The time of Keriat Shema lasts for the first three hours of the day and the time of the Amidah for four hours. “Hours” refers to proportional hours. That is to say, the day is divided into twelve equal parts, and each part is called a “proportional hour.” In the summer, when the days are long, so are the hours, and in the winter when the days are short, the hours are short too.
The question is: when do we begin to calculate the day? According to the Magen Avraham, the hours of the day are calculated according the hours of light. In other words, the calculation begins from amud hashachar and lasts until total darkness. However, according to the Gra, the calculation is based on the hours that the sun is visible, meaning from sunrise until sunset.
The time difference between amud hashachar and netz is approximately 72 minutes during the days of Nisan and Tishrei. Thus, according to the Magen Avraham, we begin calculating the three hours of the time of Keriat Shema 72 minutes before the time according to the Gra. Therefore, the last possible time to recite Keriat Shema and the Amidah according to the Magen Avraham is earlier. However, it is not 72 minutes earlier because every hour according to the Magen Avraham is longer, and that way it comes out that at the end of six hours we arrive at chatzot according to both calculations.
.The poskim who maintain that we calculate from amud hashachar are: Terumat HaDeshen, the Bach, and Eliyah Rabbah. This is also implied from a number of Rishonim, among them Rashi, Tosafot, Ramban, and Rashba (see HaZemanim BaHalachah 13:2). Although the Magen Avraham himself (233:3; 443:3) is uncertain as to whether the calculation is to be made from amud hashachar or from netz hachamah, nevertheless, he rules stringently (58:1) because Keriat Shema is a biblical mitzvah and therefore the calculation is given in his name.
In contrast to them, maintaining the Gra’s approach are Rabbeinu Chananel, Rasag, Rav Hai Gaon, Rambam, Talmidei Rabbeinu Yonah, and others. Acharonim who follow this approach include the Levush, Tosafot YomTov, and the Graz. It is also the opinion of the majority of poskim (see there 13:7-9). The Gra’s reason (Bei’ur HaGra 459, Shenot Eliyahu, the beginning of Masechet Berachot) is that the times of the day and the night are identical, and therefore one 24-hour period is divided into twelve hours for the day and twelve for the night. In calculating a whole year, the hours of the day and night are identical. This is only possible if the day is calculated according to the sun, for then the days and nights of the whole year are equal. This is not true if the calculation is made from amud hashachar until darkness, for then the days would be more than two hours longer than the nights.
. According to the Magen Avraham’s approach, there are those who calculate the hours of the day from amud hashachar until the emergence of three stars, and then the result is that before netz, 72 minutes are added, whereas after sunset only 18 or 13.5 minutes are added (Ma’amar Mordechai 233:3, Ben Ish Chai, Vayakhel 4; Divrei Yosef). However, their opinion is problematic, for based on their calculations, chatzot arrives before the sun reaches the middle of the sky, and therefore the final time for Keriat Shema according to the Magen Avraham comes out earlier than it actually is. Rav Tikochinsky, along with the prominent rabbis of Jerusalem, solved this problem by establishing the time after sunset as equal to the time before sunrise. If amud hashachar precedes netz by 72 minutes, then even after sunset 72 minutes are added. That way, it always comes out that the first six hours of the day end at chatzot, both according to the time of the Magen Avraham and according to the time of the Gra.Based on this, it is easy to calculate the difference in time between the final time according to the Magen Avraham and the final time according to the Gra. First, we must calculate the time between amud hashachar and netz and divide it into six. For example, according to the calculation of the days of Nisan and Tishrei, when amud hashachar is 72 minutes earlier than netz, every hour according to the Magen Avraham is longer by twelve minutes. If so, the difference in time between them after three hours is reduced by 36 minutes. This means, that the final time to recite Shema according to the Magen Avraham is 36 minutes earlier than the time of the Gra, and the final time to pray the Amidah according to the Magen Avraham, is 24 minutes earlier. However, based on what we learned in note 1 concerning the changes in the time of amud hashachar in the different seasons of the year, it comes out that in the height of winter, the time of Keriat Shema according to the Magen Avraham is 39 minutes earlier, and in the height of summer, it is 44 minutes earlier than the time of the Gra.
Since this is an uncertainty concerning a biblical obligation, it seems that it is correct to make the calculation according to the position of the sun at 17.5 degrees below the horizon, for at that time the first light is visible, and some poskim maintain that time as amud hashachar, as explained in note 1. Based on this, in the days of Nisan and Tishrei, amud hashachar is 78 minutes earlier than netz, and the time of Keriat Shema according to the Magen Avraham is 39 minutes earlier than that of the Gra. In the height of winter it is 42.5 minutes earlier, and in the height of summer it is 48 minutes earlier. In many calendars, the calculations are made according to the position of the sun at 19.75 degrees below the horizon, and then the time of the Magen Avraham is earlier by approximately 45 minutes in Nisan and by 56 minutes in the height of the summer. However, as I already wrote in note 1, this approach is very problematic because at that time no light is visible in the east. Additionally, some poskim calculate 90 minutes before netz and only 18 minutes after sunset, and the result is that the time of the Magen Avraham is earlier than the Gra’s time by more than an hour. Hence, as aforementioned, this opinion is very problematic.