289, 290] TIME. 367 



Article 276. This process of relative rotation is that which is 

 taken as time measuring process (Article 3), and accordingly we 

 say of this process that it takes place uniformly. Time so 

 measured is called sidereal time, and the interval in which the 

 Earth turns through four right angles relatively to the stars is a 

 sidereal day. 



Now we have said (Article 3) that the process used for 

 measuring time is the average rotation of the Earth relative 

 to the Sun. To explain this statement, consider in the first 

 place the motion of the Sun relative to a frame whose origin 

 is the centre of the Earth and whose lines of reference go out 

 thence to stars so distant as to have no observable annual 

 parallax. The path and motion of the Sun relative to this 

 frame are the same as the path and motion of the Earth relative 

 to a frame whose origin is in the Sun and whose lines of reference 

 go out thence to the same stars (cf. Example 1, p. 61), and this 

 latter motion is a planetary motion of the kind described in 

 Article 177. There is thus for the motion of the Sun, relative 

 to the frame of Earth and stars, at any instant an instantaneous 

 ellipse osculating the Sun's actual path relative to the frame, 

 and the motion is very nearly an elliptic motion about a focus 

 at the centre of the Earth. The sense in which the Sun describes 

 his orbit is the same as the sense in which any particular meridian 

 plane of the Earth turns about the polar axis, i.e. the Sun is 

 always moving from stars which have a more westerly position 

 towards stars which have a more easterly position in the plane 

 of his path. The elements of the elliptic orbit are not quite 

 constant; in particular the apse line has a small progressive 

 motion in the sense in which the orbit is described, and the 

 line of intersection of the plane of the orbit with the plane of 

 the Earth's equator (known as the line of nodes) has a small 

 progressive motion in the opposite sense. The Sun passes the 

 line of nodes at the Equinoxes, and the periodic time in the 

 orbit is a year. Now it is to be observed that, relatively to a 

 frame fixed in the Earth, the Sun makes about 365J revolutions 

 round the Earth in a year, and the stars make about 366J 

 revolutions, but the time of revolution of the Sun is not a 

 constant multiple of the time of revolution of the stars. The 

 variability arises in the first place from the fact that the motion 



