Because with the study of the earth, you uncover its secrets too. Meaning by studying geography you will also find clues and answers to the history of the world. 

They are related, because how will you find an origin of something (let's say Earth) without even studying it. And, there you go. You study history to find clues. Clues which can also be the answer/ other clue to the mysteries of the earth's composition.

In your case if we put it logically,
 • What we learn in geography and in history works the same way. "By learning something from the past, we learn things today and by learning things today we will learn things for the future." Something like that...