Hello and welcome to astronomy with Mr. Gehrin. Today we're going to look at the moon and what we can see with the naked eye, telescopes and space probes. This image is a 3D model of the moon with all heights magnified ten times. You can explore this model yourself, the link is in the video description. The moon is only about a third as large as the earth with a diameter of about 3500 kilometres. This figure is given on the GCSE fact sheet so you don't need to memorize it. The moon rotates just like all moons and planets. While gravity tries to pull it into a sphere, the centrifugal force outwards makes it bulge at the equator into a shaped like a squashed ball called an oblate spheroid. Its diameter at the equator is about 4 kilometres more than its diameter across the poles, not noticeable to the naked eye. The moon orbits the earth at about 380,000 kilometres. The distance varies by about 40,000 kilometres, bringing it quite a bit closer than average every month. When the moon is closest at the same time as a full moon, this is called a super moon. New's rooms love to make a big deal of this, but it only looks about 7% bigger than average. Again, you don't need to memorize the distance to the moon, it's on the exam fact sheet. There are lots of interesting features to look at on the moon. You can see some with the naked eye, but for others you need a telescope or binoculars. We can see several craters with the naked eye, and more with a telescope. Craters can be formed in many ways, such as volcanoes, but almost all craters on the moon are impact craters, formed when an asteroid or comet hits the moon. An impact usually leaves a circular depression. The asteroid or comet is much smaller than the crater, but the impact sends a shockwave outwards, pushing rock and dust to the sides, and leaving a mountainous ridge where the shockwave dies down. We can see this in this picture of Ticco crater, where we can also see debris from the impact in the middle of the crater. The moon does have a few elliptical craters, where the impact came in at a steep angle, but usually the circular shockwave wipes out the ellipse. Looking up at a full moon, you can clearly see light and dark patches. Some cultures see the shape of a man, a woman or a face in these patches. The light patches are called terre, singular terre, or highlands. The crust is thicker here, terre are older than maria, which we'll look at next, and so they have accumulated more craters. The far side of the moon is almost entirely terre. The dark patches are called maria, singular maria, or seas. It used to be thought they were covered in water, we now know that these are lowlands, where the crust is thinner, and a large impact broke through the crust to the molten mantle beneath. Magma erupted onto the surface and called, leaving a smooth surface. Maria formed later on and have fewer craters than terre. They are made mostly of basalt. Almost all maria are on the near side of the moon, as the crust is thinner there. Over time the mantle has called and solidified, so maria no longer form on the moon. You may be asked to name the dark patches in the exam. You can say maria or seas, but you must not call them oceans. You won't get them arc. Mountains and valleys are easy to identify, and look similar to mountains and valleys on earth. However, the moon never had plate tectonics. Mountains and valleys on the moon were formed by asteroid and comet impacts. Lastly, here are two features that you don't need to know for the GCSE, but I think you're cool. Rills are channels cut into the moon's surface, and wrinkle ridges are like upside down rills. We don't fully understand how these were formed. For the GCSE, you need to know the names of seven specific features on the moon, three craters, three maria and one mountain range. You may be asked to label them with their names, or to draw lines from the names to their location on a photo. Take care, the orientation of the picture might be different. Learn these features from these pictures, and in the exam, think about turning the exam paper to match your memory. Here is a picture of the moon, followed by the same picture with the seven features labelled. Feel free to pause the video and skip between the pictures to try to learn them. If you're on a computer, left and right arise should do the trick. So far, we've only seen pictures of one side of the moon. That's because it keeps the same side facing towards earth. The moon rotates once every 27.3 days, and revolves, or orbits around earth, every 27.3 days, these times are the same, so it keeps the same side towards us. Nobody knew what the other side of the moon looked like until the Russians sent lunar three around the moon to take photographs in 1959. Here we can see the difference between the near and far sides of the moon. You should know that the near side has more maria, and fewer craters. I'll explain the reason for this, in my video on the origin and structure of the moon. So, the period of the moon's rotation is the same as the period of its orbit, 27.3 days. You need to memorize this number, it is not given to you in the exam, and you may be asked to write it down. But why are they the same? This is down to an effect called tidal locking. In simple terms, the earth's gravity pulls the near side of the moon more strongly than the far side, and over time this creates a bulge at the equator. The centrifugal force we discussed earlier adds to this effect. As the bulge at the front of the moon turns away, the earth tries to pull it back. This actually generates friction, slowing the moon's rotation until it matches the moon's orbital rate. We think this took about 10 million years. The moon is doing the same to the earth, but much slower. It would take about 50 billion years for the earth to become tidally locked to the moon, but the Sun will have destroyed us long before that. Most means are tidally locked to their highest planet. No planets in our solar system are tidally locked to their moons, but the dwarf planet Pluto is tidally locked to its largest moon, Karen. So now you're probably thinking that without space probes we can see 50% of the moon. That makes sense, but it's not quite true. Get a big ball, a beach ball is good for this. Look at it, and then move closer and further away. You'll notice that as you get closer you can see less of it because of the angle of your lines of sight. So we can see slightly less than 50% of the moon. That's still not the whole story. There's an effect called libration, which comes in three different varieties. As we move across the earth, our angle of sight to the moon changes, letting us peek around the edges. This video shows the moon viewed at exactly the same time, but from different latitudes, from the North Pole to the South Pole. Similarly, we move from west to east as the earth rotates, so that at different times of the day or night we can peek around the sights. This is called diurnal libration. This video shows the other two types of libration, which occur throughout a month. First, because the moon's orbit is elliptical, not circular, we see it at slightly different angles, and we can sometimes see further east and sometimes further west. This is libration in longitude. Second, because the moon's axis is tilted about seven degrees from the plane of its orbit. We can see sometimes past the North Pole and sometimes past the South Pole. This is called libration in latitude. With all these effects combined, we can see a total of 59% of the moon's surface from earth. For the rest, we need spacecraft. That's all for this video. Here's a summary of what you need to know. You're given the diameter and distance to the moon in the exam, but you need to know the rotation and orbital period of 27.3 days, and that libration lets us see 59% of the moon from earth. I've got two more videos on the moon. One where I discuss the moon's phases, eclipses and tides, and one where I talk about the origin of the moon, its internal structure and the Apollo missions. Thank you for watching. Goodbye and have an excellent day. you