תהלים, פרק ק״ד, פסוק י״ט

The celestial bodies in the sky serve a much greater purpose than simply providing light. They form the very foundation of time, establishing the natural order and the rhythms of all life. In the story of creation, the moon is introduced first because the night naturally precedes the day [רד״ק].

God established the moon specifically to measure time. The primary approach among commentators is that this refers to calculating the calendar. The monthly renewal of the moon dictates the timing of the new months and sets the schedule for God's appointed festivals [רש״י, מצודת דוד, אלשיך, מאירי]. However, another perspective suggests that the focus is on the natural world rather than religious holidays. In this view, the moon's cycles, along with the sun's tilt, guide the natural seasons. These celestial movements directly affect the climate, agriculture, and the biological processes of plants and animals [אבן עזרא, רד״ק], allowing people to measure out their own units of time [ביאור שטיינזלץ].

In contrast to the moon, the sun operates with absolute predictability. While the moon's movement is complex and its appearance constantly changes, sometimes appearing early and sometimes late with varying paths, the sun follows a strictly fixed, stable, and clear course [רש״י, אלשיך, אבן עזרא, מצודת דוד, מאירי, ביאור שטיינזלץ, תורה תמימה]. The sun instinctively knows its exact path and the precise moment it must set, driving the steady annual cycle of summer and winter [אלשיך].

Another layer of meaning focuses on the source of their light. The moon is essentially a dark body with no light of its own, serving only to receive and reflect the sun's rays. The sun possesses its own independent light, while the moon relies on it entirely [רד״ק, מאירי]. Because of this relationship, the setting of the sun and the rising of the moon are perfectly linked. The sun knows that the exact moment it sets is the precise time its light will allow the moon to appear and brighten the night [מלבי״ם, רד״ק].