Astronomical Lore in Chaucer

by Florence M. Grimm

If we watch the sky for some time, or make several observations on the same night, we notice, by observing the changing positions of the constellations, that the stars move very slowly across the blue dome above us. The stars that rise due east of us do not, in crossing the dome of the sky, pass directly over our heads but, from the moment that we first see them, curve some distance to the south, and, after passing their highest point in the heavens, turn toward the north and set due west. A star rising due east appears to move more rapidly than one rising some distance to the north or south of the east point, because it crosses a higher point in the heavens and has, therefore, a greater distance to traverse in the same length of time. When we observe the stars in the northern sky, we discover that many of them never set but seem to be moving around an apparently fixed point at somewhat more than an angle of 40°[194] above the northern horizon and very near the north star. These are called circum-polar stars. The whole celestial sphere, in other words, appears to be revolving about an imaginary axis passing through this fixed point, which is called the north pole of the heavens, through the center of the earth and through an invisible pole (the south pole of the heavens) exactly opposite the visible one. This apparent revolution of the whole star sphere, as we know, is caused by the earth’s rotation on its axis once every twenty-four hours from west to east. Chaucer and his contemporaries believed it to be the actual revolution of the nine spheres from east to west about the earth as a center.

Fig. 1.

For determining accurately the position of stars on the celestial sphere astronomers make use of various circles which can be made clear by a few simple diagrams. In Figure 1, the observer is imagined to be at O. Then the circle NESW is the celestial horizon, which we have described above. Z, the point immediately above the observer is called the zenith, and Z′, the point immediately underneath, as indicated by a plumb line at rest, is the nadir. The line POP′ is the imaginary axis about which the star-sphere appears to revolve, and P and P′ are the poles of the heavens. The north pole P is elevated, for our latitude, at an angle of approximately 40° from the north point on the horizon. PP′ is called the polar axis and it is evident that the earth’s axis extended infinitely would coincide with this axis of the heavens.

In measuring positions of stars with reference to the horizon astronomers use the following circles: Any great circle of the celestial sphere whose plane passes through the zenith and nadir is called a vertical circle. The verticle circle SPNZ′, passing through the poles and meeting the horizon in the north and south points, N and S, is called the meridian circle, because the sun is on this circle at true mid-day. The meridian is the plane in which this circle lies. The vertical circle, EZ′WZ, whose plane is at right angles to the meridian, is called the prime vertical and it intersects the horizon at the east and west points, E and W. These circles, and the measurements of positions of heavenly bodies which involve their use, were all employed in Chaucer’s time and are referred to in his writings.[195]

The distance of a star from the horizon, measured on a vertical circle, toward the zenith is called the star’s altitude. A star reaches its greatest altitude when on the part of the meridional circle between the south point of the horizon, S, and the north pole, P. A star seen between the north pole and the north point on the horizon, that is, on the arc PN, must obviously be a circum-polar star and would have its highest altitude when between the pole and the zenith, or on the arc PZ. When a star reaches the meridian in its course across the celestial sphere it is said to culminate or reach its culmination. The highest altitude of any star would therefore be represented by the arc of the meridional circle between the star and the south point of the horizon. This is called the star’s meridian altitude.

The azimuth of a star is its angular distance from the south point, measured westward on the horizon, to a vertical circle passing through the star. The amplitude of a star is its distance from the prime vertical, measured on the horizon, north or south.

For the other measurements used by astronomers in observations of the stars still other circles on the celestial sphere must be imagined. We know that the earth’s surface is divided into halves, called the northern and southern hemispheres, by an imaginary circle called the equator, whose plane passes through the center of the earth and is perpendicular to the earth’s axis. If the plane of the earth’s equator were infinitely extended it would describe upon the celestial sphere a great circle which would divide that sphere into two hemispheres, just as the plane of the terrestrial equator divides the earth into two hemispheres. This great circle on the celestial sphere is called the celestial equator, or, by an older name, the equatorial, the significance of which we shall see presently. A star rising due east would traverse this great circle of the celestial sphere and set due west. The path of such a star is represented in Figure 2 by the great circle EMWM′, which also represents the celestial equator. All stars rise and set following circles whose planes are parallel to that of the celestial equator and these circles of the celestial sphere are smaller and smaller the nearer they are to the pole, so that stars very near the pole appear to be encircling it in very small concentric circles. Stars in an area around the north celestial pole, whose limits vary with the position of the observer never set for an observer in the northern hemisphere. There is a similar group of stars around the south pole for an observer in the southern hemisphere.

Fig. 2.