Mean sun - Heikin Taiyo (English spelling) Mean sun

Japanese: 平均太陽 - へいきんたいよう（英語表記）mean sun

Since the Earth revolves around the Sun in an elliptical orbit, the Sun's motion on the celestial sphere is not uniform, moving faster near the perihelion and slower near the aphelion. Since it is inconvenient to determine time based on the Sun, whose angular velocity is not constant, a hypothetical celestial body called the mean sun is considered. The first hypothetical celestial body leaves the perihelion at the same time as the real Sun and moves around the ecliptic at a constant speed with the same period as the real Sun. When this first hypothetical celestial body passes through the vernal equinox, a second hypothetical celestial body is considered that leaves the vernal equinox at the same time and moves along the equator at the same constant speed as the first hypothetical celestial body, and is called the mean sun. In contrast, the real Sun is called the true sun. Calendar time is determined based on the movement of the mean sun. The difference between the time of the mean sun and the true sun is zero four times a year, and there is an error of less than 17 minutes at other times.

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

Japanese:

地球は太陽のまわりを楕円軌道を描いて公転しているので，天球上の太陽の運行は一様ではなく，近日点近くでは速く，遠日点近くでは遅くなる。このように角速度が一定でない太陽をもとにして時を決めるのは都合が悪いので，平均太陽という仮想天体を考える。第1の仮想天体は，本物の太陽と同時に近日点を出発して黄道上を一定の速度で本物の太陽と同じ周期で年周運動する。この第1の仮想天体が春分点を通過するとき同時に春分点を出発し，第1の仮想天体と同じ一定速度で赤道上を動く第2の仮想天体を考え，これを平均太陽と呼ぶ。これに対して本物の太陽を真太陽という。暦法上の時間は平均太陽の運行を基礎にして定められる。平均太陽と真太陽の時間の差は1年に4回0となり，ほかの時期には 17分以内の誤差がある。

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