in Astronomy, a great circle of the sphere, supposed to be drawn through the middle of the zodiac, making an angle with the equinoctial of about $23^\circ 30'$ which is the sun's greatest declination; or, more strictly speaking, it is that path or way among the fixed stars which the earth appears to describe to an eye placed in the sun. The angle of inclination of the equator and ecliptic is called the obliquity of the ecliptic. This has been subject to a small irregular diminution since the time of the earliest observations on record. Its mean diminution per century is about $48''$. See ASTRONOMY.