John Calvin Commentary Psalms 102:11

"My days are like a shadow that declineth; And I am withered like grass." — Psalms 102:11 (ASV)

My days are like the shadow which declineth. When the sun is directly over our heads, that is, at midday, we do not observe such sudden changes in the shadows that its light produces; but when it begins to decline towards the west, the shadows vary almost every moment. This is the reason why the sacred writer expressly mentions the shadow which declineth. What he attributes to the afflicted Church seems indeed to be equally applicable to all people; but he had a special reason for using 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 quickly fall into decay. But the complaint here is that this happened to the people of God in the very flower of their age. The term days is to be understood as the whole course of their life; and the meaning is that the captivity was to the godly like the setting of the sun, because they quickly faded.

In the end of the verse, the comparison of withered grass, used a little before, is repeated to indicate that their life during the captivity was involved in many sorrows which dried up the very sap of life in them. Nor is this surprising, since to live in that condition would have been worse than a hundred deaths if they had not been sustained by the hope of future deliverance. But although they were not completely overwhelmed by temptation, they must have been in great distress, because they saw themselves abandoned by God.