Heliographic coordinates

Japanese: 太陽面座標 - たいようめんざひょう（英語表記）heliographic coordinates

Longitude and latitude coordinates to indicate the location of solar phenomena. As with Earth, the North Pole, South Pole, and Equator are defined on the solar solar sphere, and latitudes are measured from 0° to ±90° (+ is for the Northern Hemisphere) from the equator toward the poles, and longitudes are measured from 0° to 360° counterclockwise as viewed from the North Pole. However, unlike Earth, the Sun rotates faster at lower latitudes, and over time, the lines of longitude become curved with the equator extending further forward. Because this is inconvenient, the lines of longitude go around the axis of the Sun with a rotation period of 27.2753 days (close to the rotation period at 16° latitude) relative to the Earth, and their origin is the meridian that passes through the solar equator (ascending node) that intersects with the ecliptic plane at Greenwich mean noon on January 1, 1854.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

太陽面現象の位置を示すための経緯度座標。地球と同様に，太陽光球面上に北極，南極，赤道を定め，緯度は赤道から極に向け0゜から±90゜(＋は北半球)にとり，経度は北極側からみて反時計回りに0゜から360゜にとる。しかし太陽は地球とは異なり低緯度ほど自転速度が速く，経度線は時間が経つと赤道方向が先に伸びた曲線となる。これでは不便なので，経度線は対地球の自転周期が27.2753日(緯度16゜における自転周期に近い)で太陽の自転軸を回り，その原点は，1854年1月1日グリニジ平均正午において黄道面と交わる太陽の赤道(昇交点)を通る子午線を採用している。

出典　株式会社平凡社世界大百科事典第２版について　情報

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