According to most Rishonim the menorah in the mikdash was lit once a day towards evening and burned all night. According to these views, the Mishna's din (Menachos 49) that chinuch of the menorah, the first lighting that inaugurates the use of the menorah, must be done in the evening makes perfect sense -- this is the proper time at which the mitzvah of lighting the menorah is fulfilled.
The Rambam, however, writes (Temidim 3:10-12)
דישון המנורה, והטבת נרותיה בבוקר
ובין הערביים--מצות עשה
מה הוא דישון המנורה: כל נר שכבה--מסיר
הפתילה וכל השמן שבנר, ומקנחו, ונותן בו פתילה
אחרת ושמן אחר במידה, שהוא חצי לוג; וזה שהסיר,
משליכו במקום הדשן אצל המזבח, עם דישון המזבח
הפנימי והחיצון; ומדליק נר שכבה. והדלקת הנרות,
According to the Rambam the mitzvah
done with the menorah is called hatavah, preparing for lighting, and it
must be fulfilled twice a day, both in the morning and in the evening,
not just once. "Hadlakas hemenorah hi hatavasm," lighting the menorah is
not an independent mitzvah, but is part and parcel of this mitzvah of hatavah.
Since according to the Rambam the mitzvah
of hatavah (of which hadlakah is just one detail) must be performed in the
morning as well as the evening, why can chinuch of the menorah not
be done in the morning as well?
As we discussed once before, the Rambam
does not count the construction of the klei hamikdash as separate mitzvos
because he assumes they are all details of the larger mitzvah of building
a mikdash. Making a menorah is a kiyum of the mitzvah of making a mikdash. How do you make a menorah? The Kozhiglover suggests that a menorah that does not
provide light is not a functional menorah. It's only a lit menorah which fulfills this mitzvah of making a mikdash. Hadlakas menorah thus accomplishes a dual purpose: 1) it is a kiyum of the mitzvah of hatavah; 2) it is a kiyum of the mitzvah of binyan mikdash.
The Rambam agrees with the majority
view of the Rishonim that lighting the menorah qua the mitzvah of hatavah
can be fulfilled only toward evening. Therefore the chinuch of the menorah
can only be done at the this time. The reason the Rambam writes that
the menorah must be lit in the morning is to fulfill the miztvah of lighting
qua binyan mikdash, because a menorah that does not illuminate is a lack in the mitzvah of binyan mikdash.