The hours referred to by the Sages are seasonal hours (“sha’ot zemaniyot”). That is to say, the day is divided into twelve equal parts, and each part is called a “seasonal 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 Magen Avraham, the hours of the day are calculated according to the hours of light. That is, the calculation begins from dawn and lasts until total darkness. According to Gra, the calculation is based on the hours that the sun is visible, meaning from sunrise until sunset. This explains the two different times that appear in calendars. The earlier time follows Magen Avraham’s approach, which begins the calculation of the day from dawn, whereas the later time is in keeping with Gra’s opinion, which begins the calculation from sunrise (Peninei Halakha: Prayer, ch. 11 nn. 13-14).
In practice, most poskim follow Gra. Additionally, since the time of prayer was established by the Sages, the halakha follows the lenient opinion, and therefore the latest time for Shaĥarit is calculated according to Gra. 1
- Similarly, we follow Gra regarding the latest permissible time to eat ĥametz on Erev Pesaĥ, because the Sages are the ones who established that it is forbidden to eat ĥametz after four hours, and when there is uncertainty concerning a rabbinic mitzva, halakhic practice follows the lenient opinion. However, regarding a mitzva whose time is specified by the Torah, such as the mitzva of reciting Shema (which is obligatory for men and must be done by the end of the third hour of the day), it is proper to follow the stringent approach and recite it before the end of the third hour according to MA’s calculation. When there is uncertainty concerning a biblical commandment, we are stringent. ↩