The sun shall not smite thee by day. By these forms of expression the Psalmist magnifies the advantages which result to us from our having God present with us; and, by the figure synecdoche, under one particular, he declares in general that the faithful shall be safe from all adversities, defended as they are by Divine power. The language is metaphorical, the cold of night and the heat of day denoting all kind of inconveniences. The sense then is, that although God's people may be subject in common with others to the miseries of human life, yet his shadow is always at their side to shield them from thereby receiving any harm. The Prophet does not, however, promise the faithful a condition of such felicity and comfort as implies an exemption from all trouble; he only, for the purpose of assuaging their sorrows, sets before them this consolation -- that being interested in the Divine layout, they shall be secure from all deadly harm; a point which he unfolds more distinctly in the following verses, where he tells us that God will so keep his own people from all evils, as to maintain their life in safety. The statement in the text before us is indeed general, but he afterwards specifies the chief parts of human life.

The sun shall not smite thee by day - The Septuagint renders this, "shall not burn thee" - συγκαύσει sungkausei. So the Latin Vulgate. The Hebrew word means to smite, to strike, as with a rod or staff, or with the plague or pestilence; and then, to kill, to slay. The allusion here is to what is now called a "sun-stroke" - the effect of the burning sun on the brain. Such effects of the sun are often fatal now, as doubtless they were in the time of the psalmist.
Nor the moon by night - The psalmist here refers to some prevalent opinion about the influence of the moon, as endangering life or health. Some have supposed that he refers to the sudden cold which follows the intense heat of the day in Oriental countries, and which, because the moon rules the night, as the sun does the day, is either poetically or literally attributed to the moon. Lackmann and Michaelis suppose that there is some allusion to the influence of the moon in producing various kinds of disease, and especially lunacy - an idea which gave origin to that name. Compare the notes at Matthew 4:24. See Matthew 17:15; Mark 9:17; Luke 9:39. Knapp supposes the idea is, that from the moon's not giving a clear and full light like the sun, travelers trusting to its guidance may be led into rivers or quagmires. Macrobius refers to a custom among the Orientals of covering the faces of children when asleep, from some imagined effect of the moon on the health of the child. Andersen (Orient. Reise-Beschreib. i. 8) refers to an effect, which he says is common, and which he had often seen, of sleeping in the moon-beams, of making the neck stiff, so that it could not be turned from side to side as before. See Rosenmuller, Morgenland, in loc. Others have supposed that the allusion is to the effect of the moon, and of sleeping under the open air, in producing ophthalmia - a disease very common in the East - an effect guarded against by covering the face. The influence of the moon, in producing madness or disease - the general influence of it on health - is often referred to. Thus Shakespeare says:
"The moon, the governess of floods,
Pale in her anger, washes all the air,
That rheumatic diseases do abound."
Midsummer Night's Dream, ii. 2.
"It is the very error of the moon;
She comes more near the earth than she was wont,
And makes men mad."
Othello, v. 2.
Some of these things are evidently purely imaginary. The true idea seems to be that there were effects to be dreaded from the sudden changes from the heat of day to the cold of night, and that these effects were attributed to the moon. See Genesis 31:40. The meaning is, that God would be a Protector alike in the dangers of the day and of the night.

The sun shall not smite thee by day - Thus expressed by the Chaldee: "The morning spectres shall not smite thee by day, during the government of the sun; nor the nocturnal spectres by night, during the government of the moon." I believe the psalmist simply means, they shall not be injured by heat nor cold; by a sun-stroke by day, nor a frost-bite by night.

The sun shall not (c) smite thee by day, nor the moon by night.
(c) Neither heat nor cold, nor any inconvenience will be able to destroy God's Church, even though for a time they may molest it.

The sun shall not smite thee by day,.... With its rays, which it shoots forth like darts, and which fly swiftly, and pierce and hurt: hence Apollo, the same with the sun, is represented with a bow and arrows (o); so the rays of the sun seem to be called in Habakkuk 2:11;
nor the moon by night; this clause should be supplied, as a learned man (p) observes, thus, "neither shall the moon cool thee by night"; for that has no warmth in it, and cannot smite with heat, as the sun does: for even, as he observes, its rays focused by a magnifying glass will not communicate the least degree of sensible heat to bodies objected thereunto; yet some say (q) the moon is not only moist, but heats bodies as the sun. And Isaac Vossius (r) observes, that there can be no light, which, separately considered, does not contain some heat at least: and Macrobius (s) speaks of the lunar heat; and Plutarch (t) ascribes heat and inflammation to it, and asserts it to be fire. It is said (u) that some men of good credit, in a voyage to Guinea, strongly affirmed, that, in the night season, they felt a sensible heat to come from the beams of the moon. The Septuagint version is, "the sun shall not burn thee by day, nor the moon by night". And burning may be ascribed to the cold frosty air in a moonlight night, as to the north wind, as in the Apocrypha:
"20 When the cold north wind bloweth, and the water is congealed into ice, it abideth upon every gathering together of water, and clotheth the water as with a breastplate. 21 It devoureth the mountains, and burneth the wilderness, and consumeth the grass as fire.'' (Sirach 43)
see Genesis 31:40; and our English poet (w) expresses a sentiment to this effect; yet not what affects the bodies of men, but plants, trees, &c. and this not owing to the moon, but to the air. However, these clauses are not to be understood literally; for good men may be smitten and hurt by the heat of the one and the cold of the other, as Jacob and Jonah, Genesis 31:40; but mystically, of persecuting antichristian tyrants, which are sometimes signified by the sun and moon, as both in Rome Pagan and Papal, Revelation 6:12; and of persecution and tribulation itself, Matthew 13:6; and is sometimes applied to the perfect state of the saints, either in the New Jerusalem, or ultimate glory, when there will be nothing more of this kind, Revelation 7:15. And there are some periods in the present state, when those entirely cease; nor are the saints ever really hurt by them, they being always for their good; or, however, not so as to affect their eternal happiness. The Targum is,
"in the day, when the sun rules, the morning spirits shall not smite thee; nor the nocturnal ones in the night, when the moon rules.''
(o) Macrob. Saturnal. l. 1. c. 17. (p) Scheuchzer. Physic. Sacr. p. 976, 977. (q) Suidas in voce so Theodoret. (r) De Motu Marium & Vent. c. 6. Vid. Senecae Nat. Quaest. l. 5. c. 9. (s) Saturnal. l. 7. c. 16. (t) De Facie Lunae, in tom. 2. p. 933. (u) The Second Voyage in Eden's Travels, p. 350. 2. (w) "----The parching air----Burns frore (frosty) and cold performs the effect of fire". Milton's Paradise Lost, l. 2. v. 594.

God keeps His people at all times and in all perils.
nor the moon by night--poetically represents the dangers of the night, over which the moon presides (Genesis 1:16).

Smite - With excessive heat. Moon - With that cold and moisture which come into the air by it. Intemperate heats and colds are the springs of many diseases.