The north and south indicated on the compass are slightly different from the geographical north and south. There is a magnetic south pole in the northern hemisphere and a magnetic north pole in the southern hemisphere. The magnetic field vector usually points from south to north. The angle between the magnetic meridian plane and the geographical meridian plane is called the magnetic declination.
According to the regulations, the magnetic needle means that the magnetic declination is positive when the North Pole N is easterly, and negative when it is easterly. Magnetic declination refers to the angle between North Pole and True North when the magnetic needle is at rest. Shen Kuo, a scholar in the Northern Song Dynasty, recorded that the north and south poles of geomagnetism do not completely coincide with the north and south poles of geography, and there is a magnetic declination.
Magnetic declination varies with longitude. Geomantic omen generally only discusses the magnetic north pole, and does not use the geographical north pole. Therefore, when looking for the magnetic north pole on the map, it is necessary to convert the geographical north pole into the magnetic north pole.
2. Geomantic value of magnetic declination.
The significance of understanding this magnetic declination lies in using Google Maps or finding the geomagnetic direction in general. For example, if the map shows due south and due north, the direction measured by the compass will actually be biased, and Feng Shui is based on the direction of the compass pointer.
For example, when you look at a map in Beijing, many houses are due south and north. In fact, the direction measured by the compass is 5 50' (5.84 degrees) to the east. Taking the Changling of the Ming Tombs as an example, Feng Shui master said that "Changling is the direction of the root sea vein, and divination is consistent with examples". The direction of the mountain should be between 1 1 -20 degrees east of the north needle. The actual map shows that it is 5.75 degrees east of the north needle, and it needs to be added with the Beijing magnetic declination of 5.84 degrees, which is exactly 6520 degrees east of the north needle.
Similarly, the Forbidden City in Beijing is 2.25 degrees north latitude west and the magnetic declination is 5.84 degrees north latitude east. In fact, the location of Bauhinia City in Beijing is 3.59 degrees east of north and 3.59 degrees west of south, which is the noon direction of Zi Shan, so the location of Bauhinia City is also the entrance to grab shade.
Appendix Magnetic declination of major cities in China (north arrow is positive to the east and negative to the west)
Beijing: 5 50' (East);
Mohe: 1 1 00' (East);
Qiqihar: 9 54' (East);
Harbin: 9: 39' (East);
Changchun: 8 53' (East);
Manzhouli: 8 40' (East);
Shenyang: 7 44' (East);
Lvda: 6 35' (East);
Tianjin: 5 30' (East);
Jinan: 5 0 1' (East);
Hohhot: 4 36' (East);
Xuzhou: 4 27' (East);
Shanghai: 4 26' (East);
Taiyuan: 4 1 1' (East);
Baotou: 4 03' (East);
Nanjing: 4 00' (East);
Hefei: 3 52' (East);
Zhengzhou: 3 50' (East);
Hangzhou: 3 50' (East);
Xuchang: 3 40' (East);
Jiujiang: 3 03' (East);
Wuhan: 2 54' (East);
Nanchang: 2 48' (East);
Yinchuan: 2 35' (East);
Taipei: 2 32' (East);
Dongan: 2 29' (East);
Changsha: 2 14' (east);
Ganzhou: 2 0 1' (East);
Hengyang: 1 56' (East);
Xiamen: 1 50' (East);
Lanzhou:144' (East);
Chongqing: 1 34' (East);
Zunyi: 1 26' (East);
Dongning: 1 22' (East);
Guilin: 1 22' (East);
Guiyang: 1 17' (East);
Chengdu: 1 16' (East);
Guangzhou: 1 09' (East);
Liuzhou: 1 08' (East);
Dongsha islands: 1 05' (East);
Kunming: 1 00' (East);
Nanning: 0 50' (East);
Zhanjiang: 0 44' (East);
Pingxiang: 0 39' (East);
Haikou: 0 29' (East);
Lhasa: 0 2 1' (East);
Everest: 0 19' (east);
Dongsha islands: 0 10' (East);
Zengmu shoal:-0 24': (west);
Nansha Islands:-0 35': (west);
Urumqi:-2 44': (West).