He hath appointed the moon to distinguish seasons The Psalmist now comes to another commendation of God's providence as manifested in the beautiful arrangement by which the course of the sun and moon alternately succeeds each other; for the diversity in their mutual changes is so far from producing confusion, that all must easily perceive the impossibility of finding any better method of distinguishing time. When it is said, that the moon was appointed to distinguish seasons, interpreters agree that this is to be understood of the ordinary and appointed feasts. The Hebrews having been accustomed to compute their months by the moon, this served for regulating their festival days and assemblies, both sacred and political. [1] The prophet, I have no doubt, by the figure synecdoche, puts a part for the whole, intimating, that the moon not only distinguishes the days from the nights, but likewise marks out the festival days, measures years and months, and, in fine, answers many useful purposes, inasmuch as the distinction of times is taken from her course. As to the sentence, The sun knoweth his going down, I understand it not only of his daily circuit, but as also denoting that by gradually approaching nearer us at one time, and receding farther from us at another, he knows how to regulate his movements by which to make summer, winter, spring, and autumn. It is farther stated, that the beasts of the forest creep forth during the night, because they go out of their dens with fear. Some translate the verb rms, ramas, to walk; but its proper signification which I have given is not unsuitable; for although hunger often drives wild beasts into fury, yet they watch for the darkness of the night, that they may move abroad from their hiding-places, and on account of this fearfulness they are said to creep forth.

Footnotes

1 - "The greatest part of the Jewish feasts, as the New Moon, the Passover, the Pentecost, etc., were governed by the moon." -- Dimoch.

He appointed the moon for seasons - Genesis 1:14-18. That is, The moon, as well as the sun, is appointed to divide time; to determine its progress; to indicate the return of festival occasions, or appointed times to be observed in any manner. It is, in fact, the foundation of the division of the year into "months," and consequently the indication of all that is to be observed in the "months" of the year. But for this, there would be no natural divisions of time except those of day and night, and of the year. How great an advantage it is for the purpose of life, to have time broken up into brief intervals or periods which can be marked and remembered, both in our private life and in history, it is not necessary to say. God has been pleased to add to the natural divisions of time into days, and years, and months, an "artificial" division - the "fourth" part of the moon's course - "a week," indicated by the Sabbath, thus greatly facilitating the plans of life in regard to stated times or "seasons," and especially in regard to religious observances. The idea in the passage before us is, that the whole arrangement is one of benevolence, promoting the comfort of man, and bringing the ideas of succession, variety, and beauty into the system.
The sun knoweth his going down - As if conscious of what he is doing, he knows the exact time of setting, and never varies, but always obeys the divine command; never sets "before" his time - unexpectedly shortening the day, and leaving man in sudden darkness in the midst of his toil; and never lingers above the horizon "after" the moment has come for his setting, but withdraws at the exact time, enabling man to close his toil, and seek repose, and giving an opportunity for another class of creatures to come forth on the animated scene. Their good is regarded as well as that of man; and the operations of nature are so arranged as to promote the welfare of all.

He appointed the moon for seasons - The heathens thought that the sun and moon were gods, and worshipped them as such. The psalmist shows, 1. That they are creatures dependent on God for their being and continuance; and, 2. That they were made for the use of man. See what has been said on these luminaries in the notes on Genesis 1:14-16 (note).

He appointed (i) the moon for seasons: (k) the sun knoweth his going down.
(i) As to separate the night from the day, and to note days, months and years.
(k) That is, by his course, either far or near, it notes summer, winter and other seasons.

