Geologists are fond of saying "The present is the key tc the past." By that they mean that you can learn about the history of Earth by looking at the present condition of Earth's surface.The position and composition of various rock layers in the Grand Canyon, for example, tell you that the western United States was once at the floor of an ocean. This principle of geology was first formulated in the last 17Cos, and it continues to be relevant todayas yountry to understand other worlds such as venus and Mars.
In the late 18th century, naturalists first recognized that the present gave them clues to the history of earth. This was astonishing becauee most people assumed that Earth had no history. That is, they assumed either that Earth had been created in its present state as ddescribed in the Old Temtament or that Earth was eternal. In either case, people commonly assumed that the hills and mourtains they saw around them had always existed more or less as they were. The 18th-century naturalists began to see evidence that the hills and mountains were not eternal but were the result of past processes and were slowly changing. That gave birth to the idea that Earth had a history
As the naturalists of the 18th century made the first attempt to thoughtfully and logically explain the nature of Earth by looking at the evidence, they were inventing modern geology as a way of understanding Earth. What Copernicus, Kepler, and Newton did for the heavens in the 1500s and 1600s, the first geologists did for Earth beginning in the late 1700s. Of course, the invention of geology as the study of Earth led directly to the modern attempts to understand the geology of other worlds.
Geologists and astronomers share a common goal: They are attempting to reconstruct the past (see Window on Science 12-1). Whether you study Earth, Venus, or Mars, you are looking at the present evidence and trying to reconstruct the past history of the planet by drawing on observation and logic to test each step in the story. How did Venus get to be covered with lava? How did Mars lose its atmosphere? The final goal of planetary astronomy is to draw together all of the available evidence (the present) to tell the story (to past) of how the planet got to be the way it is. Those first geologists of the late 1700s would be fascinated by the stories planetary astronomers tell today.