The terms "astronomie" and "astrologie" are more or less interchangeable; it was understood that the astrological condition of the skies affects the weather and influences the seasons and times for planting and harvesting (hence even today the Old Farmer's Almanac provides detailed astrological information for the benefit of those whose labors may be affected by the skies. "Judicial" astronomy concerned the influence of the planets on the actions of humans, a subject of considerable debate.

John Gower provides a good general introduction to the subject in Book VII of his Confessio amantis

The cosmos consists of the earth surrounded by the nine concentric circles:

1.The First Moved (Primum mobile)
2.The Fixed Stars and the Zodiac
3. Saturn
4. Jupiter
5. Mars
6. Sun
7. Venus
8. Mercury
9. Moon
EARTH

The seven planets move through the zodiac, which consists of twelve "signs" (30 degrees each, of the 360 in the circle). The signs are, in the order in which they apppear in the solar year, starting in March:

1. Ram (Aries)
2. Bull (Taurus)
3. Twins (Gemini)
4. Crab (Cancer)
5. Lion (Leo)
6. Virgin (Virgo)
7. Scales (Libra)
8. Scorpion (Scorpio)
9. Archer (Sagittarius)
10. Goat (Capricorn)
11. Water-Bearer (Aquarius)
12. Fish (Pisces)

Here is a manuscript painting (from the Bibliotheque nationale in Paris) of the zodiac. In that image the Zodiac is shown surrounding the four elements (earth, air, fire, and water).

Here is Aries (NOTE the dryness of the dead land into which the Ram is entering; the "drought" of March is seen there, as well as a more T.S.Eiot-like waste land).

Here is Taurus (in which the action of the General Prologue takes place).

Each of the zodiacal signs dominates one of the twelve months (the match is not exact because of the precession of the equinoxes -- the movement of the constellations with which the zodiacal signs were associated, a phenomenon well known to Chaucer and his contemporaries (see note on Franklin's Tale, v. 1280, p. 899 in Riverside Chaucer.)

Here, from the Tres riches heures du Duc de Berry is the month of April. The zodiacal signs are at the top; a marriage is being contracted in the foreground. Even the rivers are apparently filled with fish, since in the background fishermen are at work.

Each of the seven planets was thought to have a particular zodiacal sign -- its house or domicile -- in which it was most powerful (its "exaltation") and one (directly opposite its house) in which it was weakest. Moreover, each planet was thought to have an affect on the character of those who were born under its influence -- when it was in the "ascendent." This was a matter of "judicial astronomy," and the subject of considerable debate not unlike the modern debate over "nurture" and "nature." See Chaucer's remarks on the ascendent in The Astrolabe (pp. 670-71 of the Riverside Chaucer.)

For a further explanation, see The Planets and their Children, compiled by Marianne Hansen at Cornell.

Further Reading:

• Walter Clyde Curry, Chaucer and Medieval Science.
  New York, 1926. PR1933.S3 C8 Widener: 12422.165
  (Old but still useful)
• J.C. Eade, The forgotten sky: a guide to astrology in English literature.
  New York, 1984, PR149.A798 E24 1984.
• John D. North, Chaucer's Universe.
  New York, 1988. PR1933.A7 N67 1988.
  (Impressive scientific knowledge but use with caution; astronomical references are often assumed when they are not necessarily there).