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Rose Chen | 陳紅穎

天球赤道（Celestial equator）：地球赤道投射到天空中的大圓圈，為天球三種主要的坐標系統之一。緯度的投影稱為赤緯（Declination），經度的投影稱為赤經（Right Ascension）（自天球赤道與黃道[Ecliptic]的交會點——牡羊座開始起算）。

古代文獻中，赤道常與黃道相提並論。黃道為太陽運行的軌道。由於赤道與黃道並非平行，中間有一斜交角，因此太陽在黃道運行時，從牡羊座到處女座會在赤道的北方，亦稱北緯，天秤座到雙魚座則在赤道的南方，即南緯。也因為如此，當太陽在北緯時，北半球的天氣較南北球溫暖，反之亦然。

以下為古典名家欽迪（Al-Kindi）描述氣候冷熱變化的片段。

冷與熱的增加與減少

The intensity of heat and cold may be measured in four “degrees” or stages. At the extreme of northern declinations (i.e., Cancer), hot is at four degrees, and cold in its first degree. As the Sun moves towards the equinox and then to the extremes of southern declination (i.e., Capricorn), heat decreases and cold increases, to become cold to the fourth degree and hot to the first; likewise as he moves back up towards the northern signs. At the equinoctial points, heat and cold are evenly balanced at two degrees.

冷與熱的強度可分為四種程度或階段。在北緯的最高點時（即巨蟹座），熱度會來到四級，冷度只有一級。當太陽往秋分點移動，然後來到南緯的頂端時（即魔羯座），熱度會降低來到一級，冷度則會增加來到四級的程度，當太陽重返北緯的星座則又重複一樣的歷程。若太陽來到交會點，熱度與冷度則是相當的。

And I say that if the circuit of the Sun was in the degrees of circles equidistant to the equator of the day, there would be a season of only one nature, hot or cold. And the Sun, from the beginning of Libra to the beginning of Capricorn, is elongated from the northern portion, and that portion is made cold, and what is in it of moisture, flows, and it is coldness to the greatest extent while he comes to the beginning of Capricorn.

若太陽繞行的軌道與天體赤道相同，氣候就只會有一種型態，不會有冷和熱等四季的產生。而太陽從天秤座初度數繞行至魔羯座初度數時，與北方的距離會愈來愈遠，此時北方的氣候會變得冷與潮濕，或有大水，而當太陽來到魔羯座零度時將為最寒冷的時候。北緯的星座則又重複一樣的歷程。若太陽來到交會點，熱度與冷度則是相當的。

（以上引自戴克博士的《世界占星學I》）