At Mission San Jose and missions stretching from northern California to South America, a twice yearly orientation of the sun triggers an extraordinarily rare and fascinating event.  At sunset a sunbeam enters the west window of the church, traverses the length of the church, and bathes the gilded altar in brilliant light. 

This phenomenon has only been discovered in recent decades. During the Spanish mission era, Mission churches were designed with an orientation such that the illumination coincided with a particular saint’s feast day or religious holiday.  The illumination conveyed to Native California converts the rebirth of light, life and reinforced the divinity of Christ. In the case of Mission San Jose, the spring illumination occurs on or about the feast day of St. Joseph, and the fall illumination, coincides with the feast of St. Francis, the patron of the Spanish Franciscan missionaries. 

For further information, see a paper published in Smithsonian Magazine.