In this answer I cited R. Yechiel Michel Epstein's explanation for rejecting R. Yaakov of Karlin's suggestion that the times for Shema are not based on sha'ot zemaniot. In truth, however, I don't really see how his explanation addresses the issues. Here is the quote reproduced:
מיהו יש בזה שאלה דנהי דודאי לכל דיני התורה כן הוא מ"מ לענין ק"ש דכתיב בה ובשכבך ובקומך הלא השכיבה והקימה לא ישתנו בהתארך היום או יתקצר כלומר כשהיום והלילה שוים הוי זמן ק"ש עד אחר שעה תשיעית דהם ג' שעות על היום משום דבני מלכים ישנים עד ג' שעות וא"כ אטו בתקופת תמוז שרביע היום הוא בחצי שעה שמינית יקומו אז הבני מלכים הרי הם ישינים עד אחר שעה התשיעית וא"כ ליהוי זמן ק"ש תמיד עד אחר שעה תשיעית שהרי קרינן אז ובקומך וראיתי לאחד מן הגדולים שעלה על דעתו לומר כן [משכנות יעקב סי' ע"ו וכתב אלולי דמסתפינא ע"ש] אמנם באמת אין זה שאלה כלל ודיני התורה לא ישתנו לעולם כלומר לראות לפי מנהג המקום והדברים התלוים ביום ובלילה צותה לנו התורה שכל אחד במקום שהוא כשאצלו יום ינהוג דיני יום אף שבמקום אחר הוי לילה וכשאצלו לילה ינהוג דיני לילה אף שבמקום אחר עדיין יום ועל זה נאמר שבת היא לה' בכל משבתיכם כלומר תקבלו השבת בכל מקום שאתם יושבים שכשבמקומכם יום הששי הגיע הלילה אצליכם שבת אף שבמקום אחר עדיין הוא יום ששי ולהיפך בשבת בערב כשהוא לילה אצליכם חול אף שבמקום אחר עדיין הוא שבת וכן גבי ק"ש שהתורה אמרה ובשכבך ובקומך אינו תלוי בכל אדם מתי שישכב ומתי שיקום אלא זמן שכיבה לרוב העולם וזמן קימה לרוב העולם ושיערו חז"ל שבזמן הנץ רוב העולם קמים ממטתם ויש שקמים מעמוד השחר ובני מלכים ישנים עד ג' שעות שהוא רביעית היום ואין משגיחים על מה שיש כמה ערים שבנץ החמה עדיין כולן ישינים וכיוצא בזה ולפ"ז הוי כאלו התורה אמרה שבנץ החמה תקרא ק"ש ועד רביע היום הוה זמן ק"ש ואז ממילא דבכל מקום ומקום לפי מה שעומד בכדור הארץ כששם תנץ החמה הוה זמן ק"ש וכששם יעבור רביע היום עברה זמן ק"ש כמו בשבת ולא משגחינן אם שם כבר עמדו או עדיין ישנים דלא לפי מנהג בני אדם ניתנה התורה אלא גזירה היא כך וכך תעשה ביום ובלילה וממילא שכל אחד מתי שהוא אצלו יום הוה מצות היום עליו ובלילה מצות הלילה עליו
However, there is a question with this: Granted that this is certainly true with regard to all the laws of the Torah, nevertheless regarding Keriat Shema where it is written "when you lie down and when you get up", the [times of] lying down and getting up don't change with the lengthening or shortening of the day. That is to say that when the day and night are equal [i.e. twelve hours each] the time for Keriat Shema would be until after 9:00 which is three hours into the day, for the children of kings sleep until three hours into the day. If so, during the summer when a quarter of the day is 8:30, do the sons of kings get up [at 8:30]? They still sleep until after 9:00! So the time for Kerias Shema should always be until after 9:00, because that is when it is called "when you get up". And I saw one of the great [rabbis] who thought to say this (Mishkenot Yaakov # 76, and he wrote "if I was not afraid", see there).
However, in truth this is not a question at all, and the laws of the Torah never change, meaning, to look at the practice of the locale. For the things that are dependent on day and night the Torah commanded us that each individual — in the place where he is — when it is day for him he should observe the laws of day even though elsewhere it is night. And when it is night for him he should observe the laws of night even though elsewhere it is still day. And about this it was said "it is Shabbat for God in all of your dwelling-places", meaning that you should accept Shabbat in any place where you are dwelling such that when in your place on Friday the night arrives it is Shabbat for you even though elsewhere it is still Friday. And conversely on Saturday in the evening when it is night, for you it is non-Shabbat even though elsewhere it is still Shabbat.
And so too by Kerias Shema, where the Torah said "when you lie down and when you get up", it is not dependent on when the individual person lies down and gets up. Rather, it is [based on] the time of lying down for the majority of the world, and the time of getting up for the majority of the world. And the Sages assessed that at the time of sunrise most of the world gets up from their beds, and there are some who get up at dawn, and the sons of kings sleep until three hours [into the day] which is a quarter of the day. And we don't care that there are many cities where everyone is still sleeping at sunrise, or the like.
Accordingly, it is as if the Torah said that sunrise is when you should read Shema, and until a quarter of the day is the time for Keriat Shema. And then automatically, in each place depending on where it is located on the globe, when the sun rises there it is the time for Keriat Shema, and when a quarter of the day passes there the time for Keriat Shema passes, just like by Shabbat. And we don't care if there they already got up or are still sleeping because the Torah was not given according to the practice of the people; it is simply a decree [that] such-and-such you do by day or by night. And automatically whenever it is day for each individual the day mitzvot are upon him, and at night the night mitzvot are upon him.
What I don't understand is how any of this relates to sha'ot zemaniot. The suggestion was that since the Torah specified the time for Shema in terms of getting up from bed, the time for Shema should be fixed throughout the year since the sons of kings don't modify their sleeping habits based on how long the sha'ot zemaniot of a given day are. That doesn't seem to have anything to do with whether laws can change, or whether they can be different for different individuals. R. Yaakov of Karlin agrees that Shabbat is determined by when night begins in your locale. But that's because Shabbat is simply a day; whenever the day begins (starting with the night) Shabbat begins. Had the Torah said "read Shema at night and by day" then the times for Shema would also fluctuate over the year when the times for day and night fluctuate.
So how exactly did R. Yechiel Michel Epstein refute R. Yaakov of Karlin's suggestion? Furthermore, R. Epstein argues that we only look at the majority and we don't care about the fact that other groups of people might be getting up later. But isn't this explicitly contradicted by the very Talmudic passage under discussion? The whole debate stems from the fact that the time for Shema ends when the sons of kings get up, because as long as there is a (significant) group of people not getting up it can still be called "a time of getting up". But surely the sons of princes do not constitute the majority of the population! In fact, he said himself that most of the world gets up at sunrise.
How, then, did R. Epstein reach the conclusion that the time for Shema is always fixed as the time between sunrise and the end of the third solar hour, and it's as if the Torah said "read Shema between sunrise and a quarter of the solar day"? That doesn't seem to follow from what was said. If everyone would get up at noon, how would we know that the Torah meant sunrise?
