CAPRICORN, CAPRICORNE, CAPRICORNUS is the constellation the Goat, which lies in the southern hemisphere near Sagittarius. The tenth sign of the zodiac, it is the day house of Saturn, the exaltation of Mars, and the depression of Jupiter (Confessio Amantis VII.1169-84; Tetrabiblos I.19).

The sun, Phoebus, is in Capricorn, the winter solstice, and bitter frosts destroy the green in every yard, FranklT 1248-1251. Capricorn is called the winter solstice or the tropic of winter, Astr I.17, because the sun enters the sign on December 13, the shortest day in the northern hemisphere in Chaucer's time. Today, the sun enters Capricorn on December 21. The circle of Capricorn is the widest circle of the three principal circles, Astr I.17, the others being the tropic of Cancer and the equator. The head or the beginning of Capricorn is the sun's most southern declination, that is, its southernmost point, Astr I.17. According to Ptolemy, the sun's southernmost latitude or declination is 23 degrees and 50 minutes in Capricorn, Astr I.17; Skeat notes that, in Ptolemy's time, the true value for both latitudes of the sun, north (Cancer) and south (Capricorn), was about 23 degrees and 40 minutes, but in Chaucer's time it was 23 degrees and 31 minutes. The signs found from the head or beginning of Capricorn to the end of Gemini are called tortuous signs, oblique signs, or obedient signs; that is, they obey the signs of right ascension or the western or sovereign signs: Capricorn obeys the sovereign (western) sign Sagittarius, Astr II.28.38-39. A sign is said to rise obliquely when a similar part of the equator rises with it. Every sign between the head or beginning of Capricorn and the end of Gemini rises in less than two equal hours, Astr II.28.25-26. The longitude and latitude of Venus are calculated by measuring its degree from Capricorn, Astr II.40.10-42.

Capricorn, the English variant, appears in FranklT 1248 and throughout the Treatise on the Astrolabe. Capricorne, the French variant, and Capricornus, the Latin variant, appear only in the Treatise on the Astrolabe.