The rotation of the Earth along with its transitional movement around the sun, leads to the appearance of night and day and creation of different seasons throughout the year. On the other hand, the inclination of the axis of the Earth and its unsteady speed in the transitional movement around the sun create certain changes in the religiously important times throughout the year which have been examined along with the temporal and spatial factors that cause them in the two cities of Tehran and Shiraz. In view of the importance of the issue for Muslims who are bound to perform prayers at the first true legal time, the results of these changes can be used to clarify the public's mind and to motivate the related fields’ enthusiasts to pay more attention to the astronomy and geographic issues.