Astronomical Lore in Chaucer

by Florence M. Grimm

The wydeste of thise three principal cercles is cleped the Cercle of Capricorne, by-cause that the heved of Capricorne turneth evermo consentrix up-on the same cercle. (That is to say, the Tropic of Capricorn meets the ecliptic in the sign Capricornus, or, in other words, the sun attains its greatest declination southward when in the sign Capricornus.) In the heved of this for-seide Capricorne is the grettest declinacioun southward of the sonne, and ther-for is it cleped the Solsticioun of Winter. This signe of Capricorne is also cleped the Tropik of Winter, for thanne byginneth the sonne to come agayn to us-ward.”

VI. The moon’s orbit around the earth is inclined at an angle of about 5° to the earth’s orbit around the sun. The moon, therefore, appears to an observer on the earth as if traversing a great circle of the celestial sphere just as the sun appears to do; and the moon’s real orbit projected against the celestial sphere appears as a great circle similar to the ecliptic. This great circle in which the moon appears to travel will, therefore, be inclined to the ecliptic at an angle of 5° and the moon will appear in its motion never far from the ecliptic; it will always be within the zodiac which extends eight or nine degrees on either side of the ecliptic.

The angular velocity of the moon’s motion in its projected great circle is much greater than that of the sun in the ecliptic. Both bodies appear to move in the same direction, from west to east; but the solar apparent revolution takes about a year averaging 1° daily, while the moon completes a revolution from any fixed star back to the same star in about 27¼ days, making an average daily angular motion of about 13°. The actual daily angular motion of the moon varies considerably; hence in trying to test out Chaucer’s references to lunar angular velocity it would not be correct to make use only of the average angular velocity since his references apply to specific times and therefore the variation in the moon’s angular velocity must be taken into account.

VII. On the line “In two of Taur,” etc., Skeat has the following note: “Tyrwhitt unluckily altered two to ten, on the plea that ‘the time (four days complete, l. 1893) is not sufficient for the moon to pass from the second degree of Taurus into Cancer? And he then proceeds to shew this, taking the mean daily motion of the moon as being 13 degrees, 10 minutes, and 35 seconds. But, as Mr. Brae has shewn, in his edition of Chaucer’s Astrolabe, p. 93, footnote, it is a mistake to reckon here the moon’s mean motion; we must rather consider her actual motion. The question is simply, can the moon move from the 2nd degree of Taurus to the 1st of Cancer (through 59 degrees) in four days? Mr. Brae says decidedly, that examples of such motion are to be seen ‘in every almanac.’

For example, in the Nautical Almanac, in June, 1886, the moon’s longitude at noon was 30° 22′ on the 9th, and 90° 17′ on the 13th; i. e., the moon was in the first of Taurus on the former day, and in the first of Cancer on the latter day, at the same hour; which gives (very nearly) a degree more of change of longitude than we here require. The MSS all have two or tuo, and they are quite right. The motion of the moon is so variable that the mean motion affords no safe guide.” [Skeat, Notes to the Canterbury Tales, p. 363.]

VIII. The moon’s “waxing and waning” is due to the fact that the moon is not self-luminous but receives its light from the sun and to the additional fact that it makes a complete revolution around the earth with reference to the sun in 29½ days. When the earth is on the side of the moon that faces the sun we see the full moon, that is, the whole illuminated hemisphere. But when we are on the side of the moon that is turned away from the sun we face its unilluminated hemisphere and we say that we have a ‘new moon.’ Once in every 29½ days the earth is in each of these positions with reference to the moon and, of course, in the interval of time between these two phases we are so placed as to see larger or smaller parts of the illuminating hemisphere of the moon, giving rise to the other visible phases.

When the moon is between the earth and the sun she is said to be in conjunction, and is invisible to us for a few nights. This is the phase called new moon. As she emerges from conjunction we see the moon as a delicate crescent in the west just after sunset and she soon sets below the horizon. Half of the moon’s surface is illuminated, but we can see only a slender edge with the horns turned away from the sun. The crescent appears a little wider each night, and, as the moon recedes 13° further from the sun each night, she sets correspondingly later, until in her first quarter half of the illuminated hemisphere is turned toward us. As the moon continues her progress around the earth she gradually becomes gibbous and finally reaches a point in the heavens directly opposite the sun when she is said to be in opposition, her whole illumined hemisphere faces us and we have full moon. She then rises in the east as the sun sets in the west and is on the meridian at midnight. As the moon passes from opposition, the portion of her illuminated hemisphere visible to us gradually decreases, she rises nearly an hour later each evening and in the morning is seen high in the western sky after sunrise. At her third quarter she again presents half of her illuminated surface to us and continues to decrease until we see her in crescent form again. But now her position with reference to the sun is exactly the reverse of her position as a waxing crescent, so that her horns are now turned toward the west away from the sun, and she appears in the eastern sky just before sunrise. The moon again comes into conjunction and is lost in the sun’s rays and from this point the whole process is repeated.

IX. That the apparent motions of the sun and moon are not so complicated as those of the planets will be clear at once if we remember that the sun’s apparent motion is caused by our seeing the sun projected against the celestial sphere in the ecliptic, the path cut out by the plane of the earth’s orbit, while in the case of the moon, what we see is the moon’s actual motion around the earth projected against the celestial sphere in the great circle traced by the moon’s own orbital plane produced to an indefinite extent. These motions are further complicated by the rotation of the earth on its own axis, causing the rising and setting of the sun and the moon. These two bodies, however, always appear to be moving directly on in their courses, each completing a revolution around the earth in a definite time, the sun in a year, the moon in 29½ days. What we see in the case of the planets, on the other hand, is a complex motion compounded of the effects of the earth’s daily rotation, its yearly revolution around the sun, and the planets’ own revolutions in different periods of time in elliptical orbits around the sun. These complex planetary motions are characterized by the peculiar oscillations known as ‘direct’ and ‘retrograde’ movements.

Fig. 4.