(346.) In the foregoing chapters, it has been shown that the apparent path of the sun is a great circle of the sphere, which it performs in a period of one sidereal year. From this it follows, that the line joining the earth and sun lies constantly in one plane; and that, therefore, whatever be the real motion from which this apparent motion arises, it must be confined to one plane, which is called the plane of the ecliptic.

(347.) We have already seen (art. 146.) that the sun's motion in right ascension among the stars is not uniform. This is partly accounted for by the obliquity of the ecliptic, in consequence of which equal variations in longitude do not correspond to equal changes of right ascension. But if we observe the place of the sun daily throughout the year, by the transit and circle, and from these calculate the longitude for each day, it will still be found that, even in its own proper path, its apparent angular motion is far from uniform. The change of longitude in twenty-four mean solar hours averages 0° 59′ 8″°.33; but about the 31st of December it amounts to 1° 1′ 9″°.9, and about the 1st of July is only 0° 57′ 11″°.5. Such are the extreme limits, and such the mean value of the sun's apparent angular velocity in its annual orbit.

(348.) This variation of its angular velocity is accompanied with a corresponding change of its distance from us.