As you know, the stars shift their relative positions in the heavens; but though the real distances traversed are beyond all ordinary comprehension, the effects as we see them are small.

Nevertheless, they are susceptible of measurement, not by years, indeed, but by centuries.

It was by this means that Sir John Herschel arrived at the date of the building of the Great Pyramid--a date fixed by the time necessary to change the star of the true north from Draconis to the Pole Star, and since then verified by later discoveries.

From the above there can be no doubt whatever that astronomy was an exact science with the Egyptians at least a thousand years before the time of Queen Tera.

Now, the stars that go to make up a constellation change in process of time their relative positions, and the Plough is a notable example.