Demography (from prefix demo- from Ancient Greek δῆμος dēmos meaning "the people", and -graphy from γράφω graphō, implies "writing, description or measurement") is the statistical study of populations, especially human beings. As a very general science, it can analyze any kind of dynamic living population, i.e., one that changes over time or space (see population dynamics). Demography encompasses the study of the size, structure, and distribution of these populations, and spatial or temporal changes in them in response to birth, migration, aging, and death. Based on the demographic research of the earth, earth's population up to the year 2050 and 2100 can be estimated by demographers. Demographics are quantifiable characteristics of a given population.
Geography (from Greek γεωγραφία, geographia, literally "earth description") is a field of science devoted to the study of the lands, the features, the inhabitants, and the phenomena of Earth. The first person to use the word "γεωγραφία" was Eratosthenes (276–194 BC). Geography is an all-encompassing discipline that seeks an understanding of the Earth and its human and natural complexities—not merely where objects are, but how they have changed and come to be.
Living organisms including humans are social when they live collectively in interacting populations, whether they are aware of it, and whether the interaction is voluntary or involuntary.
Since my youth geography has been for me the primary object of study. When I was engaged in it, having applied the considerations of the natural and geometric sciences, I liked, little by little, not only the description of the earth, but also the structure of the whole machinery of the world, whose numerous elements are not known by anyone to date.
Gerardus Mercator (1578), Introduction to Ptolemy's Geography
As a young man, my fondest dream was to become a geographer. However... I thought deeply about the matter and concluded that it was far too difficult a subject. With some reluctance, I then turned to physics as a substitute.
Duane F. Marble, Professor of Geography, posted this on his office door at SUNY at Buffalo, jestingly misattributing it to Albert Einstein; it has since been quoted as if it were a genuine quote of Einstein, and debunked at sites online and in The Ultimate Quotable Einstein (2010) edited by Alice Calaprice, p. 474.