Relative dating cratering distribution
The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer.
There's no absolute age-dating method that works from orbit, and although scientists are working on age-dating instruments small enough to fly on a lander (I'm looking at you, Barbara Cohen), nothing has launched yet. Relative age dating has given us the names we use for the major and minor geologic time periods we use to split up the history of Earth and all the other planets.The chronostratigraphic scale is an agreed convention, whereas its calibration to linear time is a matter for discovery or estimation. We can all agree (to the extent that scientists agree on anything) to the fossil-derived scale, but its correspondence to numbers is a "calibration" process, and we must either make new discoveries to improve that calibration, or estimate as best we can based on the data we have already.To show you how this calibration changes with time, here's a graphic developed from the previous version of Fossils give us this global chronostratigraphic time scale on Earth.We have no idea how much older thing B is, we just know that it's older.
That's why geologic time is usually diagramed in tall columnar diagrams like this.
That last, pink Precambrian column, with its sparse list of epochal names, covers the first four billion years of Earth's history, more than three quarters of Earth's existence. Paleontologists have used major appearances and disappearances of different kinds of fossils on Earth to divide Earth's history -- at least the part of it for which there are lots of fossils -- into lots of eras and periods and epochs.