The very first commandment given to Jews while they were still in Egypt was to create a calendar based on the cycle of the moon: "And G‑d said to Moses … in the land of Egypt … This month is for you, the head of the months; first it is for you among the months of the year.”1

The Midrash points out that the Hebrew words for “this month” are הַחֹדֶשׁ הַזֶה, lit., “this renewal.” In other words, G‑d showed Moses the moon in its time of renewal and said to him, “When the moon renews itself, you will have a new month.”2

However, it should be noted that the Jewish calendar is actually a luni-solar calendar.

Although the months of the year follow the lunar cycle, we are commanded to keep the holidays in their proper seasons. If we were to only go by the lunar cycle (as some cultures do), we would have a problem. The solar cycle is approximately 365 days, while the lunar year is approximately 354 days, so the lunar calendar would fall behind the solar calendar by 11 days each year, throwing off the holidays. For example, the holiday of Passover, which is meant to be celebrated in the spring, would eventually fall out in the winter.

To align our lunar calendar with the solar cycle and keep the holidays in the right seasons, we add a thirteenth month every few years to make a “leap year.” Thus, technically the Jewish calendar is considered a luni-solar calendar.

Having said that, we can now return to the question of why we mostly follow the lunar calendar.

Waxes and Wanes

Our sages tell us that the Jews count based on the moon and are compared to the moon.3

Unlike the sun, which gives off a steady and unchanging light, the moon waxes and wanes in cycles. So, too, throughout history we Jews wax and wane. We were redeemed from Egypt and received the Torah, but then sinned with the Golden Calf and the twelve spies. We entered the Land of Israel and built the Holy Temple, but it was destroyed and we went into exile. We rebuilt the Temple, only for it to be destroyed again. During the long exile itself, our standing has waxed and waned. The lesson is that whenever our life feels dark, we know that ultimately it will brighten up again, just like the moon.4

But there is a much deeper connection to the moon.

Humble and Majestic

The mystics compare the moon to the divine attribute of malchut (“royalty”). Somewhat counterintuitively, malchut is considered the last of the 10 attributes. Just as the spiritual "light" of malchut is only attained through humbling and nullifying itself to receive from the sefirot above it, so too the moon’s light is only a reflection of the light of the sun.

In a similar way, we are able to become a vessel for the divine light through humbling ourselves.

For more on this, see Moonlight Illuminations.

G‑d’s Sin-Offering

Regarding the creation of the sun and moon, the verse states: “And G‑d made the two great lights,” but then continues, “the greater light … and the lesser light.”5 So were there two great lights, or was just one light great?

The Talmud explains that at first they were indeed created equal. But then the moon complained to G‑d, “Sovereign of the Universe! Is it possible for two kings to wear one crown?”

G‑d answered: “Go then and make yourself smaller.”

“Sovereign of the Universe!” cried the moon, “Because I have suggested that which is proper, must I then make myself smaller?”

G‑d replied: “Go, and you will rule by day and by night.”

“But what is the value of this?” cried the moon. “Of what use is a lamp in broad daylight?”

G‑d replied: “Go, and Israel will reckon by you the days and the years.”6

But the moon was still not consoled.

Ultimately, G‑d consoled the moon by saying: “Bring an atonement for Me for diminishing the moon.”

This, the Talmud tells us, is the meaning of the following statement:

Rabbi Shimon ben Lakish said, “Why is the goat offering of the New Moon unique? Why does it say it is a sin offering for G‑d?7 Because the Holy One, blessed be He, said, ‘This goat will be an atonement for my having diminished the moon.’ ”8

If G‑d needed an atonement for His “sin” of diminishing the moon, then why did He do it? Or at the very least, why didn’t He change things back?

Because it was critical that G‑d diminish the moon. The purpose of creation is to make a dwelling place for G‑d in even the lowest and darkest of places. It is precisely when we transform a seemingly broken and diminished place that G‑d’s true greatness is revealed. This will be fully realized in the messanic era, when the moon will once again shine brightly.9

The Lubavitcher Rebbe explains that although one may be under the impression that the second half of the month, when the moon wanes, is a sign of diminished light and spirituality, on the contrary, it is actually a time of increased humility and self-abnegation. The waning of the moon is not a sign that we decrease in all other positive areas of growth. On the contrary, it is precisely when we reach our peak of perfection, the full moon in all of its brightness, that we are in need of the second half of the month, with its message of humility.

Furthermore, astronomically, when the moon is waxing, it is actually farther away from the sun, whose light it reflects. However, in the second part of the month, the reason the moon wanes and the light is diminished is that the moon is actually moving closer to the light’s source, the sun. In the first half of the lunar cycle we have more sunlight; in the second half we have more sun—the luminary itself!10

For more on this, see The Lunar Files.