Psalms 121:6

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.