Shalom Josh,

At first glance your point is very reasonable. Chevron is south, and somewhat west, of Yerushalayim, so if the Mishnah were measuring the very first appearance of dawn in the east, mentioning Chevron would seem later, not earlier.

However, the Rishonim understand the phrase "until Chevron" in different ways.

The Yerushalmi, brought by Rashi, explains that Chevron was mentioned in order to recall the merit of the Avos, the "Yeshenei Chevron." According to this approach, the primary check was that the eastern horizon was illuminated, and the mention of Chevron carries an added spiritual dimension.

The Me'iri explains that the requirement of "until Chevron" represents a stronger and clearer light than the simple illumination of the east mentioned by the Tana Kama. After an incident in which the light of the moon was mistaken for dawn, they required a level of daylight strong enough that one could see by that light as far as Chevron. The Me'iri then cites the Yerushalmi that Chevron was chosen specifically in order to recall the merit of the Avos. According to this, Chevron serves both as a practical measure of the intensity of the light and as a symbolic reference.

The Ritva emphasizes the practical side even more. When the light had spread to the point that it was visible even toward Chevron, the dawn was already clearly established. In that sense, your question actually sharpens the point. They did not rely on a small glimmer in the east, which could easily be confused with moonlight, but required a broader and stronger light, extending beyond the immediate eastern horizon.

So the mention of Chevron is not about where the sun first rises. It either reflects the merit of the Avos, or it serves as a test of the strength and spread of the daylight, or both.

I hope this helps,

Aharon Steiner