Because the old theory demonstrably did not explain the observations.
The Theory of Heredity
soon
what theory did fleming come upwith
Newton
Continental drift
in 1856
oh have forgotte
They do the experiment again, double check all their calculations, then they come up with a new theory if it's necessary. That's why there's no such thing as a scientific fact, only scientific theories.
If observations don't match the theory, then either the theory or the experiment is wrong. Maybe a measuring device isn't sensitive or accurate enough. If the experiment is right, but results doesn't match theory, then it's time to come up with a new theoru that matches the experiment's results.
Geologists use the geologic column to classify layers of rocks and fossils that make up the Earth's crust.
When he saw that the plates could fit.
Do you mean that you have to come up with your own theory about some aspect of astronomy? That is wonderful! What is your thinking so far?