Knowest thou the ordinances of heaven? - The laws or statutes by which the motions of the heavenly bodies are governed. These were wholly unknown in the time of Job, and the discovery of some of those laws - for only a few of them are yet known - was reserved to be the glory of the modern system of astronomy. The suggestion of the great principles of the system gave immortality to the name Copernicus; and the discovery of those laws in modern times has conferred immortality on the names of Brahe, Kepler, and Newton. The laws which control the heavenly bodies are the most sublime that are known to man, and have done more to impress the human mind with a sense of the majesty of God than any other: discoveries made in the material universe. Of course, all those laws were known to God himself, and he appeals to them in proof of his greatness and majesty. The grand and beautiful movements of the heavenly bodies in the time of Job were fitted to produce admiration; and one of the chief delights of those that dwelt under the splendor of an Oriental sky was to contemplate those movements, and to give names to those moving lights. The discoveries of science have enlarged the conceptions of man in regard to the starry heavens far toward immensity; have shown that these twinkling lights are vast worlds and systems, and at the same time have so disclosed the laws by which they are governed as to promote, where the heart is right, intelligent piety, and elevate the mind to more glorious views of the Creator.
Canst thou set the dominion thereof in the earth? - That is, "dost thou assign the dominion of the heavens over the earth?" The reference is, undoubtedly, to the influence of the heavenly bodies upon sublunary objects. The exact extent of that cannot be supposed to have been known in the days of Job, and it is probable that much more was ascribed to the influence of the stars on human affairs than the truth would justify. Nor is its extent now known. It is known that the moon has an influence over the tides of the ocean; it may be that it has to some extent over the weather; and it is not impossible that the other heavenly bodies may have some effect on the changes observed in the earth which is not understood. Whatever it is, it was and is all known to God, and the idea here is, that it was a proof of his immense superiority over man.

Knowest thou the ordinances of heaven? - Art thou a thorough astronomer? Art thou acquainted with all the laws of the planetary system? Canst thou account for the difference of their motions, and the influence by which they are retained and revolve in their orbits? And canst thou tell what influence or dominion they exercise on the earth? Sir Isaac Newton has given us much light on many of these things; but to his system, which is most probably the true one, gravity is essential; and yet what this gravity is he could neither explain nor comprehend; and his followers are not one whit wiser than he. No man has ever yet fully found out the ordinances of heaven, and the dominion thereof on the earth.

Knowest thou the ordinances of heaven? canst thou set the (u) dominion thereof in the earth?
(u) Can you cause the heavenly bodies to have any power over the earthly bodies?

Knowest thou the ordinances of heaven?.... Settled by the decree, purpose, and will of God, and are firm and stable; see Psalm 148:6; the laws and statutes respecting their situation, motion, operation, influence, and use, which are constantly observed; these are so far from being made by men, and at their direction, that they are not known by them, at least not fully and perfectly;
canst thou set the dominion thereof in the earth? or over it; of the heavens over the earth; not such an one as judicial astrologers ascribe unto them, as to influence the bodies of men, especially the tempers and dispositions of their minds; to affect their wills and moral actions, the events and occurrences of their lives, and the fate of nations and kingdoms; their dominion is not moral and civil, but physical or natural, as to make the revolutions of night and day, and of the several seasons of the year; and to affect and influence the fruits of the earth, &c. see Genesis 1:16; but this dominion is solely under God, and at his direction, and is not of men's fixing.

ordinances--which regulate the alternations of seasons, &c. (Genesis 8:22).
dominion--controlling influence of the heavenly bodies, the sun, moon, &c., on the earth (on the tides, weather) (Genesis 1:16; Psalm 136:7-9).

Ordinances - The laws which are firmly established concerning their order, motion, or rest, and their powerful influences upon this lower world. Didst thou give these laws? Or dost thou perfectly know them? Canst thou - Manage and over rule their influences.