Rabbi Yitzchak raised a sharp astronomical objection to a proposed method of calculating the calendar. If you followed a certain interpretation, he argued, the moon would already be in the middle of the sky at the start of the new month — which would be observationally absurd. The new month is supposed to begin when the moon is barely visible as a thin crescent, not when it is already high overhead.
This forced the discussion back to the original, more sound formulation: a month is intercalated (extended), and a year is intercalated. The principle is that just as days are added to a month only at its end, so too is a month added to a year only at its end. You do not insert extra time in the middle of a period — you append it.
This seemingly technical debate reveals something profound about how the rabbis approached the Jewish calendar. They treated it not as an arbitrary human convention but as a system with internal logic that had to be self-consistent. If a proposed rule produced an absurd astronomical result, the rule had to be wrong. Observable reality served as a check on legal interpretation.
Rabbi Yitzchak's intervention also demonstrates the intellectual rigor of Mekhilta discussions. A single empirical observation — "the moon would be in the wrong position" — was enough to overturn an entire line of reasoning and send the rabbis back to first principles. Theory had to match what you could see in the sky.