Relative and Standard Hours — Eruvin 56a
תלמוד בבלי מסכת עירובין דף נו/א
תקופת ניסן ותקופת תשרי חמה יוצאה בחצי מזרח ושוקעת בחצי מערב שנאמר הולך אל דרום וסובב אל צפון הולך אל דרום ביום וסובב אל צפון בלילה סובב סובב הולך הרוח אלו פני מזרח ופני מערב פעמים מהלכתן ופעמים מסבבתן
We are accustomed to assuming that hours of the day, for purposes of Halacha are relative hours (sha'os zemanios) — viz., the day is divided into twelve equal parts, from dawn to dusk according to the Magen Avraham, and from sunrise to sunset according to the Gra and Baal HaTanya. Hence, a sha'ah zemanis in
It ain't necessarily so.
In the first place, the Pnei Yehoshua in the Kunteres Acharon to Berachos goes so far as to say that sha'os zemanios were an invention of the Rambam!
Secondly, the Meshech Chochmah (Bamidbar 25:23) derives that the Halachic hours of the day are standard, 60 minute hours (sha'os shavos) from our Gemara.
He asks why the Torah never uses the term olas ha'boker except in regard to Pesach ("milvad olas ha'boker asher l'olas ha'tamid). He explains that according to the Zohar (Vayakhel 195b; see also R' Tzadok HaKohen, Sefer HaZichronos, Mitzvas Kiddush HaChodesh 3:2 at length) the hours of the Halachic days are indeed standard hours. Thus, zman tefillah is always local standard time, and that is considered the end of the Halachic morning.
Now, as our Gemara notes, it is only in Nisan and Tishrei (equinox days) that the actual night and actual day are equal and the relative hours correspond to the standard hours. Thus, it is only at these times that the four-hour time-frame of the morning tamid is congruent with the actual morning. On the other hand, in the middle of the summer, for example, the actual day might begin with a sunrise, and its end (even if measured in relative hours) is well before 8:30 a.m. Hence, the term olas ha'boker is only appropriate for Nisan and Tishrei. That the Torah only uses it the context of Nisan is because the calendar is always adjusted so Nisan falls at the right time, while Tishrei is not taken into account.
[Much more can be — and has been — written on the topic. One interesting point: According to the Zohar it is easier to understand R' Moshe Feinstein's position that chatzos ha'yom is at the same time — — every day of the year, even though — as he admits — the two "halves" of a day may not be equal. R' Moshe actually bases his position on our Gemara (see Igros Moshe, Orach Chaim and ,32.]