The moon as contracted attainable light

איתא ישראל מונין ללבנה.

It is taught: Yisrael count by the moon.

The Sefas Emes notes that Bnei Yisrael reckon their calendar by the lunar cycle, which carries deep meaning.

והוא המאור הקטן.

And it is the lesser luminary.

The moon is called the "lesser light," diminished and reflecting rather than producing its own brilliance.

פי' שכביכול צמצם הקב"ה אורו להיות יכולין בנ"י להשיג השגת משהו.

The meaning is that, as it were, HaKadosh Baruch Hu contracted (tzimtzem) His light so that Bnei Yisrael could grasp some small comprehension.

The "lesser light" represents a tzimtzum — Hashem's deliberate dimming of His infinite light so that limited human beings can attain at least a measure of perception.

ובמד' הירח אינו נברא להאיר רק למועדים.

And in the Midrash: the moon was not created to give light, but only for the appointed times (mo'adim).

The moon's purpose is not primarily illumination but the fixing of the festivals and sacred times.

כמ"ש ירח עשה למועדים.

As it is written: "He made the moon for the appointed times" (Tehillim 104:19).

The pasuk confirms that the moon's function is to mark the mo'adim.

זה השגת הדעת.

This is the attainment of daas (knowledge).

The moon's measured, attainable light corresponds to the kind of daas — graspable divine awareness — that Bnei Yisrael can actually reach.

וזה החודש הזה לכם בשבילכם.

And this is "This month is for you" (Shemos 12:2) — for your sake.

The first mitzvah of sanctifying the new moon was given "for you" — it is a gift placed within Bnei Yisrael's reach and power.

דאיתא כזה ראה וקדש.

For it is taught: "Like this, see and sanctify."

Hashem showed Moshe the form of the moon and said that Bnei Yisrael are to see it and sanctify the month accordingly — the perception is theirs to make.

א"כ יכולין בנ"י לראות ולהשיג זה ההתחדשות של הלבנה.

If so, Bnei Yisrael are able to see and to grasp this renewal (hischadshus) of the moon.

Because the light is contracted to their level, Bnei Yisrael can perceive and take hold of the moon's constant renewal — the capacity for spiritual renewal.

ובני ישראל זכו לזה ביציאת מצרים:

And Bnei Yisrael merited this at yetzias Mitzrayim (the exodus from Egypt).

This power of renewal — to grasp the contracted divine light and constantly begin anew — was granted to Bnei Yisrael at the redemption from Mitzrayim, which is why the mitzvah of the new moon was their very first.

Summary: Bnei Yisrael count by the moon because the "lesser luminary" represents Hashem's tzimtzum — His contraction of infinite light into a measure that limited human beings can actually grasp. The moon exists "for the appointed times," embodying attainable daas. Through the first mitzvah, "This month is for you," Bnei Yisrael were given the power, merited at yetzias Mitzrayim, to perceive and take hold of the moon's constant renewal — the capacity for ongoing spiritual hischadshus.