Rabbi Slifkin’s final mistake pertains to what he claims Chazal held about the opacity, or lack thereof, of the celestial-rakia. He points to the Gemora in Pesachim 94a as proof that Chazal universally held that the celestial rakia has an opaque surface (in addition to being a solid dome), such that would block the sight of anything behind it, specifically the sun in its imagined travels. That this is erroneous—i.e., that not all Chazal held this, and perhaps none held this—can be plainly seen from a Midrash about Avraham Avinu with which Rashi has made us all familiar:
מדרש רבה בראשית פרשה מד
Said Rebbi Yehudah in the name of Rebbi Yochonon: [When Hashem wanted to impress Avraham with how numerous his descendents will be, he had him look at the stars—but not up at the stars.] He raised him above the כיפת הרקיע, the rakia’s sphere [or “dome,” and Avraham gazed down from there and saw the stars]. This is why it says “הבט נא השמימה”, “Gaze [down] at the heavens—for the term “הבט” connotes gazing from a higher vantage point towards something at a lower level.
Chazal (or at least those individuals among Chazal who promulgated this teaching) did not visualize the part of the rakia behind the stars as opaque, such that it would block our sight of anything on the other side of it (such as the stars, or the sight of the sun in its imagined course). In describing Avraham’s prophetic vision, when Hashem wanted him to see the uncountable number of stars, Chazal visualized Avraham as being positioned above the dome of the rakia—the same place that the Gemora Pesachim places the sun at night—and looking downwards to see the stars. Now, to look down from the back of the rakia and see the stars—and certainly to see more stars than what are visible from earth—requires visualizing the rakia as being transparent, not opaque. Indeed, if the back of the rakia were opaque, it would be the worst place from which to see the stars!
(Furthermore, we note that Avraham is not depicted—neither literally nor even poetically—as shattering anything on his way up through the rakia, and nor is Hashem depicted as “opening the heavenly spheres” for him to pass through them. This again indicates that Chazal visualized the rakia as a non-solid substance—meaning no more solid than, say, the atmosphere; and probably, as the rishonim took it, as even less solid.)
Secondly, Chazal understood that the moon’s light is only due to the sun’s. If at night the sun is behind an opaque surface that prevents us from seeing it, how can one account for the moon’s lit surface? And of course, there is the admission made by Rabbi Shimon bar Yochai that we simply do not know if the movement of the stars are indeed related to any celestial spheres. 
These questions would all be answered by accepting that at least some of Chazal, if not all of Chazal, did not think of the rakia as intrinsically opaque or solid. And indeed, we have seen that there are statements from Chazal indicating that at least some of them, if not all of them, thought of the rakia as non-solid and transparent. These facts certainly belie Rabbi Slifkin’s claim that Chazal had a universal mesorah that the earth is covered by a solid, opaque dome.
In the next posts, I will suggest that even according to the Gemora Pesachim that deals with the sun’s path at night, we need not say the reason the sun is unseen at night is due to any opacity of the rakia.
 Even if they would have held that the celestial sphere is opaque, they may have envisioned it as a some substance such as a thick cloud (dark or otherwise) that blocks out the sun—not at all necessarily a solid, impenetrable material. Regardless, all this investigation would be irrelevant if not for the assertion (unwarranted, not only in my opinion, but obviously in that of the rishonim) that since the rakia is something mentioned in the Torah, whatever Chazal said about it must have been their mesorah of what the word meant. The fact is, Rambam and many others deny that the sayings of Chazal that touch upon the celestial region’s makeup were matters of mesorah (facts transmitted by the prophets), which would render them incontrovertible.
 This teaching was also reported by Rebbi Yehuda bar Rebbi Seemone in the name of Rebbi Chanin (Sh’mos Rabbah 38), Rebbi Shmuel Bar R. Yitzchak (Bamidbar Rabbah 2) and Rebbi Levi (Yalku Shimoni, Breishis 18: § 76.
 We are consistently following the Rambam’s “rationalist” perspective, which includes the view that all this episode was a prophetic vision, and not a physical occurrence.
 מדרש אגדה בראשית פרק א ד"ה [טז] ויעש אלהים
וגזר עליה שלא תהא מאירה אלא מכח השמש.ס
Midrash Aggada Breishis on Breishis 1:16
Hashem decreed that the moon could only shine through the power of the sun
זהר במדבר קפא
דהא לא אנהיר סיהרא אלא מנהורא דשמשא.ס
Zohar on B’Midbar 181
For behold, the moon only shines through the shining of the sun
Also, see Philo: (91) … it is on the fifteenth day that the moon is rendered full of light, borrowing its light from the sun at the approach of evening, and restoring it to him again in the morning [if meant literally, not quite our take—ZL] so that during the night of the full moon the darkness is scarcely visible, but it is all light.
 בראשית רבה ו:ח
"ויתן אותם אלהים ברקיע השמים." כיצד גלגל חמה ולבנה שוקעים ברקיע? ר' יהודה בר אלעאי ורבנן. רבנן אמרין מאחורי הכיפה ולמטה. ור' יהודה בר אלעאי אמר מאחורי הכיפה ולמעלה. א"ר יוחנן נראין דברי ר"י בר אלעאי--דהוא אמר מאחורי הכיפה ולמעלה--בימות החמה, שכל העולם כולו רותח ומעיינות צוננין. ומילהון דרבנן--דאמרינן מאחורי הכיפה ולמטה--בימות הגשמים, שכל העולם כולו צונן ומעיינות פושרין. אמר רבי שמעון בן יוחאי אין אנו יודעין אם פורחין הן באויר ואם שפין ברקיע ואם מהלכין הן כדרכן הדבר קשה מאד ואי אפשר לבריות לעמוד עליו
Rabbi Shimon bar Yochai said, We do not know if they fly in the air, if they glide in the rakia,, or if they go on their own way; the matter is exceedingly difficult and it is impossible for humans to determine.