Lo in the orient when the gracious light
Lifts up his burning head, each under eye
Doth homage to his new-appearing sight,
Serving with looks his sacred majesty;
And having climbed the steep-up heavenly hill,
Resembling strong youth in his middle age,
Yet mortal looks adore his beauty still,
Attending on his golden pilgrimage:
But when from highmost pitch, with weary car,
Like feeble age, he reeleth from the day,
The eyes, 'fore duteous, now converted are
From his low tract, and look another way:
The Greek god Phoebus was said to drive his chariot towards Helios, the sun. Picking up the reference, now he has reached the summit of the sky — the highmost pitch — the chariot or “car” is exhausted. 
 The concept of the exhausted chariot is known as  hypallage , a transferred epithet. It is the Greek god who is weary,  not his vehicle.
Sonnet 7