The radius is the size of the sphere as measured from the center of the explosion.
Maybe this analogy helps.
You throw a ball at a wall and it bounces back towards you. If the ball reaches you or passes you then you die.
You can throw a ball (the fireball) 150ft.
At the end of your throw it hits a wall (center of explosion). The ball bounces back towards you 92.832ft (explosion radius/edge of explosion). The ball is now 57.168 ft away from you. Ball did not reach you therefore you live.
The radius is the size of the sphere as measured from the center of the explosion.
Maybe this analogy helps.
You throw a ball at a wall and it bounces back towards you. If the ball reaches you or passes you then you die.
You can throw a ball (the fireball) 150ft. At the end of your throw it hits a wall (center of explosion). The ball bounces back towards you 92.832ft (explosion radius/edge of explosion). The ball is now 57.168 ft away from you. Ball did not reach you therefore you live.
First, of course, we must use the wish spell to make the room 150ft long with the alter at the other side.