To get back to our Chanukah vs. shabbos issue (see previous posts), I want to expand on what Bluke posted because I think I have a good answer to a kashe asked by HaRav Shlomo Fischer. The gemara tells us the shiur Chanukah candles must burn for is from nightfall until the time the marketplace clears, which in the days of Chazal was about a half-hour after dark (R’ Soloveitchik noted that today the streets are filled with people until much later and therefore candles should burn later). The Rashba writes that one need not light the menorah during this time; one can light it earlier and have it burn until after dark (as previously discussed), but one can also light later if one missed the time and still gain some mitzvah. The Mishna tells us that mitzvos that must be done at night can be performed all night, and Chanukah candles are no exception. The Rambam, however, holds that if the time is missed and it is passed the time the marketplace is clear, the candles should not be lit.
The issue being debated becomes clearer in light of the Rambam’s formulation of the mitzvah of lighting menorah (and here I am going beyond Bluke a bit). In Hil Chanukah (3:3) the Rambam places the obligation of lighting neiros Chanukah in the context of the obligation to recite hallel on Chanukah; the Rambam also adds superfluously that the Chanukah candles are obligatory “just like reading the megillah on Purim”. R’ Soloveitchik explained (cited in Moadei HaRav) that the Rambam’s choice of words and placement is deliberate. Chazal tell us that no hallel is recited on Purim because “keriysa zu hi haleila”, reading the Megilah is itself a fulfillment of hallel. The Rambam categorizes lighting menorah also as a fulfillment of hallel and pirsumei nisa; its role is to publicize the miracle and declare, just as we do in hallel and via reading the purim megillah, our thanks to Hashem.
According to the Rashba, the mitzvah of lighting the menorah and the mitzvah of pirsumei nisa are distinct kiyumim. One may light early or late because one can fulfill hadlaka even in an absence of pirsumei nisa. The Rambam disagrees and sees both fulfillments as intertwined and inseparable.
The Rambam’s opinion (which the GR”A quotes) is that the menorah is lit immediately after shkiya, even before actual nightfall. Why, asked R’ Soloveitchik, can we fulfill the mitzvah of menorah even before dark, during bein hashemashos, while we must wait until actual nightfall to do other mitzvos that must be done only at night, such as eating matzah or sitting in sukkah? R’ Soloveitchik explained that the Rambam does not define lighting menorah as a mitzvah of night per se. The halacha simply mandates that one light Chanukah lights – since the rule is sharaga b’tihara mai ahanei, a candle lit during the day (when it is light out) is functionally useless, we have no choice but to wait until is begins to get dark to light our menorah. The shiur does not define the time of one’s obligation (chovas hagavra) as occurring only at night (like matzah or sukkah); rather, the shiur of shkiya defines for us what defines a ner (cheftza).
The Rambam (Temidim 3:10) famously writes that in the Bais HaMikdash the menorah was not just kindled during the evening (as most rishonim hold), but was lit during the morning as well. R’ Shlomo Fischer shlit”a asks – if the gemara uses the shiur of shkiya to define a ner, how can the Rambam possibly hold that there is an obligation to light the menorah during the day? How does one have a cheftza of a ner during that time?
I think perhaps the answer relates to the dual nature of the mitzvah of ner Chanukah. The shiur of shkiya is a definition of ner only with respect to its function as a cheftza shel mitzvah for the mitzvah of pirsumei nisa, which is fulfilled by the Chanukah candles. But with respect to the simple act of hadlakah, which is all that was required in Mikdash, even a candle lit during the day serves as a kiyum mitzvah.