He appointeth the moon for seasons,.... Or, "he made" (e); for the moon is the work of his hands, Psalm 8:3 as is likewise the sun. From the rain the psalmist passes to the luminaries; for this reason, as Aben Ezra thinks, because they are the cause of rain: the moon is taken notice of in the first place, because, as Kimchi observes, the night was before the day; and in the night of the fourth day were the sun, moon, and stars; but the sun rose in the morning. The moon was made for seasons as well as the sun, Genesis 1:16 or that times might be numbered by it, as the Targum, both months and years; one of its courses and revolutions making a month, and twelve of these a year; which lunar years were in use among some nations: as also it is supposed to have an influence on the ebbing and flowing of the tides; and served to regulate the festivals of the Jews, their set appointed times, as the word signifies, and is used of them, and which were governed by it. And this Jarchi takes to be the sense of the passage; though Aben Ezra more truly remarks, that it purely belongs to the work of creation, and the original design and use of this luminary. It was an emblem of the ceremonial law; which consisted, among other things, in the observation of new moons; which gave some light in the time of Jewish darkness, though but a dim one, in comparison of the Gospel; had its imperfections, was changeable, waxed old, and vanished away; and which the church is said to have under her feet, being abolished, Revelation 12:1. Though some think the world is meant, which is changeable and fading. It was also an emblem of the church, Song 6:10 which receives her light from Christ, the sun of righteousness; has its different phases and appearances; sometimes being in prosperity, and sometimes in adversity; has its spots and imperfections, and yet beautiful, through the grace of God and righteousness of Christ.
The sun knoweth his going down; not the going down of the moon, which is the sense of some, according to Kimchi; but his own going down; and so he knows his rising, to which this is opposed, Psalm 50:1 and every revolution, diurnal or annual, he makes; and which he constantly and punctually observes, as if he was a creature endued with reason and understanding; see Psalm 19:5. He knows the time of his setting, as the Targum, Syriac, and Arabic versions; and also the place where he is to set, at the different seasons of the year, and indeed every day. This luminary is an emblem of Christ, the sun of righteousness, Psalm 84:11 the fountain of all light; the light of nature, grace, and glory; and of all spiritual life and heat, as well as fruitfulness. He arose at his incarnation, and set at his death, the time of both which he full well knew; and he has his risings and settings, with respect to the manifestation of himself to his people, or hiding himself from them, which depend on his pleasure.
(e) "fecit", Pagninus, Montanus, Junius & Tremellius, Piscator, &c.

We are to praise and magnify God for the constant succession of day and night. And see how those are like to the wild beasts, who wait for the twilight, and have fellowship with the unfruitful works of darkness. Does God listen to the language of mere nature, even in ravenous creatures, and shall he not much more interpret favourably the language of grace in his own people, though weak and broken groanings which cannot be uttered? There is the work of every day, which is to be done in its day, which man must apply to every morning, and which he must continue in till evening; it will be time enough to rest when the night comes, in which no man can work. The psalmist wonders at the works of God. The works of art, the more closely they are looked upon, the more rough they appear; the works of nature appear more fine and exact. They are all made in wisdom, for they all answer the end they were designed to serve. Every spring is an emblem of the resurrection, when a new world rises, as it were, out of the ruins of the old one. But man alone lives beyond death. When the Lord takes away his breath, his soul enters on another state, and his body will be raised, either to glory or to misery. May the Lord send forth his Spirit, and new-create our souls to holiness.

The fifth decastich, in which the poet passes over from the third to the fourth day, shows that he has the order of the days of creation before his mind. The moon is mentioned first of all, because the poet wishes to make the picture of the day follow that of the night. He describes it in Psalm 104:19 as the calendarial principal star. מועדים are points and divisions of time (epochs), and the principal measurer of these for civil and ecclesiastical life is the moon (cf. Sir. 43:7, ἀπὸ σελήνης σημεῖον ἑορτῆς), just as the sun, knowing when he is to set, is the infallible measurer of the day. In Psalm 104:20 the description, which throughout is drawn in the presence of God in His honour, passes over into direct address: jussives (תּשׁת, ויהי) stand in the hypothetical protasis and in its apodosis (EW. 357, b). It depends upon God's willing only, and it is night, and the wakeful life of the wild beasts begins to be astir. The young lions then roar after their prey, and flagitaturi sunt a Deo cibum suum. The infinitive with Lamed is an elliptical expression of a conjugatio periphrastica (vid., on Habakkuk 1:17), and becomes a varying expression of the future in general in the later language in approximation to the Aramaic. The roar of the lions and their going forth in quest of prey is an asking of God which He Himself has implanted in their nature. With the rising of the sun the aspect of things becomes very different. שׁמשׁ is feminine here, where the poet drops the personification (cf. Psalm 19:1-14). The day which dawns with sunrise is the time for man. Both as to matter and style, Psalm 104:21 call to mind Job 24:5; Job 37:8; Job 38:40.

For seasons - To distinguish the times, the seasons of divers natural events, as of the ebbing and flowing of waters, and other seasons for sacred and civil affairs, which were commonly regulated by the moon.