What is relative age dating
The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer.
Of course, this only works for rocks that contain abundant fossils.
The science of paleontology, and its use for relative age dating, was well-established before the science of isotopic age-dating was developed.
Nowadays, age-dating of rocks has established pretty precise numbers for the absolute ages of the boundaries between fossil assemblages, but there's still uncertainty in those numbers, even for Earth.
On Earth, we have a very powerful method of relative age dating: fossil assemblages.On other solid-surfaced worlds -- which I'll call "planets" for brevity, even though I'm including moons and asteroids -- we haven't yet found a single fossil.Something else must serve to establish a relative time sequence. Earth is an unusual planet in that it doesn't have very many impact craters -- they've mostly been obliterated by active geology.Conveniently, the vast majority of rocks exposed on the surface of Earth are less than a few hundred million years old, which corresponds to the time when there was abundant multicellular life here.Look closely at the Geologic Time Scale chart, and you might notice that the first three columns don't even go back 600 million years.