Reference for Pesachim 187:7

לימא תיהוי תיובתא דר' יוחנן אמר לך אנא ביממא הוא דאמרי ורבנן הוא דקא טעו דקא חשבן דקדמא וחשוכא

The proo is this: what is an [average] man's journey in a day? Ten parasangs, and from daybreak until the rising sun four mils, [and] from sunset until the stars appear four mils,: hence the thickness of the sky is one tenth of the day['s journey].<span class="x" onmousemove="('comment',' The one tenth is of the inclusive figure, i.e., four in forty, whereas one sixth mentioned before was exclusive: six in thirty. But in any case they disagree.');">⁸</span>

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