 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk and introduce the two most Earth-like planets and that would mean Mars and Venus. Those are the two planets in the solar system that have the greatest similarities to the Earth and properties. So they are planets we have studied in both the two planets we have studied really in the most details. We've actually looked at them more than we have studied any of the other planets in the solar system and for that reason they have atmospheres, they have solid surfaces and they are very similar in many ways to the Earth. So let's look at them in a little bit more detail. First of all what do we see from the Earth? Well Venus is the third brightest object in the sky. Only the Sun and the Moon would be brighter. So it's one of the very easiest objects to see either early in the morning right before sunrise or in the evening right after sunset. Very prominent in the sky and very hard to miss because it is such a bright object. When we look at it we can see phases through a telescope. So Venus does go through a set of phases and we can see that it not only does it change its phase but it changes its size as well. So here an image about a quarter phase. We see that Venus is very small in angular diameter and as it goes towards the crescent phase it gets larger and larger eventually reaching the new phase which isn't completely invisible for Venus because Venus does have an atmosphere. So here we're seeing the results of light coming around through the atmosphere around Venus. So even when it's completely the completely illuminated side is facing away from the Earth we can still see a little bit there that small ring around it. But we see that the size changes and that the phase changes and recall that was one of the things that told us that the Venus had to orbit around the Sun. Now when we look at Mars Mars has a distinct red color so let's take a look at that there. Mars does have this distinct red color to it so even when seen through a small telescope it has a very distinct red color and you could see various surface markings on it as well. So not only is it a planet very similar to Earth in a number of ways except that it is a little bit smaller but we can see surface features and those surface features sometimes are seen to undergo changes. So there are changes going on on Mars that can cause those. At one point it might have been thought that they were perhaps features that were caused by seasonal plant changes. Now of course now we know that there is no plant life on Mars and not necessarily any life at all. Nothing that has been able to be detected but we still see those changes we see polar caps that come and go with the seasons so it has some other very similar similar things to the Earth. Now one of the big things on Mars that was originally that was thought long ago was that Mars were their canals on Mars. I'll take a minute just to look at the idea of canals on Mars and this came up by an Italian astronomer Giovanni Schiaparelli who observed in 1877 and he wrote in his writings that he saw canali on Mars. Now canali is literally translated as channels. However you can see from its name just the way it's written how it could be mistranslated perhaps as a canal. Now a channel can be a natural surface feature but when we talk about canals we tend to think of things like what? The Panama canal or the Erie canal things that are man-made so not necessarily things that are that are natural in cause. A channel can be a natural channel not necessarily implying any intelligent intervention so this is one of the reasons that you know this looked at as an artificial origin for the canals things that were not natural and another astronomer the late 1800s Percival Lowe championed the canals as an evidence for a possible Martian civilization and here we see the drawings of Schiaparelli where he sketched in what he was able to see for these supposed canals of Mars and at one point it was thought that maybe they were away there was a dying civilization as the planet dried out and it was a way to bring water to the central regions from the polar regions which would still have water now let's look a little bit at just the general properties of these two planets as well so what are some the properties of Venus and Mars? Venus is closest in size to the earth if we look at the table here in terms of mass it is 82 percent of the mass of the earth very close in mass it is only about 600 kilometers smaller in diameter its density is about the same its surface gravity is about the same so very many of the things are very close to what the earth has the things that are different of course are the atmospheric pressure 90 times the earth's atmospheric pressure very different and what's not put on here would be the surface temperature which is also much much hotter so in many ways it is like the earth but it has that very high surface temperature and a very high pressure the other thing that's different is the rotation rotational period is 243 days so very very long day night cycle because of its very slow rotation Mars on the other hand is more earth like in many ways because it does not have the high surface temperature and pressure in fact its pressure is very low much too low to be able to breathe even if it had oxygen and it is much smaller than the earth if we look at the size it's nearly half the size of the earth its density is a lot less showing a higher concentration of rock and less of metal and its surface gravity is also less again related to the mass being significantly less however the day on Mars is just about the same as a day here on earth 24 and a half hours and versus about 24 hours here on the earth so the big difference with Mars is that is significantly smaller and has lost its atmosphere so let's finish up here with our summary and what we find is that Venus and Mars are the two closest planets to the earth so that the closest and they are the most earth like they have the biggest similarities to the earth neither of them is exactly like the earth but they have the closest similarities to planets on to of any planets to the earth canals on Mars never existed it was a combination of mistranslation of the word from one of the original observers and essentially an optical illusion recall that things were done by drawing here so you're looking at things with your eye you're not making photographs with the technology was just coming up in photography around this time so your mind tends to put reason and pattern to things that are essentially randomness so just to finish Venus and Mars both have some similarities to the earth but do have very significant differences as well so that concludes our lecture on the 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