The following is the principle and method to tell the direction with a watch: the earth rotates 360 degrees in 24 hours, 1 hour rotation 15 degrees, and the hour hand of a watch always rotates at twice the speed of the sun. According to this principle, the direction can be roughly determined by the watch and the sun. At 6 o'clock in the morning, the sun is in the east and the shadow points to the west. At this time, the hour hand on the watch points to the sun, but what about the dial? 12? The word points to the west. If the dial rotates 90 degrees, it is 6: 30, then what is on the dial? 3? The figure faces the sun. 12? The characters point to the north. At noon 12, the sun is in the south. If 12 is folded in half, what about the dial? 6? Facing the sun, and then what? 12? Still refers to the north.
When determining the direction according to this method, we should consider the local time difference. Beijing time and conversion time should become local time. Based on the line of east longitude 1 20th degree, every 1 5th degree in east longitude, plus1hour in Beijing time, every15th degree in west longitude and minus1hour in Beijing time is the local time. If the geographical coordinate of Urumqi is 87 degrees 40 minutes east longitude, then (120-87 degrees? 15 degrees = 2 hours and 9 minutes, Beijing time minus 2 hours and 9 minutes) is the local time in Urumqi. The above two methods cannot be used in summer in the south of the Tropic of Cancer in Chiayi, Taiwan Province (23 degrees 27 minutes north latitude), Nan 'ao Island in the northeast of Shantou, Guangdong, Wuzhou City in Guangxi and Gejiu City in Yunnan.
By the way, we know the principle that the relative position of the earth and the sun moves 15 degrees when the earth rotates. We can use the watch and the sun to roughly determine the direction. On the contrary, we can also use the needle facing north and the sun to measure the approximate time. The method is to point the pointer to the north and divide the reading of the sun position by 15. In China, Beijing time shall prevail. Beijing is on the east longitude120th degree line, and other areas will make corrections according to the longitude difference to get the correct solar time. So I think the principle of using a watch to set the direction in Detective Conan is to rely on the sun.