Here's why:
* Common nouns are general names for people, places, things, or ideas. They are not capitalized unless they are part of a proper noun.
* Proper nouns are specific names for people, places, things, or ideas. They are always capitalized.
Examples:
* Common Noun: city
* Proper Noun: London
Since "geography" refers to the general study of the Earth's surface, it's a common noun.