Commentary for Pesachim 187:6
מיתיבי רבי יהודה אומר עוביו של רקיע אחד מעשרה ביום תדע כמה מהלך אדם בינוני ביום עשר פרסאות ומעלות השחר עד הנץ החמה ארבעת מילין משקיעת החמה ועד צאת הכוכבים ארבעת מילין נמצאת עוביו של רקיע אחד מעשרה ביום תיובתא דרבא תיובתא דעולא תיובתא
Ten parasangs: from daybreak until the first sparklings of the rising sun five mils, and from sunset until the stars appear five mils: hence the thickness of the heaven is one sixth of the day['s journey].<span class="x" onmousemove="('comment',' The periods from daybreak until the rising sun is in the heavens, and again from sunset until the stars appear, were regarded as the time during which the sun was passing through the sky, which was conceived as a solid vault stretched out above the earth. Hence it follows from Rabbah's dictum that since five mils can be walked in each of these two periods, while thirty mils can be walked during the day excluding these periods (ten parasangs=forty mils) , the thickness of the sky is one sixth of the world's diameter.');"><sup>7</sup></span> An objection is raised: Rab Judah said: The thickness of the sky is one tenth of the day's journey.