Al-Birjandi

Al-Birjandi is a brilliant astronomer, mathematician and physicist whose answer to one of Qutb al-Din al-Shirazi's arguments was this: 'The small or large rock will fall to the Earth along the path of a line that is perpendicular to the plane (sath) of the horizon, this is witnessed by experience (tajriba). And this perpendicular is away from the tangent point of the Earth's sphere and the plane of the perceived (hissi) horizon. This point moves with the motion of the Earth and thus there will be no difference in place of fall of the two rocks.'. He has written many books, including Sharh al-tadhkirah or 'A commentary on al-Tusi's memoir' discussing astronomical elements.