As of 1542, that's the year before Copernicus published De Rev, there were two questions before on the forefront of astronomy. What's required in the way of updating and revision of the Alfonsine tables to eliminate the large discrepancies in longitude and latitude that they currently exhibit? That was pointed out by Regiomontanus.

Can an account of celestial motions of the sort Rheticus has indicated Copernicus to be proposing in which the Earth is in motion save the salient orbital phenomenon as well as the Ptolemaic account does? Remember, Tychic, Copernicus De Rev wasn't out yet. So those were real questions in the community and it was fairly wide spread.

Go 100 years later. I'm gonna go through this slide even though it's gonna keep you a minute or two. Insofar as Copernican and Tychonic systems are both fully consistent with all accessible astronomical observations, and several leading astronomers adhere to the latter, can any decisive evidence be adduced to settle the question whether the Earth is in orbit about the sun, or vice versa?

Second, granted that Kepler's claims about planetary orbital motion hold at least a high approximation, should they be taken to hold, one, for bodies beyond those now known to be orbiting the Sun, and two, for the bodies now known indefinitely far into the past and future, and should they be taken to hold exactly, or only essentially exactly, or merely approximately, and if they do not hold exactly, should they be regarded as idealizations of some sort, or do they at least hold in the mean?

That question was developed entirely at the end last time, so I'm not gonna motivate it further. Three, granted that questions about relative distances of the planets, sun, and earth from one another have largely been resolved in units of the mean distance to the Earth from the sun, what do these distances amount to in earthly units, in units of the radius of the Earth?

Okay? That's both sizes and distances. Is orbital astronomy perfectible at all? Can the motions be mathematically characterized in a way that assures the conclusions drawn from the remote past and the remote future will hold at least to the same level of precision as conclusions about the present era?

That question really drove. He did not want astronomy not to be perfectible. Fifth, can the apparent motion of the the moon be mathematically characterized to the same level of precision as been achieved for the planets? And six, what are comets? What trajectories do they describe in their observed motions, and are they governed by the same physical processes, whatever those may be that govern the motions of the planets?

Now the point I'm making is a century has produced a totally different family of questions. And they're all on the floor. They all get answered in the Principia. That's not quite true. They all receive answers in the Principia except the third. Newton does not try to resolve the third, he depends on others.

But for the others, those are questions the Principia answers. And now we're gonna go from 1642, we're actually going to start in 1638 in Galileo's two new sciences of motion and move to 1642. This, in effect, to just before Newton, and then come back to astronomy to see what happened in astronomy between 1642 and 1680.

And, finally, we will continue. Fair enough? I just hope you appreciate how much has changed in the 100 years from before De Rev to this time. It's a different world.