Answer:
 the Sun is the object that is at a very great distance and the focus point of the sun's shape is the distance to the magnesia, this image is equal to the seal distance
Step-by-step explanation:
The method for measuring the focal length of a lens is based on the use of the constructor's equation 
 

 
where q and q are the distance to the object and the image respectively, f is the focal length. 
If we place the object very far away (infinity) the equation remains 
 
 
 
Therefore with this we can devise a means for measuring the Sun is the object that is at a very great distance and the focus point of the sun's shape is the distance to the magnesia, this image is equal to the seal distance