The twenty-four jieqi, the solar terms, divide the tropical year into arcs of fifteen degrees of solar longitude. They are solar, not lunar, and they are the part of the Chinese calendar that keeps the agricultural year in step with the seasons while the lunar months drift against it.
Because they are cut at fifteen-degree intervals from the vernal point, every second solar term falls on a multiple of thirty degrees, and thirty degrees is the width of a tropical zodiac sign. The correspondence is therefore exact rather than approximate. Chunfen, the spring equinox, begins at zero degrees, which is the cusp of Aries. Xiazhi, the summer solstice, begins at ninety degrees, the cusp of Cancer. Qiufen begins at one hundred and eighty, and Dongzhi at two hundred and seventy.
The twelve terms falling on the cusps are the zhongqi, the principal or middle terms; the twelve falling halfway between are the jieqi proper, the sectional terms. The two alternate.
The alternation does real calendrical work. A lunar month must contain the zhongqi assigned to it, and a lunar month that contains none becomes an intercalary month, repeating the number of the month before it. This is the rule that keeps the Chinese lunisolar calendar aligned, and it is why leap months fall where they do rather than at a fixed point in the year.
The exactness of the sign correspondence is a consequence of arithmetic, not a coincidence or a later harmonisation. Both systems cut the same tropical circle, one into twenty-four parts and the other into twelve.
What the two systems do with their divisions has nothing in common. The solar terms name weather and agricultural work. The tropical signs carry an entirely separate body of meaning. They share a set of boundaries and nothing else.