Thou hast fixed [1] all the boundaries of the earth. What is here stated concerning the boundaries or limits assigned to the earth, and concerning the regular and successive recurrence of summer and winter every year, is to the same effect as the preceding verse. It is doubtful whether the prophet means the uttermost ends of the world, or whether he speaks of the particular boundaries by which countries are separate from each other. Although the latter are often disturbed by the violence of men, whose insatiable cupidity and ambition cannot be restrained by any of the lines of demarcation which exist in the world, but are always endeavoring to break through them; yet God manifests his singular goodness in assigning to each nation its own territory upon which to dwell. I am, however, rather of opinion, that the clause is to be understood of those bounds which cannot be confounded at the will of men, and consider the meaning to be, that God has allotted to men as much space of earth as he has seen to be sufficient for them to dwell upon. Farther, the well regulated successions of summer and winter clearly indicate with what care and benignity God has provided for the necessities of the human family. From this, the prophet justly concludes, that nothing is more improbable than that God should neglect to act the part of a father towards his own flock and household.

Footnotes

1 - The original word implies "to settle, to place steadily in a certain situation or place." See Parkhurst's Lexicon on ytsv

Thou hast set all the borders of the earth - Thou hast established all the boundaries of the world; that is, the boundaries of the earth itself; or the natural bonndaries of nations and people, made by seas, mountains, rivers, and deserts. The language in regard to the first of these - the earth itself - would be derived from the prevalent mode of speaking, as if the earth were a plane, and had limits - a common mode of expression in the Scriptures, as it is in all ancient writings, and in the common language of men, even of philosophers. In regard to the latter idea, the language would imply that God had fixed, by his own power and will, all the natural boundaries of nations, or that his dominion is over all the earth. There are natural boundaries, or arrangements in nature, which tend to break up the one great family of man into separate nations, and which seem to have been designed for that. Compare Acts 17:26. Over all these God presides, and he has his own great plans to accomplish by the arrangement.
Thou hast made summer and winter - literally, as in the margin, "Summer and winter, thou hast made them." That is, he has so made the earth that these various seasons will occur. The fact that there are different seasons of the year, or that the year is divided into seasons, is to be traced to the agency of God. He has so made the world that these changes will take place. Nothing is the result of chance; all things in the arrangements of nature are by his design.

Thou hast set all the borders of the earth - Thou alone art the Author of all its grand geographical divisions.
Thou hast made summer and winter - Thou hast appointed that peculiarity in the poise and rotation of the earth, by which the seasons are produced.

Thou hast set all the borders of the earth,.... Of the whole world, and each of the nations, as of the land of Canaan, so of others, Deuteronomy 32:8, and even has fixed and settled the bounds of every man's habitation, Acts 17:26,
thou hast made summer and winter; see Genesis 8:22, which, taken literally, are great benefits to the world; and, figuratively understood, may represent the two dispensations of the law and Gospel; see Song 2:11, and the different frames of God's people when under temptations, and clouds, and darkness, and when they enjoy peace and comfort; and the different state of the church, when affected with affliction, persecution, false doctrine, deadness, and formality, which is now greatly the case; but there is a summer coming, when it will be otherwise; see Luke 21:30.

Set - Thou hast fixed the bounds of the habitable world in general, and of all the countries and people upon the earth. And as this clause shews God's power over all places, so the next displays his dominion over all times and seasons.