 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to begin our study of the two Earth-like planets in the solar system, two that are at least most like Earth, and that would be Mars and Venus. So what do we know about these planets? Well, let's see what we can start out with from Earth. Venus is the third brightest object in the sky, and we see it here in the central part of this image. The next brightest object in the sky down below would be the moon, or the full moon specifically. This is a thin crescent moon, and those would be the other, of course, the brightest object would be the sun. Now Venus also has phases which are visible through a small telescope, and we can see those here. These are one of the things that were discovered by Galileo, showing that Venus had to orbit our sun. So here we see the phases from a very small phase, that is about half full, up to a very close to new phase that we see here. Now Mars, on the other hand, is a little different. It has a distinct red color, it isn't near as bright as Venus, but it has a distinct red color that we can see here, and even through a small telescope we can see some of the surface features. And here we see that we can see a little bit of lighter and darker areas on the surface of Mars. Best visible when it's at opposition, and that's when it is opposite to the sun in the sky, making it at its closest to Earth, and also making it high up in the nighttime sky when the sky is darkest, so we can see the most detail. Now looking at Mars, one of the things that always comes up is the idea of canals on Mars. Now this was actually started, and the very first reference to anything like this was in 1877, and that was Giovanni Cciaparelli, who wrote of seeing canali on Mars. Now as you might expect, canali, which means channels in Italian, was mistranslated as canal, and you can certainly see why, because it looks like canal, and a canal and channel have a similar meaning, but there is a very big difference between them. A channel is a natural occurrence, so a river channel or things like that would be something that would be completely natural. A canal means an artificial origin. We think of things like the Panama Canal or the Suez Canal that are manmade, so things that are not natural, and that kind of gave the idea that maybe there was some kind of life on Mars, as this became thought that there were canals on Mars. Maybe there was some kind of intelligent civilization on Mars. And this was championed by Percival Low, and he championed the idea of canals for a long time as regions, and perhaps a way that some Martian civilization, dying civilization, was bringing water from the north and southern polar regions, where water was frozen into the caps, down to the warmer equatorial regions. So that was thought that maybe that would be the case. Now, of course, we know now that no canals, no such canals exist. It was really just an optical illusion that our minds want to put patterns. So when you start to see some things there, you'd start to see, you would try to make yourself see patterns. However, in order to see any kind of canal, these things would have had to been tremendous in size to be visible from Earth. Many large features on Mars are completely invisible from Earth, even under the best conditions. So to see a canal, even something a mile across, would not be visible from Earth. So it was really just an optical illusion. What our minds do is try to put some kind of sense to random patterns. And since there are some random patterns and some different colorations there, our minds tried to put them there. And once one person sees something, then others start to see the same thing. But to be fair, nothing was ever, you could never photograph canals. And of course, once we visited Mars in the 1960s, our images showed that there were definitely no canals, or in most cases, even channels, certainly nothing that can be seen from Earth. So let's look at some of the basic properties of these two planets. They are relatively similar. Venus, almost a twin of Earth, if you look at the numbers here. It has a mass, only a little bit less than Earth's. Its diameter is very close. Its density is very close. Its gravity is very close. Escape velocity is very close. Surface area is close. The things that are different are the rotational period, which are very different. Earth taking 24 hours, or just under. And Venus taking 243 days. We also notice that atmospheric pressures are very different. Earth's being one, and Venus's being 90 times the density of Earth's atmosphere. And we will look at this a little bit more detail when we talk about Venus. Mars, on the other hand, is a lot smaller. About 10% of Earth's mass, about half of its diameter. A lot less dense. Gravity is lower. Escape velocity is lower. However, the rotational period is almost the same. And that makes it convenient for a rover, because a rover will then have days just like it would have on Earth and was able to recharge things like solar cells efficiently, something that would not happen on our moon. And essentially, no atmosphere. Very, very thin atmosphere. Not near enough to even come close to being able to breathe. So we'll look at both of these in more detail in the coming lectures. This is just an overview to start out this lesson. So let's go ahead and finish up with our summary. And what we looked at. Mars and Venus are the two closest planets to Earth. And the two that are probably most Earth-like in many ways. We talked about the canals on Mars, which never existed. And we're really a combination of a mistranslation and an optical illusion. And then we looked at the different properties, how they have some, each have some similarities to Earth, but they each have significant differences as well. So that concludes this lecture on Earth-like planets. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.