Geology has traditionally been considered the science (the body of descriptions and explanations) that deals with the earth. Of late, with the exploration of the solar system, its scope has widened, and has come to include other planets and objects.
Although relatively recent, the birth and growth of geology parallels that of other sciences. It did not emerge as a cohesive science (i.e. a consistent body of descriptions and explanations as opposed to a collection of practical ideas and techniques) until the concept of a real, regular, reasonable and consistent nature was applied to the earth. Most would agree that that happened at the end of the XVIIIth Century, when a Scotsman named James Hutton (1726-1797), formulated the doctrine of uniformitarianism. In brief, this doctrine states that "the present is the key to the past."
Prior to uniformitarianism, geologic change was, with few exceptions, viewed as catastrophic, due to sudden, random events such as the Great Flood of Noah. The central problem with catastrophism is that this model does not allow events to be analyzed logically. It denies an orderly progression of events. Because it considers the geologic record to be an accumulation of unpredictable events, such a point of view closes the past to investigation and the future to prediction.
In contrast, uniformitarianism asserts that the same laws which operate today also operated in the past. Therefore, the present earth's appearance and behavior is the result of past events. This concept allows us to understand the earth and, like Janus (the Roman god of doors and gates represented by a head with two faces), see and understand the past and predict the future.
With uniformitarianism, Hutton provided the first global model for geology, as Newton's idea of force had done for physics and astronomy, Daltonís notion of the atom for chemistry, and as Darwin's natural selection would do for biology. Hutton stated that when we look at the earth, "we find no vestige of a beginning, no prospect of an end." This concept of an ancient, but consistently changing earth, obeying immutable chemical and physical laws, is a legacy and foundation upon which generations of geologists have relied to gain an ever better understanding of our world.
The story of the Earth is intertwined with the story of the Solar System. We are, of course, the third planet outward to orbit our sun, a smallish star whose proper Latin name is Sol.