Adam Clarke on Psalms 102:11

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, Jer 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.