Declination (English spelling)

Japanese: 偏角 - へんかく（英語表記）declination

One of the three components of geomagnetism. The angle between the direction of the horizontal component of the geomagnetic field and the geographic North Pole (or South Pole). In other words, the angle between the geomagnetic meridian and the geographic meridian. Measuring declination is the simplest and most accurate way to determine direction, so it has been measured all over the world since ancient times. E. Halley, an astronomer famous for Halley's Comet, already published a map of declination for the entire world in 1702. Currently, the declination in the Tokyo area is about 6° west (the magnetic needle is tilted 6° west of the north). However, this value has changed over time, and in the 1600s and 1700s, the declination in Tokyo (Edo) was 6 to 4° east.

Declination
argument

When a point on a plane is expressed in polar coordinates by the distance r from the pole (the origin) and the direction θ, we get x = r cos θ and y = r sin θ. This angle θ is called the argument.

Declination
Angle of deviation

The angle between the incident light and the reflected light when light is refracted by a prism.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

地磁気の3要素の1つ。地磁気の水平分力の方向と地理学的北極 (または南極) とのなす角度。すなわち地磁気子午線と地理学上の子午線とのなす角度。偏角の測定は方角を知るうえで最も簡単で正確な方法を与えるので，古くから世界各地で測定がなされている。ハレー彗星で有名な天文学者の E.ハレーはすでに 1702年に全世界の偏角図を発表している。現在東京付近での偏角は約6°西 (磁針の向きが北に対して西のほうへ6°傾く) である。しかしこの値は時代とともに変化し，1600～1700年代には東京 (江戸) における偏角は6～4°東であった。