What is "tekufah"

Tekufot ( Hebrew: תקופות, singular: tekufah, literally, "turn" or "cycle") are the four seasons of the year recognized by Talmud writers. According to Samuel Yarḥinai, each tekufah marks the beginning of a period of 91 days 7½ hours. The four tekufot are:

1. Tekufat Nisan, the vernal equinox, when the sun enters Aries; this is the beginning of spring, or "eit hazera" (seed-time), when day and night are equal.
2. Tekufat Tammuz, the summer solstice, when the sun enters Cancer; this is the summer season, or "et ha-katsir" (harvest-time), when the day is the longest in the year.
3. Tekufat Tishrei, the autumnal equinox, when the sun enters Libra, and autumn, or "et ha-batsir" (vintage-time), begins, and when the day again equals the night.
4. Tekufat Tevet, the winter solstice, when the sun enters Capricornus; this is the beginning of winter, or "et ha-ḥoref" (winter-time), when the night is the longest during the year.

Notice that in the early 20th century the tekufot fell from fourteen to eighteen days later than the true solar equinox or solstice. However, the Jewish calendar follows the figures of R. Ada.