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.