Laws of the "Shema" - Part 2
The time to recite the morning Shema extends until the third hour, which is one-fourth of the day. (Shulchan Aruch Orach Chaim 58:1)
Although the above law is clear, straightforward and without machloket (dispute), its application is not so simple. There is a well-known dispute among the Rabbis about when a day starts and ends. One opinion maintains that a day begins at dawn and ends when three stars appear (Terumat HaDeshen, Magen Avraham and others). The second opinion maintains that the day begins at sunrise and ends at sunset (Lavush, Gra, Shulchan Aruch HaRav and others).
The general rule when dealing with a Biblical command is to follow the stricter opinion. In this case that would mean that one should calculate the hours from dawn to the appearance of three stars, since this time will pass first, and only if this time has already passed may one rely on the second opinion and say the Shema before the end of the third hour by calculating the hours from sunrise to sunset (Kaf HaChaim). The Mishnah Berurah writes that in an ideal case there is no practical difference between the two opinions since the law is that one must not delay saying the Shema once its time has arrived.
Rabbi Moshe Feinstein writes that the custom in most places in his country (“Europe”) followed the opinion of the Gra, and that it is considered the main opinion. Only individuals who were strict in this matter followed the Magen Avraham. However, today most communities follow the Magen Avraham in accordance with the majority opinion of the later Rabbis, and this is the custom in Eretz Yisrael (Piskei Teshuvot).
How to Calculate an Hour
According to halacha the length of a day, as mentioned above, is either from sunrise to sunset or from dawn to the appearance of three stars. In the summer the days are longer while in the winter they are shorter. Therefore, the exact time of day changes, based on the time of the year. To properly calculate the exact time of day until when one may recite the Shema prayer, one must take the total amount of time (i.e. the total minutes) from sunrise to sunset, or from dawn to the appearance of three stars, and divide it into twelve equal parts. This will give you the length of one “halachic hour”. You will notice that in the summer, when the days are long, a halachic hour can be as long as seventy minutes, and in winter as short as fifty minutes. You can then calculate when the 3rd hour ends to know the latest time to say the Shema. Nowadays we have tools such as myzmanim.com to make this task much simpler.