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 - 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.