The zman of krias shma is an argument between the Magen Avraham and the GRA. First there is an argument in the length of the day. The Magen Avraham’s view is that the day starts from alos hashachar, dawn and ends at tzetz hakochovim, the coming out of the middle size stars. The GRA holds that the day starts at sunrise and ends at sunset. There is 72 min between dawn and sunrise. Also 72 min between sunset and tzetz hakochivim making the day according the Magen Avraham 12 hrs + 2 x 72 min or 12 hrs + 2 hrs + 24 min = 14 hrs and 24 min = 14.4 hrs. Divide 14.4 by 12 to find the shaos zmaniyos = 14.4/12 = 1.2. Zman of krias shma is 3 hours from alos hashachar 6 AM normal sunrise – 72 min = 6 – 1.2 = 4.8 + 3 x 1.2 = 4.8 + 3.6 = 8.4 which would be 8 + 2 x 12 min = 8 hr 24 min AM whereas according to the GRA is 6 AM + 3 hr 9 AM a difference of .6 or 36 min
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