Astronomical Lore in Chaucer

by Florence M. Grimm

The angle of elevation of the celestial equator to the horizon varies according to the position of the observer. If, for example, the observer were at the north pole of the earth, the north celestial pole would be directly above him and would therefore coincide with the zenith; this would obviously make the celestial equator and the horizon also coincide. If the observer should pass slowly from the pole to the terrestrial equator it is clear that the two circles would no longer coincide and that the angle between them would gradually widen until it reached 90°. Then the zenith would be on the celestial equator and the north and south poles of the heavens would be on the horizon.

We have still to define a great circle of the celestial sphere that is of equal importance with the celestial equator and the celestial horizon. This is the sun’s apparent yearly path, or the ecliptic. We know that the earth revolves about the sun once yearly in an orbit that is not entirely round but somewhat eliptical. Now since the earth, the sun, and the earth’s orbit around the sun are always in one plane, it follows that to an observer on the earth the sun would appear to be moving around the earth instead of the earth around the sun. The sun’s apparent path, moreover, would be in the plane of the earth’s orbit and when projected against the celestial sphere, which is infinite in extent, would appear as a great circle of that sphere. This great circle of the celestial sphere is the ecliptic. The sun must always appear to be on this circle, not only at all times of the year but at all hours of the day; for as the sun rises and sets, the ecliptic rises and sets also, since the earth’s rotation causes an apparent daily revolution not only of the sun, moon, and planets but also of the fixed stars and so of the whole celestial sphere and of all the circles whose positions upon it do not vary. The ecliptic is inclined to the celestial equator approximately 23½°, an angle which obviously measures the inclination of the plane of the earth’s equator to the plane of its orbit, since the celestial equator and the ecliptic are great circles on the celestial sphere formed by extending the planes of the earth’s equator and its orbit to an infinite distance. Since both the celestial equator and the ecliptic are great circles of the celestial sphere each dividing it into equal parts, it is evident that these two circles must intersect at points exactly opposite each other on the celestial sphere. These points are called the vernal and the autumnal equinoxes.

We shall next define the astronomical measurements that correspond to terrestrial latitude and longitude. For some reason astronomers have not, as we might expect, applied to these measurements the terms ‘celestial longitude’ and ‘celestial latitude.’ These two terms are now practically obsolete, having been used formerly to denote angular distance north or south of the ecliptic and angular distance measured east and west along circles parallel to the ecliptic. The measurements that correspond in astronomy to terrestrial latitude and longitude are called declination and right ascension and are obviously made with reference to the celestial equator, not the ecliptic. For taking these measurements astronomers employ circles on the celestial sphere perpendicular to the plane of the celestial equator and passing through the poles of the heavens. These are called hour circles. The hour circle of any star is the great circle passing through it and perpendicular to the plane of the equator. The angular distance of a star from the equator measured along its hour circle, is called the star’s declination and is northern or southern according as the star is in the northern or southern of the two hemispheres into which the plane of the equator divides the celestial sphere. It is evident that declination corresponds exactly to terrestrial latitude. Right ascension, corresponding to terrestrial longitude, is the angular distance of a heavenly body from the vernal equinox measured on the celestial equator eastward to the hour circle passing through the body.

The hour angle of a star is the angular distance measured on the celestial equator from the meridian to the foot of the hour circle passing through the star.

Fig. 3.

It remains to describe in greater detail the apparent movements of the sun and the sun’s effect upon the seasons. In Figure 3, the great circle MWM′E represents the equinoctial and XVX′A the ecliptic. The point X represents the farthest point south that the sun reaches in its apparent journey around the earth, and this point is called the winter solstice, because, for the northern hemisphere the sun reaches this point in mid-winter. When the sun is south of the celestial equator its apparent daily path is the same as it would be for a star so situated. Thus its daily path at the time of the winter solstice, about December 21, can be represented by the circle Xmn′. The arc gXh represents the part of the sun’s path that would be above the horizon, showing that night would last much longer than day and the rays of the sun would strike the northern hemisphere of the earth more indirectly than when the sun is north of the equator. As the sun passes along the ecliptic from X toward V, the part of its daily path that is above the horizon gradually increases until at V, the vernal equinox, the sun’s path would, roughly speaking, coincide with the celestial equator so that half of it would be above the horizon and half below and day and night would be of equal length. This explains why the celestial equator was formerly called the equinoctial (Chaucer’s term for it). As the sun passes on toward X′ its daily arc continues to increase and the days to grow longer until at X′ it reaches its greatest declination north of the equator and we have the longest day, June 21, the summer solstice. When the sun reaches this point, its rays strike the northern hemisphere more directly than at any other time causing the hot or summer season in this hemisphere. Next the sun’s daily arc begins to decrease, day and night to become more nearly equal, at A the autumnal equinox[196] is reached and the sun again shapes its course towards the point of maximum declination south of the equator. The two points of maximum declination are called solstices.

The two small circles of the celestial sphere, parallel to the equator, which pass through the two points where the sun’s declination is greatest, are called Tropics; the one in the northern hemisphere is called the Tropic of Cancer, that in the southern hemisphere, the Tropic of Capricorn. They correspond to circles on the earth’s surface having the same names.

II. By “artificial day” Chaucer means the time during which the sun is above the horizon, the period from sunrise to sunset. The arc of the artificial day may mean the extent or duration of it, as measured on the rim of an astrolabe, or it may mean (as here), the arc extending from the point of sunrise to that of sunset. See Astrolabe ii.7.

There has been some controversy among editors as to the correctness of the date occurring in this passage, some giving it as the 28th instead of the 18th. In discussing the accuracy of the reading “eightetethe” Skeat throws light also upon the accuracy of the rest of the passage considered from an astronomical point of view. He says (vol. 5, p. 133):