There were major advances in observational astronomy during this time. The first we already discussed last week. I'm now showing you both Huygens' astronomical clock on your left, and his maritime clock on the right. You'll notice the cleverness on the maritime clock of having two strings to try to stabilize it.

The maritime clock is really important to determine longitude at sea. And I cannot stress too much how many boats went down. They really didn't know where they were at sea, and when they were in the Atlantic, and it was a very big deal to start finding some way that they could locate where they were longitudinally at sea.

That's what the clock was for. We'll talk about it later in the, actually talk about it next semester. The clock on the left though, which is the one from Horologiam. It's not his first clock for observatories. His first one didn't have the cheeks, but his success with his escapement then altered all time keeping inside observatories.

Not everybody used a Huygens' clock, you'll see later the Greenwich Observatory was using a Tompion clock but, the point is all of a sudden time keeping in observatories meant you had to be good to at least within 15 seconds per day, versus the old clocks, 15 minutes per day.

Errors as large as 15 minutes, so you could not rely on them with confidence for better than plus or minus ten minutes. Here you can rely on them for plus or minus ten seconds out of 86,400 seconds, and ten seconds is within observational error on timing things. Ten to fifteen minutes is not necessarily depending on how fast the object is moving so that's a major step forward that Huygens initiated, though once again, he designed clocks and then handed them over to other people to manufacture.

And the best I can tell, he did not care whether he made money off of it. It was not something of concern to him. The other thing Huygens did in the 1650's, he and his brother Constantine, a year or two older than him, who then left science and became Ambassador later in life too.

At the time of Newton's Principia, he's Ambassador to England from Holland. Constantine and Christian started grinding their own lenses, and Christian got the clever idea of what you need is a long, long focal length, but who wants to build a 120 foot tube. So he started realizing, we don't need a tube there, what we need is to have the business end of the refracting telescope far away, 120 feet away.

It set up on a platform, and down below align it to the other end of the thing with the eyepiece and you've got, one of them is 127 feet long. And you've got the two brothers producing, for about a three year period, the best lenses of anybody. So that's a breakthrough in telescopes, meaning you're getting a degree of magnification that we haven't remotely seen before this moment.

No telescope before the mid 1650's gave you anything like this degree of magnification. I'll go on with what Huygens discovered within the second. The other thing at the bottom is Huygens' compound negative eyepiece. This is pure engineering. Though he of course knew a lot from Descartes' optics. So he knew Snell's law.

The primary thing he's doing with that pair of lenses forming an eyepiece. That's the eyepiece at the bottom. Those pair of lenses will cancel out spherical aberration almost entirely. They're designed to do that and they largely eliminate chromatic aberration. Which hadn't even been properly explained yet. We'll get to that later tonight.

But he managed to get far better images with a combination of his IP's plus the enormous increase in magnification you get from very, very long focal lengths. There are a lot of other tricks he did. This is again, Huygens the engineer now focusing on telescopes. But with these telescopes from the mid 1650's, the refracting telescope had become a fairly mature piece of technology.

Huygens himself introduced cross-hairs in the eyepiece. Uzo and Picard very soon after. Put in fixed micrometer and then they put in a screw micrometer, so you can move the two little points and on the side actually measure the for example, the width of Mars as you're looking at it etc.

So we're here talking now about a fairly mature technology. From here on is all gonna be fairly subtle refinements until we hit the reflecting telescope that Newton introduces, and that doesn't make any difference til way into the 18th century. Bradley didn't use reflector, right? So Huygens made at least two significant discoveries in the mid 1650's.

They were both originally presented to the Paris discussion group while he was visiting Paris. This is the discussion group that Mersenne had started, Gassendi had taken over when Gassendi died, took it over. It’s the one satirized, as I always like to say, in Moliere’s Learned Ladies. The first thing he announces is that Saturn has a satellite.

It’s not just Jupiter and the Earth with satellites. It’s Saturn too, it’s Titan. It’s a fairly good sized object, it’s nearly Earth size as I recall. I think it’s the largest satellite in our system. You guys can check that. But it's often compared to the Earth though, it's so incredibly cold since it's very, very far away.

But it's been an object that people have looked at as comparable in size to the Earth. And then, this is just detective work. I don't know that his telescope helped him achieve this. He looked at all the different diagrams that people had drawn, those are on the outside starting with Galileo, of Saturn and the bulges on Saturn.

And he figured out Saturn has rings. And the inner, this is his drawing by the way. This is not a modern drawing, this is right out of System Saturnium, Huygens' 1659 publication on Saturn. What he shows is, looking from the Earth, how the rings are gonna look. As Mars goes around its orbit.

And the Earth goes around its. And simply works backward. And says all these drawings we've seen can be given rise to, very simply by a ring structure around Saturn at an angle. At an angle to the ecliptic. Much like the Earth, of course rotates at an angle to the ecliptic too, so that's not a big surprise.

So those are both major breakthroughs. The important thing that starts happening here, and you can see it at the bottom, once Cassini gets hold of Componi telescopes, this is actually crediting Componi himself, you start seeing further things. You see for example, the remark here which is out of Al Van Helden's paper on, his article in the Cambridge history of astronomy that I used back when you were looking at Galileo, Compani was able to see the shadow of the rings on Saturn.

Which is very nice. Again I'm gonna be featuring Compani in a couple of moments.