余年数
diff --git a/tests/test_nakshatra_native_dasha_clock.py b/tests/test_nakshatra_native_dasha_clock.py
index df07f0a4..2b61f3bc 100644
--- a/tests/test_nakshatra_native_dasha_clock.py
+++ b/tests/test_nakshatra_native_dasha_clock.py
@@ -1,4 +1,13 @@
 """The interpretation helper must consume the real, unclipped MD/AD clock."""
+import math
+from fractions import Fraction
+from types import SimpleNamespace
+
+import pytest
+
+from scripts.dasha_analyzer import lon_to_nakshatra
+from scripts.jyotish_engine import cmd_dasha
+from scripts.nakshatra_advanced import nakshatra_index_and_progress
 from scripts.nakshatra_dasha import calc_nakshatra_dasha_breakdown
 
 
@@ -15,3 +24,27 @@ def test_first_remaining_md_is_not_repartitioned_into_new_ads():
     assert result['antardasha']['progress'] > 0
     # Missing natal positions are never silently fabricated as 0 degrees.
     assert {row['period'] for row in result['pada_windows']} <= {'AD Ketu', 'AD Rahu'}
+
+
+@pytest.mark.parametrize('index', range(27))
+def test_native_vimshottari_exact_star_birth_balance(index):
+    packet = cmd_dasha(SimpleNamespace(
+        moon_lon=index * 40 / 3, nakshatra=None, pada=None,
+        birthdate='2000-01-01', year=2000, month=1, day=1, hour=12, minute=0, second=0,
+        lat=None, lon=None, tz=0, today='2000-01-01', dasha_year_days=365.25))
+    first = packet['timeline'][0]
+    assert first['years'] == first['full_years']
+    assert first['antardasha_timeline'][0]['lord'] == first['lord']
+
+
+def test_exact_star_neighbors_keep_adjacent_indexes():
+    start = float(Fraction(40, 3))
+    before = math.nextafter(start, 0.0)
+    after = math.nextafter(start, 360.0)
+    assert nakshatra_index_and_progress(before)[0] == 0
+    assert nakshatra_index_and_progress(start)[0] == 1
+    assert nakshatra_index_and_progress(after)[0] == 1
+    assert lon_to_nakshatra(before)[0][0] == 'Ashwini'
+    assert lon_to_nakshatra(start)[0][0] == 'Bharani'
+    assert lon_to_nakshatra(after)[0][0] == 'Bharani'
+    assert lon_to_nakshatra(start)[1] == 0.0