The Beit Yosef (O.C. 670) famously raises the question of why Chanukah was instituted as an eight-day celebration, given that the first night that the menorah burned in the Beit Ha-mikdash was not miraculous. After all, the Gemara (Shabbat 21b) relates that the pure oil discovered by the Chashmonaim sufficed to sustain the candles for one night, and thus the miracle occurred for only seven days, not eight.
This question was already posed by the Tosafot Ha-Rosh (Masekhet Shabbat), who, interestingly enough, gave the same three famous answers proposed by the Beit Yosef. The first possibility suggested is that the kohanim in the Beit Ha-mikdash realized that it would take eight days before new oil could be produced and shipped to Jerusalem, and so from the outset they divided the small supply of pure oil into eight portions. They lit one portion each night, and thus even on the first night, a miracle occurred, as one-eighth of the amount of oil normally needed to kindle the menorah sufficed for the entire night.
Already the Peri Chadash (671) questioned this theory, noting the famous principle of “ein somkhin al ha-neis” – that we do not rely on miracles, neither in general, nor in our performance of mitzvot. Seemingly, the Peri Chadash asserts, it would have been improper for the kohanim to light the menorah with only one-eighth the amount of oil needed to fulfill the mitzvah, hoping that it would miraculously sustain the flames for the entire night. As we are expected to fulfill mitzvot within the constraints of the natural order, it would seem that the Chashmonaim should have used the available oil for the first night to at least satisfy that night’s requirement.
Rav Asher Weiss explores several possible approaches to answer this question. One possibility, he writes, is that the kohanim initially supplied one-eighth the normal amount of oil hoping for a miracle, but stood nearby ready to add more oil to the oil lamps if they saw that the supply was dwindling. If so, then they hoped for – but did not rely upon – a miracle, devising a strategy which would enable them to fulfill the mitzva all eight nights if a miracle occurred, while at least fulfilling the mitzva the first night if no miracle occurred.
However, as Rav Weiss notes, this theory works off the assumption that kindling the menorah in the Beit Ha-mikdash in this fashion is valid – meaning, that the required amount of oil did not need to be supplied at the outset, when the candles were lit. When it comes to Chanukah candles, for example, Halakha demands that already at the time of lighting, the candles must be capable of burning for the minimum required duration. As such, when the candles are lit, there must be enough oil to sustain the candles for the required period of time (Shulchan Arukh, O.C. 675:2). The question thus becomes whether this requirement applied also to the kindling of the menorah in the Beit Ha-mikdash.
Rav Weiss writes that this question would appear to hinge on the different interpretations of the Gemara’s discussion in Masekhet Menachot (89a) concerning the method by which the kohanim initially determined how much oil to place in the menorah’s lamps. According to one view cited by the Gemara, when the menorah was first used for lighting, the kohanim placed in the lamps before lighting an amount a revi’it of oil, and if this did not suffice, they would add more oil. In order not to risk wasting precious olive oil, they began with this small amount, and then added more if they saw that a revi’it did not suffice. (According to the other view in the Gemara, the opposite was done – the kohanim began with a larger amount, and then reduced the amount if they saw that the initial amount was more than necessary.) Rashi explains this to mean that if they saw during the night that the oil supply was being depleted, the kohanim added more oil to ensure the candles would burn throughout the night. If so, then indeed, it would have been acceptable for the Chashmonaim to initially kindle the menorah on the first night with just one-eighth the regular amount of oil, as the Rosh and Beit Yosef explain, and then add more oil later if they saw this was necessary.
However, in a different version of Rashi’s commentary to Masekhet Menachot (called “Rashi ketav yad,” which appears on the page in the standard editions of Masekhet Menachot), this comment is explained to mean that more oil was placed in the lamps the next night, if necessary. In other words, the kohanim would not add oil during the first night if they saw that a revi’it did not suffice, but would rather learn from this failed “experiment” that a higher amount of oil was needed. This view, apparently, felt that it would not have helped to add oil during the night, since the menorah needed to be lit with the required amount of oil already in place.
It would thus seem that the theory proposed by the Tosafot Ha-Rosh and the Beit Yosef would hinge on these different opinions as to whether the full supply of oil needed to be in place at the time of lighting the menorah.