The Torah tells us the sun, moon, and stars are for "signs and seasons, days and years." Targum Pseudo-Jonathan on (Genesis 1:14) lets this sentence breathe. The luminaries, in the Targumist's reading, were made for festival times, "for the sanctifying of the beginning of months, and the beginning of years," the passing of months, the revolutions of the sun, the birth of the moon, and the revolvings of seasons.
This is not astronomy. It is liturgy. The sky is a Jewish calendar.
Rosh Chodesh, Pesach, Sukkot, Shabbat — every sacred moment in Jewish life is keyed to the movement of these lights. Before there is a people to keep the calendar, the calendar is already up there, spinning. The Targumist is quietly insisting that holy time was created first, before any human being could mark it. When Israel later counts new moons and consecrates the months, they are not inventing a practice. They are reading the clock God hung on day four.