Tamid, Chapter 3, Mishnah 2. Our chapter follows the kohanim in sequence, moving them out of the drawing of lots and into the hands-on labor of the early service. We have now reached the point where the morning's daily offering, the Tamid shel shachar, is about to be brought.
The Tamid was brought twice each day, once in the morning and once toward the end of the afternoon. One condition governs both: it may be offered only in the daytime. The pasuk speaks of "beyom zivchachem," on the day of your offering, teaching that the avodah of this korban belongs to the day and not to the night. The kohanim therefore could not simply assume that day had come. They had to establish it.
What counts as day
Day, for this purpose, begins at dawn. Once the first rays of light have entered the atmosphere, it is already considered daytime, even though the sun itself has not yet been seen rising over the horizon. The kohanim were looking for that earliest light, not for sunrise.
The appointed kohen sends a lookout
In the mishnah's words, "Amar lahem hamemuneh": the memuneh, the officer charged with directing the service, addressed the kohanim. His instruction was "Se'u ur'u im higia zeman hashechita": ascend and check whether the hour of shechitah had come. One kohen would then climb to an elevated spot within the Beit Hamikdash, a place from which the horizon in the east lay open to view.
"Im higia, haro'eh omer: barkai." Should the moment indeed have arrived, the watchman would call down a single word, "Barkai," whose sense is brightness: light is here. He kept his eyes fixed eastward until the first shine broke over the horizon, and at that point he proclaimed that dawn had come.
Matya ben Shemuel's higher standard
"Matya ben Shemuel omer: he'ir penei hamizrach." Matya ben Shemuel, who was one of the appointed officials of the Beit Hamikdash during the Second Temple era, records the call in a different formulation: the lookout announced that the eastern face of the sky had grown bright. According to him, a first shine breaking in from that direction does not yet permit the shechitah of the Tamid. The kohen on watch had to hold off until light had spread across the whole eastern expanse, and only at that stage could he report back down.
All the way to Chevron
The kohanim standing below would press him further, as the mishnah continues, "Ad shehu beChevron": has the brightness reached as far as Chevron, a town lying at a genuine distance from Yerushalayim? "Vehu omer: hen", and the watchman confirmed it: yes.
And why single out Chevron? Chevron carries the title Ir Ha'avot, for it is there that the Avot lie buried. Invoking that name precisely as the day's service was about to begin wove the Avot into what was unfolding in the Beit Hamikdash, joining them, in a sense, to the avodah of that day.