Here's why:

* Two independent clauses: The sentence contains two complete thoughts that could stand alone as separate sentences:

* "By the 1970s scientists have accepted the idea of moving continents."

* "Today geologists refer to the movement as plate tectonics."

* Incorrect comma usage: A comma alone cannot connect two independent clauses. It needs a conjunction (like "and," "but," "or") or a semicolon.

Here are a few ways to fix the sentence:

* Add a conjunction: "By the 1970s scientists have accepted the idea of moving continents, and today geologists refer to the movement as plate tectonics."

* Use a semicolon: "By the 1970s scientists have accepted the idea of moving continents; today geologists refer to the movement as plate tectonics."

* Make two separate sentences: "By the 1970s scientists have accepted the idea of moving continents. Today geologists refer to the movement as plate tectonics."