diff --git a/main.py b/main.py
index f946f0c..04810ba 100644
--- a/main.py
+++ b/main.py
@@ -5,9 +5,10 @@ import random
 import subprocess
 import sys
 import time
-from datetime import datetime
+from datetime import datetime, timedelta
 
 import emoji
+import ephem
 import pygame
 import pytchat
 from playsound import playsound
@@ -47,6 +48,16 @@ class Main:
         screen: pygame.Surface = pygame.display.set_mode (
                 (CWindow.WIDTH, CWindow.HEIGHT))
 
+        # 大月ヨヨコの観測値
+        observer = ephem.Observer ()
+        observer.lat, observer.lon = '35', '139'
+
+        # き太く陽
+        sun = ephem.Sun ()
+
+        # 大月ヨヨコ
+        moon = ephem.Moon ()
+
         # 吹き出し
         balloon = pygame.transform.scale (pygame.image.load ('talking.png'),
                                           (CWindow.WIDTH, 384))
@@ -55,13 +66,13 @@ class Main:
 
         # 背景（夕方）
         bg_evening: pygame.Surface = pygame.transform.scale (
-            pygame.image.load ('bg-evening.jpg'),
-            (CWindow.WIDTH, CWindow.HEIGHT))
+                pygame.image.load ('bg-evening.jpg'),
+                (CWindow.WIDTH, CWindow.HEIGHT))
 
         # 背景（夜）
         bg_night: pygame.Surface = pygame.transform.scale (
-            pygame.image.load ('bg-night.jpg'),
-            (CWindow.WIDTH, CWindow.HEIGHT))
+                pygame.image.load ('bg-night.jpg'),
+                (CWindow.WIDTH, CWindow.HEIGHT))
 
         # 音声再生器の初期化
         pygame.mixer.init (frequency = 44100)
@@ -94,7 +105,27 @@ class Main:
         histories: list = []
 
         while (True):
-            cls.draw_bg (screen, bg_evening, bg_night)
+            # 観測値の日づけ更新
+            observer.date: datetime = datetime.now ().date ()
+
+            # 日の出開始
+            sunrise_start: datetime = (
+                    (ephem.localtime (observer.previous_rising (sun))
+                     - timedelta (minutes = 30)))
+
+            # 日の出終了
+            sunrise_end: datetime = sunrise_start + timedelta (hours = 1)
+
+            # 日の入開始
+            sunset_start: datetime = (
+                    (ephem.localtime (observer.next_setting (sun))
+                     - timedelta (minutes = 30)))
+
+            # 日の入終了
+            sunset_end: datetime = sunset_start + timedelta (hours = 1)
+
+            cls.draw_bg (screen, bg_evening, bg_night,
+                         sunrise_start, sunrise_end, sunset_start, sunset_end)
 
             # 左上に時刻表示
             for i in range (4):
@@ -217,7 +248,9 @@ class Main:
                         if not double_mode or random.random () < .5:
                             break
 
-                        cls.draw_bg (screen, bg_evening, bg_night)
+                        cls.draw_bg (screen, bg_evening, bg_night,
+                                     sunrise_start, sunrise_end,
+                                     sunset_start, sunset_end)
 
                         chat_item.author = {'name':     'ゴートうひとり' if goatoh_talking else '伊地知ニジカ',
                                             'id':       '',
@@ -243,14 +276,29 @@ class Main:
     @classmethod
     def draw_bg (
             cls,
-            screen:     pygame.Surface,
-            bg_evening: pygame.Surface,
-            bg_night:   pygame.Surface) \
+            screen:         pygame.Surface,
+            bg_evening:     pygame.Surface,
+            bg_night:       pygame.Surface,
+            sunrise_start:  datetime,
+            sunrise_end:    datetime,
+            sunset_start:   datetime,
+            sunset_end:     datetime) \
     -> None:
-        if 17 <= (h := datetime.now ().hour) < 18:
-            screen.blit (bg_evening, (0, 0))
-        elif (h < 6) or (18 <= h):
+        dt: datetime = datetime.now ()
+
+        if (dt < sunrise_end) or (sunset_start <= dt):
             screen.blit (bg_night, (0, 0))
+
+            if sunrise_start <= dt < sunrise_end:
+                bg_evening.set_alpha (((dt - sunrise_start) * 255)
+                                      / (sunrise_end - sunrise_start))
+            elif sunset_start <= dt < sunset_end:
+                bg_evening.set_alpha (((sunset_end - dt) * 255)
+                                      / (sunset_end - sunset_start))
+            else:
+                bg_evening.set_alpha (0)
+
+            screen.blit (bg_evening, (0, 0))
         else:
             screen.fill ((0, 255, 0))