My days are like the shadow which declineth [1] When the sun is directly over our heads, that is to say, at mid-day, we do not observe such sudden changes of the shadows which his light produces; but when he begins to decline towards the west the shadows vary almost every moment, This is the reason why the sacred writer expressly makes mention of the shadow which declineth What he attributes to the afflicted Church seems indeed to be equally applicable to all men; but he had a special reason for employing this comparison to illustrate the condition of the Church when subjected to the calamity of exile. It is true, that as soon as we advance towards old age, we speedily fall into decay. But the complaint here is, that this befell the people of God in the very flower of their age. By the term days is to be understood the whole course of their life; and the meaning is, that the captivity was to the godly as the setting of the sun, because they quickly failed. In the end of the verse the similitude of withered grass, used a little before, is repeated, to intimate that their life during the captivity was involved in many sorrows which dried up in them the very sap of life. Nor is this wonderful, since to live in that condition would have been worse than a hundred deaths had they not been sustained by the hope of future deliverance. But although they were not altogether overwhelmed by temptation, they must have been in great distress, because they saw themselves abandoned by God.

Footnotes

1 - It is literally, "My days are like a shadow, stretched out." As the sun descends in the firmament, the shadow of any terrestrial object gradually lengthens, and grows fainter as it becomes longer, until shooting out to an unmeasurable length, it disappears. The Psalmist complains that his days were like a shadow nearly stretched to its utmost length, and at the point of being lost in total darkness. He felt that he had far passed his meridian, that the sun of life was about to set, and the dark night of death to fall down upon him" -- See Psalm 109:23.

My days are like a shadow that declineth - The shadow made by the gnomon on a sun-dial, which marks the hours as they pass. See 2-Kings 20:10. The idea is that the shadow made by the descending sun was about to disappear altogether. It had become less distinct and clear, and it would soon vanish. It would seem from this, that the dial was so made that the shadow indicating the hour ascended when the sun ascended, and declined when the sun went down. See the notes at Isaiah 38:8.
And I am withered like grass - See the notes at Psalm 102:4.

My days are like a shadow that declineth - Or rather, My days decline like the shadow. I have passed my meridian, and the sun of my prosperity is about to set for ever. There may be here an allusion to the declination of the sun towards the south, which, by shortening their days, would greatly lengthen their nights. Similar to the exclamation of a contemporary prophet, Jeremiah 8:20 : "The harvest is past, the summer is ended, and we are not saved." There is now scarcely any human hope of our deliverance.

My days are like a shadow that declineth,.... Or, "that is stretched out" (s), which, though it may appear long, is soon at an end; as it does appear longer when the sun sets (t), and departs from the earth: he reckons his life not by months and years, but by days; and these he compares to a "shadow", which has no substance in it; his age being as nothing before the Lord, and has much darkness and obscurity in it; his days being days of darkness, affliction, and trouble, and quickly gone, as man's life is; there is no abiding; see 1-Chronicles 29:15. Pindar (u) calls man the dream of a shadow:
and I am withered like grass; which in the morning is flourishing, is cut down at noon, and withered at evening: this is the case of all flesh, however beautiful and goodly it may look; it is weak, frail, and mortal; cannot stand before the force of afflictions, which quickly consume strength and beauty, and much less before the scythe of death; see Psalm 90:5.
(s) "inclinata", Pagninus, Montanus, Piscator, Musculus, Cocceius; "extensa", Michaelis. (t) "Et sol crescentes decedens duplicat umbras", Virgil. Bacol. Eclog. 2. (u) Pyth. Ode 8.

shadow . . . declineth--soon to vanish in the darkness of night.