<human>:  When did Galileo write the letter to the Grand Duchess Christina?
<bot>: The letter to the Grand Duchess was written in 1613. Galileo was an Italian astronomer, physicist, and mathematician. He was born in Pisa in 1564, and died in Arcetri, Florence, Italy in 1642. He is best known for his contributions to the field of mechanics, including his law of free fall, and for his experiments with falling bodies.