 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are beginning our discussion of the moon, and we will have a couple of lectures looking at the various properties of the moon, other than our Earth, probably the best explored object in the solar system, and the only one to date that humans have actually landed on. So, let's get started looking at the moon, and let's give it some numbers here, and as I said, we tend to give these relative to Earth in many cases, because it makes the numbers more comprehensible. So, the mass of the moon is 0.0123 Earth masses, a little over 1,100 the mass of Earth, so not very much there. Diameter is a little over a quarter Earth of Earth's diameter. So, if you put the foot four moons side by side touching each other, those would stretch across the diameter of Earth. Its surface gravity is much less, just 17% of Earth's, and its density is also lower. Now, you know, density we don't do relative to Earth, density is a much smaller number as is rotation periods, so we give those in standard units 3.3 grams per cubic centimeter, and the rotational period of 27.3 days. Now, if you remember previously, we talked about the cycle of the moon being 29.5 days. That is the cycle of phases which is relative to the Sun. The actual rotation period, or the sidereal period, is 27.3 days. Now, let's talk a little bit about how the moon has been explored. We've certainly been able to see the moon since ancient times, but have not been able to see the far side until 1959, when the Luna 3 spacecraft traveled around and looked at the other side of the moon and gave us this first image. Now, while the image looks horrible, by today's standards, it was amazing at the time because it was the first time we had seen this part of our nearest neighbor in space. A few years later, in 1966, we had the first soft landing on the moon, and then in between 1969 and 1972, we had the Apollo landings, one of those shown here, where we landed on the moon a total of six times to explore and collect samples. Now, the exploration did not stop after the Apollo landings, and continued exploration went on, including the Luna Reconnaissance Orbiter in 2009, which was able to actually take images and see the Apollo landing sites from lunar orbit. So while these are not visible from Earth, they are visible from lunar orbit by the Reconnaissance Orbiter, which mapped not just this, but the entire moon's surface in detail. And then we talk about rovers on Mars, and we'll look at those again when we get to Mars, but there's also been rovers on the moon. Now, it's much more difficult to have a rover on the moon because the conditions on the moon are far more extreme than on Mars. The moon has no atmosphere to protect it, and the temperature swings are tremendous, from extremely hot, baking in the sun during the day, to bitterly cold at night. And that makes it difficult. In addition, the days and nights are 14 days long each. So while you have 14 days of sunlight to power solar panels on a craft, you then also have to hibernate for 14 days. And rovers such as this one usually last a few lunar days, which remember a lunar day is a month, before the intense pressure temperature changes become too much for the mechanism. So what is our moon made up of? What have we learned about our moon? Well, it's primarily rocky material. It has far less metals than Earth, and we will come back to this when we look at how our moon might have formed. It has no atmosphere to speak of, but it does have some water ice in craters. Now, water ice cannot exist on most of the moon because the sun would vaporize it during the lunar day. However, the sun can exist near the polar regions in craters that are permanently shielded from direct sunlight. So there are areas on the moon that never get direct sunlight. At some point, if a comet happened to crash into the moon, it could have deposited materials. So there is some water ice there on the moon, but much, much less than we see on Earth. Now, the moon is divided into parts. We see that here, and we'll see it here in another image. The moon is divided into two areas. The maria, which are the darker regions, and have few impact craters. And remember that fewer impact craters means that they are younger regions. We also have the lighter-colored highlands, which are many impact craters, heavily cratered. So when we look here, we can see, for example, part of the maria here, and see hardly any cratering. Any of those. There's some cratering, but not a whole lot in the maria. When we look at the lighter-colored highlands, we can find craters on top of craters, on top of craters in similarly-sized regions. So the areas, there are two distinct regions on the moon, one being a lot older, the highlands, the other being a lot younger, the maria. Now, when we talk about younger on the moon, we're still talking about 3.5 billion years old. And that is much older than anything, much almost anything we have here on Earth. The highlands can be 4 to 4.5 billion years old. Now, let's look at the different types of rocks that we see from these. And we get three different types. We have the basalts that we find in the maria. These are dark-colored basalt volcanic rocks that we see here. And that gives the maria their distinct dark color. They are also called vesicular, meaning that they had trapped air bubbles in them, and you can see the bubbly kind of formation here as they melted, were molten from the lava, and the air bubbles, at least the remnants of them, remained in the rock. Now, in the highlands, we see a different type of rock. We call an orthocyte. An orthocyte is the lighter-colored, and that gives the distinct lighter-colored to the highlands. These are also volcanic rocks. They are a lower density rock and older, so they are the higher regions being a lower density. And we see these, again, are both volcanic rocks. We do not see sedimentary rocks on the moon as we see on Earth. We see only volcanic rocks, and we see brecchias. Brecchias are created on the moon by impacts. So let's take a look at one here. And they are created by impacts, and the material is melted and fused together to make a new rock. So, a brecchia would be an example of a metamorphic type rock changed by heat or pressure of the volcanic rocks, and in this case, the heat is from a massive impact smashing in to the moon. Now, how do we get changes on the moon? What things change? The moon is really essentially unchanged from billions of years ago. Yeah, there have been a few impact craters, but overall, it's essentially the same. So, erosional processes on the moon are very, very different than what we have here on Earth. We have no wind, no water. The only thing we have on the moon for erosion is meteorite impacts, micrometeorite impacts, something we do not have on Earth. On Earth, we have an atmosphere, and our atmosphere will burn up all these micrometeorites, little grains of sand, in the upper atmosphere, so they never make it down to our surface. However, on the moon is constantly being bombarded with these over billions of years, and has built up a soil, a powdered soil on the moon, that we see here in the image of the moon's footprint. This is what we call the regolith, or lunar soil, which may be an inch or so thick, deep, as you see, with the footprint here, and it covers the entire surface of the moon. So these micrometeorite impacts will slowly erode away at the moon's surface, but it takes a long time. It is much, much slower than the wind or water erosion to see this footprint. This is what the footprint would look like today. This footprint made 50 years ago would look the same if we were to go back to the moon today and take a look at it. It would be unchanged. That is how slow the erosional processes are. A footprint on Earth would not last 50 years, because it would be eroded away very quickly. On the moon, it lasts until we go ahead and finish up our beginning lecture on the moon with our summary. What we've looked at, the moon, our closest neighbor, but is very, very different than Earth, has very different regions. It has the maria, which are the darker colored craterless regions, and the highlands, which are the heavily cratered lighter colored regions. We looked at some of the different erosional methods that occur on the moon. So that concludes this lecture on the structure of the moon. 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.